Air Brake Manual Comp

102
Air Brake Manual Working with drivers to make our roads safer.

Transcript of Air Brake Manual Comp

Page 1: Air Brake Manual Comp

Air Brake ManualWorking with drivers to make our roads safer.

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ForewordThe Air Brake Manual has been prepared by Manitoba Public Insurance, to assist drivers in understanding the basic operation and function of an air brake system. The study of this manual, together with practical instruction, is recommended for a driver who is preparing for the air brake examination. A large illustration of a complete dual air brake system is located on the inside cover and can be folded out and referred to when studying this manual. Study questions are included at the end of each section so that readers may self–test their understanding of the subject matter. Drivers who have qualified and are authorized to operate air brake equipped vehicles are encouraged to review this manual on a periodic basis to ensure they are fully aware of the proper method of inspecting an air brake system and identifying problems that can occur when the system malfunctions.

The illustrations and explanations of various types of brake system designs are provided for instructional purposes only. Most air gauges measure in imperial units. Therefore the measurements used and relating to the air brake system will be in imperial units. This manual has no legislative sanction. For interpreting and applying the law, consult The Drivers and Vehicles Act, The Highway Traffic Act and its regulations. Sales of this manual are final and not refundable.

We gratefully acknowledge the contributions of all jurisdictions, particularly British Columbia.

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Air Brake Endorsement• Permits the holder to drive vehicles

equipped with air brakes in class of vehicle for which the driver is licenced.

• To adjust manual or automatic slack adjusters, the operator must hold an “S” brake endorsement.

Requirements for Air Brake Endorsement• Must complete an Air Brake Written Test.

• Must complete an Air Brake Practical Test.

An air brake endorsement is not required for a licensed holder to drive a Class 3 or 5 air brake equipped vehicle if it is registered as a farm vehicle. Class 3 or 5 air brake equipped vehicles, fitted with dealer plates, may be driven by farmers for demonstration purposes.

Use of Licence as a Learner PermitA holder of any licence may be authorized to operate vehicles equipped with air brakes as a learner after meeting the required written standards. The learner must be supervised by someone who has held, for at least two years, an air brake endorsement.

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Dual Air Brake System

1 2 3 4 5 6 7 8 9 10 11

11 1223242526282729

31

3332

34

iii

30

6 6

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12 13 14 15

16

17

6

18 15141920212212

1. Compressor 2. Governor 3. Air dryer 4. Safety valve 5. Supply/wet reservoir 6. Drain valves 7. One–way check valves 8. Primary/dry reservoir 9. Low pressure indicator 10. Secondary/dry reservoir 11. Rear service brake chambers 12. Spring parking brake chambers 13. Tractor relay valves 14. Trailer service brake chamber 15. Trailer spring parking brake chamber 16. Trailer reservoirs 17. Trailer relay valve 18. Trailer spring brake valve 19. Anti–compound lines 20. Glad hands

21. Supply (emergency) line 22. Control (service) line 23. Spring brake modulator valve 24. Tractor protection valve 25. Stop lamp switch 26. Two–way check valves 27. Spring parking brake control valve 28. Trailer supply valve 29. Reservoir air pressure gauges 30. Trailer brake hand valve 31. Foot valve 32. Front service brake chambers 33. Quick release valve 34. Automatic front brake limiting valve

This illustration has an automatic front brake limiting valve (34), and therefore the control valve (35), for a manual front brake limiting valve (36) are not shown here, but appear later in the manual.

Legend

blue–supply/wet

green–primary/dry

red–secondary/dry

yellow–spring parking

brake system

dark green–trailer system

In–cab portion is

highlighted

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CHECK THE SLACK!

It is up to YOU,

the DRIVER,

to ensure that

your vehicle has

safe, properly

adjusted brakes.

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1 • Air Brake Manual

Table of Contents

Foreword Foldout i AirBrakeEndorsement Foldout iiRequirementsforAirBrakeEndorsement Foldout iiUseofLicenceasaLearnerPermit Foldout ii DualAirBrakeSystemIllustration Foldout iiiHowtoBookYourTestAppointment 3KeyPointsAboutYourTests 4

Section 1—Brakes and Braking 5

Heat–Energy–Traction–Friction 6Speed–Weight–Distance 7BrakingForce 7UseofAirPressure 8LeverageandAirPressure 8StoppingDistance 8SectionSummaryQuestions 9

Section 2—The Componentsof an Air Brake System 11

TheComponentsofanAirBrakeSystem 12 CompressorandGovernor 12 Reservoirs 14 AirDryer 15 SafetyValve 16 FootValve 16 BrakeChambers,SlackAdjusters andBrakeLining 16 WedgeBrakes 20 DiscBrakes 21 Air–Over–HydraulicBrakeSystems 21 Air–Actuated–HydraulicBrakeSystem 21 Air–Boost–HydraulicBrakeSystem 22 SectionSummaryQuestions 24

Section 3—How the Basic System Works 25

BasicAirBrakeSystem 26 One–wayCheckValve 26 AirPressureGauge 27 BrakeApplicationGauge 27 LowPressureWarningDevice 27 StopLightSwitch 27 QuickReleaseValve 28 RelayValve 28 ManualFrontBrakeLimitingValve 28 AutomaticFrontBrakeLimitingValve 29 TandemRearAxles 30 SectionSummaryQuestions 30

Section 4—Spring Parking Brakes 31

SpringParkingBrakeSystems 32 UsingaSpringParkingBrake 33 MechanicalRelease(Caging) 35 SectionSummaryQuestions 35

Section 5—Trailer System 37

GladHands 38 ApplicationLine 38 TrailerBrakeHandValve 39 Two–wayCheckValves 40 TractorProtectionSystem 41 TractorProtectionValve 42 TrailerSupplyValve 43 AutomaticTrailerSupplyValveSystem 44 TractorandTrailerCoupled 46 ChargingtheTrailerSystem 47 FootorHandValveBrakeApplication 47 EmergencyApplication 48 Supply(Emergency)LineRupture 49 Control(Service)LineRupture 49 LossofReservoirAirPressure 50 ManualTrailerSupplyValve 51 TrailerSpringParkingBrakes 52 SectionSummaryQuestions 52

1 Air Brake Manual

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2 • Air Brake Manual

Section 6—Dual Air Brake System 53

BasicDualAirBrakeSystem 54 DualAirBrakeSystemwith SpringParkingBrakes 56 SpringParkingBrakeswithModulatorValve 57 CombinationTractorandTrailer

withSpringParkingBrakes 58SectionSummaryQuestions 59

Section 7—Electronic ControlledBraking and Traction Systems 61

Anti–lockBrakeSystem(ABS) 62 AutomaticTractionControl(ATC) 64 SectionSummaryQuestions 64

Section 8—Brake Adjustment and In–Service Check 65

BrakeAdjustment 66 BrakeAdjustment—It’sCritical 66 BrakeAdjustment—It’stheLaw 66 Definition 67 CheckingandAdjustingS–CamBrakes —ManualSlackAdjusters 69 PryMethodofFreeStrokeMeasurement 69 AppliedStrokeMethod (servicebrakeapplication) 70 BrakeAdjustmentIndicators 70 BrakeAdjustment—ManualSlackAdjuster 71 Adjustment 71 AdjustingS–CamBrakeswithAutomatic SlackAdjusters—Why—When—How 72 EmergencyManualBrakeAdjustment —AutomaticSlackAdjusters 73 DiscBrakeAdjustment 75 WedgeBrakeAdjustment 75 AfteraBrakeAdjustment 75 In-ServiceChecks 76 MaintenanceandServicing oftheAirBrakeSystem 76 AirBrakeAdjustmentMyths 77 SectionSummaryQuestions 78

Section 9—Pre–Trip and Post–Trip Air Brake Inspection 79

VehicleInspection 80 AirBrakePracticalTest 80 Pre–TripInspection—CombinationUnit 81 BrakeAdjustment—CombinationUnit 82 Post–TripInspection—CombinationUnit 84 Pre–TripInspection—SingleUnit 85 BrakeAdjustment—SingleUnit 86 Post–TripInspection—SingleUnit 87 Pre–TripInspection—Air–Over–Hydraulic (AirActuated)BrakeSystem 88 Post–TripInspection—Air–Over–Hydraulic (AirActuated)BrakeSystem 89 SectionSummaryQuestions 89

MetricConversionTable 89Pre-tripInspectionTearOutSheet 91OrganandTissueDonation 93

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Test Appointments How to Book Your Knowledge or Road Test

1 Visit any Autopac agent* to pay for your test and to schedule your test appointment.

2 The agent can provide you details of available appointments at any testing location.

3 You can schedule your test appointment up to eight weeks in advance.

Test Locations

Winnipeg• 15 Barnes Street (at Bison Drive) • 40 Lexington Park (at Gateway Road) • 1284 Main Street • 930 St. Mary’s Road (French Language Services available) • 125 King Edward Street East • 1103 Pacific Avenue • 420 Pembina Highway

Outside Winnipeg• Arborg | 323 Sunset Boulevard• Beausejour | 848 Park Avenue • Brandon | 731–1st Street • Dauphin | 217 Industrial Road • Portage la Prairie | 25 Tupper Street• Selkirk | 1008 Manitoba Avenue • Steinbach | 165 Park Road West, Unit 2, Clearspring Village Mall• Thompson | 53 Commercial Place • Winkler | 355 Boundary Trail

Mobile Test UnitsMobile Test Units serve rural customers living outside Service Centre communities. Locations and schedules are available from any Autopac agent.

*For a list of Autopac agents, see mpi.mb.ca.

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Key Points About Your Tests

Cost*• A knowledge test costs $10.• Costs for road tests: • Class 1 licence: $50 • Class 5 licence: $30 • Class 2 or 3 licence: $45 • Class 6 licence: $30 • Class 4 licence: $35 • Air Brake: $30**

*Cost may change ** There is no additional charge for the air brake test if you are taking a road test for a class

1, 2, 3, 4 or 5 licence and the vehicle you are using for the test is equipped with air brakes

• No cost applies for tests in these two situations: • You’re 65 or older; or • Our Medical Compliance and Assessments department requires you

to take the test

General Testing Information • There is a 30 minute time limit to complete a knowledge test

• Both knowledge and road tests are conducted during weekdays only (with the exception of holidays) at most locations. Saturday appointments are available at some Winnipeg locations

• Only one knowledge or road test of the same class may be completed per day

• There is a waiting period between the knowledge and road test for some classes of licences

• If you are in Graduated Driver Licensing and are not successful on your knowledge test, there is a seven day waiting period before you may take the test again

• If you are in Graduated Driver Licensing and are not successful on your road test, there is a 14 day waiting period before you may take the test again

• The knowledge test is not an open book test. Cellular phones and electronic devices are not allowed in the test area

• Please check-in 15 minutes prior to your scheduled appointment time

If you need to call one of these locations other than to book a test appointment:• In Winnipeg call 204–985–7000• Outside Winnipeg call 1–800–665–2410

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Air Brake Manual • 5

BrakesandBraking

Section1

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6 • Air Brake Manual

Heat–Energy–Traction–FrictionForavehicletomovealongthehighway,aninternalcombustionenginemustconvertitsheatenergyintomechanicalenergy.Thismechanicalenergygoesfromtheenginetothedrivingwheeltiresbymeansofasystemofconnectingrods,shaftsandgears.Thefinalfactorthatmovesthevehicleistheamountoftractionitstireshaveontheroadsurface.

Frictionistheforcethatresistsmovementbetweentwosurfacesincontactwitheachother.Tostopavehicle,thebrakeshoeliningsareforcedagainstthemachinedsurfacesofthebrakedrums,creatingfriction.Thisfrictionproducesheat.

Theengineconvertstheenergyofheatintotheenergyofmotion;thebrakesmustconvertthisenergyofmotionbackintotheenergyofheat.Thefrictionbetweenbrakedrumsandliningsgeneratesheatwhilereducingthemechanicalenergyoftherevolvingbrakedrumsandwheels.Theheatproducedisabsorbedbythemetalbrakedrums,whichdissipatetheheatintotheatmosphere.Theamountofheatthebrakedrumscanabsorbdependsonthethicknessofthemetal.Whenenoughfrictioniscreatedbetweenthebrakeliningandthedrums,thewheelsstopturning.Thefinalfactorthatstopsthevehicleisthetractionbetweenthetiresandtheroadsurface.

Ifa200–horsepowerengineacceleratesavehicleto100km/hinoneminute,imaginethepowerneededtostopthissamevehicle.Also,considerthatthevehiclemighthavetostopinanemergencyinaslittleassixseconds(just1/!)thetimeittooktoreach100km/h).

Tostopthevehiclein1/!)thetimeittooktoacceleratewouldrequireastoppingforceof10timestheaccelerationforce—theequivalentofapproximately2,000horsepower.Ifthevehiclehadsixwheels,eachwheelwouldhavetoprovide1/^thebrakingforce.Ifoneortwoofthewheelshadbrakesthatwerenotproperlyadjusted,theotherwheelswouldhavetodomorethantheirshareofthebraking,andthatmightbemorethantheirbrakeswereconstructedtostand.Excessiveuseofthebrakeswouldthenresultinabuildupofheatgreaterthanthebrakedrumscouldabsorbanddissipate.Toomuchheatresultsinbrakedamageandpossiblefailure.

Mostbrakeliningsoperatebestataround250°Candshouldnotexceed425°C.It’simportanttounderstandthatthepowerneededtostopgeneratesheatwhichcoulddamagethebrakes.

100 km/h

10X

Brake Drums

250°C Normal

1100°C Panic!

425°C Maximum

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Air Brake Manual • 7

Speed–Weight–DistanceThedistancerequiredtostopavehicledependsonitsspeedandweight,inadditiontoenergy,heatandfriction.Thebrakingforcerequiredtostopavehiclevariesdirectlywithitsweightandspeed.Forexample,iftheweightisdoubled,thebrakingforcemustbedoubledtobeabletostopinthesamedistance.Ifthespeedisdoubled,thebrakingforcemustbeincreasedfourtimestobeabletostopinthesamedistance.Whenweightandspeedarebothdoubled,thebrakingforcemustbeincreasedeighttimestobeabletostopinthesamedistance.

Forexample,avehiclecarryingaloadof14,000kgat16km/hisbroughttoastopin30metreswithnormalapplicationofthebrakes.Ifthissamevehiclecarried28,000kgat32km/h,itwouldrequireeighttimesthebrakingforcetostopthevehiclein30metres.Thiswouldbemorebrakingforcethanthebrakescouldprovide.Novehiclehasenoughbrakingforcewhenitexceedsitslimitations.

Braking Force

Mechanical

Brakingsystemsusedevicestogainamechanicaladvantage.Themostcommondeviceforthispurposeisleverage.Aleverisplacedonapivotcalledthefulcrum.AsthedistancefromAtoCisfourfeet,andfromCtoBisonefoot,theratioisfourtoone(4:1).Forcehasbeenmultipliedbytheleverageprinciple.

Lookatthissimpleleversystem:

Ifa100lbdownwardforceisappliedatpointA,thentheupwardforceatpointBis400lb.

B = 400 lb

C

A = 100 lb

C

A 100 lb

S–cam brake

B 400 lb

B 400 lb

A B

4 feet 1 foot

C

Applied force = 100 lb

Delivered force = 400 lb

A

BC

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8 • Air Brake Manual

Use of Air PressureForcecanalsobemultipliedbytheuseofairtogainfurthermechanicaladvantage.Everyonehasfelttheforceofaironawindyday.Aircanbecompressed(squeezed)intoamuchsmallerspacethanitnormallywouldoccupy,forinstance,aircompressedintirestosupporttheweightofavehicle.Thesmallerthespaceintowhichairissqueezed,thegreatertheair’sresistancetobeingsqueezed.Thisresistancecreatespressure,whichisusedtogainmechanicaladvantage.

Ifaconstantsupplyofcompressedairisdirectedthroughapipethatisoneinchsquare,andifaoneinchsquareplugwereplacedinthepipe,thecompressedairwouldpushagainsttheplug.Ascalecanbeusedtomeasurehowmanypoundsofforcearebeingexertedbytheairagainsttheplug.

Ifthescaleregisters10pounds,forexample,thenitcouldbesaidtheforceis10poundsontheonesquareinchsurfaceoftheplugor10poundspersquareinch(psi).

Themorecompressedtheairinthesupplyreservoir,thegreatertheforceexertedonthefaceoftheplug.

Leverage and Air PressureInactualoperation,pipesareroundandplugsarediaphragmsofflexiblematerialactingagainstpushrods.Ifcompressedairof120psiactsonadiaphragmof30squareinches,3,600lbofforceisproduced(120x30).Applythisforcetoapushrodtomovea6–inchslackadjusteroperatingacamandthetotalforceequals21,600inchpoundstorque(3,600x6),or1,800footpoundstorque(21,600÷12).Itrequires25to30footpoundsoftorquetotightenthewheelonacar.Thiscomparisonillustratestheforceobtainedfromusingmechanicalleverageandairpressurecombined.

Stopping DistanceStoppingdistanceconsistsofthreefactors:

•driver’sreactiontime

•brakelag

•brakingdistance

Driver’sreactiontime:Reactiontimeisoftencalled“thinkingtime”.Thetimeittakesfromthemomentahazardisrecognizedtothetimethebrakeisapplied,approximatelyPofasecond.

Brakelag:Asairishighlycompressible,itrequiresarelativelylargevolumeofairtobetransmittedfromthereservoirtothebrakechamberbeforethereisenoughpressureforthebrakestoapply.Itcanbesaidthatbrakelagisthetimeittakestheairtotravelthroughaproperlymaintainedairbrakesystem(approximately4/!)ofasecond).

1 square inch

10 psi

30 square inches

120 psi

6 inches

1 inch

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Air Brake Manual • 9

Passenger car

Brakes applied

Loaded truck

Actual stop

Actual stop

Brakingdistance:Theactualdistancethevehicletravelsafterthebrakeisapplieduntilthevehiclestops.

Thedistancedependsontheabilityofthebrakeliningtoproducefriction,thebrakedrumstodissipateheatandthetirestogriptheroad.

Driversshouldnevertaketheirbrakesforgranted.Thebrakingsystemmustbetestedandtheadjustmentcheckedbeforeplacingthevehicleintoservice.Driversmustunderstandthebrakingsystem,realizeitscapabilitiesandlimitations,andlearntousethemtothebestadvantage.

Heavyvehiclesrequirepowerfulbrakingsystemsthatareobtainedbyuseofmechanicalleverageandairpressure.Brakesmustbeusedkeepinginmindtheheatgeneratedbyfriction.Iftheheatbecomestoogreat,brakingeffectivenesswillbelost.Theheaviertheloadandthefasterthespeed,thegreatertheforceneededtostop.

Itisimportanttorememberthatanairbrakeequippedvehicle,evenwithproperlyadjustedbrakes,willnotstopasquicklyasapassengercar.

Comparative Stopping Distances

Section Summary Questions

1. Whatisthefinalfactorthatwilldetermineifthevehiclewillmove?

2. Whatisthefinalfactorthatwilldetermineifthevehiclewillstop?

3. Howistheheatthatisgeneratedbythebrakesdissipated?

4. Ifonesetofbrakeshoesispoorlyadjusted,whateffectcouldithaveontheremainingsetsofbrakeshoesinthesystem?

5. Whatismeantbytheterm“friction”?

6. Iftheweightofthevehicleisdoubled,howmanytimesmustthestoppingpowerbeincreased?

7. Ifthespeedofthevehicleisdoubled,howmanytimesmustthestoppingpowerbeincreasedtobeabletostopatthesamedistance?

8. Ifbothweightandspeedofthevehiclearedoubled,howmanytimesmustthestoppingpowerbeincreasedtostopatthesamedistance?

9. Whatiscompressedair?

10. Whatdoestheabbreviation“psi”standfor?

11.If40psiisexertedagainstadiaphragmof30squareinchesinarea,whatarethetotalpoundsofforcethatcouldbeexerted?

12.Stoppingdistanceconsistsofwhatthreefactors?

13.Definethefollowingterms?

•Driver’sReactionTime

•BrakingDistance

•BrakeLag

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10 • Air Brake Manual

Notes

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Air Brake Manual • 11

TheComponentsofanAirBrakeSystem

Section 2

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12 • Air Brake Manual

SectionOneofthismanualhasexplainedthatitispossibletogainamechanicaladvantagethroughtheuseofleversandthatairunderpressurecanbeusedtogainamechanicaladvantage.SectionTwowillexplainhowairunderpressurecanbeusedtooperatetheairbrakesofavehicle.

Pipingillustrationshavebeenkeptsimpleinordertobeeasilyunderstood.Thepipingarrangementsfoundonvehiclesinactualuseonthehighwaymightdiffersomewhatfromtheillustrationsinthismanual.

The Components of an Air Brake System

Abasicairbrakesystemcapableofstoppingavehiclehasfivemaincomponents:

1.Acompressortopumpairwithagovernortocontrolit.

2.Areservoirortanktostorethecompressedair.

3.Afootvalvetoregulatetheflowofcompressedairfromthereservoirwhenitisneededforbraking.

4.Brakechambersandslackadjusterstotransfertheforceexertedbythecompressedairtomechanicallinkages.

5.Brakeliningsanddrumsorrotorstocreatethefrictionrequiredtostopthewheels.

Itisnecessarytounderstandhoweachofthesecomponentsworkbeforestudyingtheirfunctionsintheairbrakesystem.

Compressor and Governor

Compressedairisusedtotransmitforceinanairbrakesystem.Thesourceofthecompressedairisacompressor(1).Acompressorisdesignedtopumpairintoareservoirwhichresultsinpressurizedair.

Thecompressorisdrivenbythevehicle’sengine,eitherbybeltsandpulleysorshaftsandgears.Invehicleswherethecompressorisdrivenbybelts,theyshouldbecheckedregularlyforcracksandtension.Also,checkthecompressorforbrokenmountingbracketsorloosebolts.

Thecompressorisinconstantdrivewiththeengine.Whenevertheengineisrunning,soisthecompressor.Whenpressureinthesystemisadequate,anywherefromalowof80psitoahighof135psiitisnotnecessaryforthecompressortopumpair.Agovernor(2)controlstheminimumandmaximumairpressureinthesystembycontrollingwhenthecompressorpumpsair.Thisisknownasthe“loading”or“unloading”stage.Mostcompressorshavetwocylinderssimilartoanengine’scylinders.Whenthesystempressurereachesitsmaximum,whichisbetween115and135psi,thegovernorplacesthecompressorinthe“unloading”stage.

Thecompressormustbeabletobuildreservoirairpressurefrom50to90psiwithinthreeminutes.Ifunabletodosothecompressorrequiresservicing.Acompressormaynotbeabletobuildairpressurefrom50to90psiwithinthreeminutesiftheairfilterispluggedorifthebeltwasslipping,ifthesewerenotatfaultthecompressorcouldbefaulty.

Governor

Pressure setting spring

Exhaust port

Unload port

Reservoir port

Exhaust port

Unload port

Reservoir port

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Air Brake Manual • 13

Placingthecompressorintheunloadingstageisdonebydirectingairpressuretotheinletvalvesofthecompressor,holdingthemopen,allowingtheairtobepumpedbackandforthbetweenthetwocylinders,insteadofcompressingtheair.Whenthepressureinthesystemdrops,theinletvalvesclose,returningthecompressortothe“loading”stage.Thegovernormustplacethecompressorinthe“loading”stageatnolowerthan80psi.Duringthe“unloading”stage,thecompressorisabletocool.

Usuallycompressorsarelubricatedfromtheenginelubricationsystem,althoughsomecompressorsareself–lubricatingandrequireregularchecksofthelubricantlevel.

Itisveryimportanttheairthatentersthesystembekeptascleanaspossible.Theairmustfirstpassthroughafiltertoremoveanydustparticles.Theairfiltermustbecleanedregularly.Adirtyfilterwillrestricttheflowofairintothecompressor,reducingitsefficiency.Somevehicleshavetheinletportofthecompressorconnectedtotheintakemanifoldandreceiveairthathasbeenfilteredbytheengineaircleaner.

Apistontypecompressoroperatesonthesameprincipleastheintakeandcompressionstrokesofanengine.

• Intakestroke:Thedownwardstrokeofthepistoncreatesavacuumwithinthecylinderwhichcausestheinletvalvetoopen.Thiscausesatmosphericairtoflowpasttheinletvalveintothecylinder.

Piston

From governor

Intake air filter

Unload plunger

Inlet valve

Discharge valve

Compressor (Unloading stage)

Intake air filter

Unload plunger

Inlet valve

Discharge valve

Compressor (Intake stroke)

Piston

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14 • Air Brake Manual

•Compressionstroke:Theupwardmotionofthepistoncompressestheairinthecylinder.Therisingpressurecannotescapepasttheinletvalve(whichthecompressedairhasclosed).Asthepistonnearsthetopofthestroke,thepressurizedairisforcedpastthedischargevalveandintothedischargelineleadingtothereservoir.

Reservoirs

Reservoirsortanksholdasupplyofcompressedair.Thenumberandsizeofthereservoirsonavehiclewilldependonthenumberofbrakechambersandtheirsize,alongwiththeparkingbrakeconfiguration.Airbrakevehiclesareequippedwithmorethanonereservoir.Thisgivesthesystemalargervolumeofmainreservoirair.Thefirstreservoirafterthecompressorisreferredtoasthesupplyorwet(5)reservoir.Theotherreservoirsareknownasprimary(8)andsecondary(10)ordry(8)(10)reservoirs.Whenairiscompressed,itbecomeshot.Theheatedaircoolsinthereservoir,formingcondensation.Itisinthisreservoirthatmostofthewateriscondensedfromtheincomingair.Ifoilleakspastthepistonringsofthecompressorandmixeswiththismoisture,itformssludge,which

accumulatesinthebottomofthereservoir.Ifallowedtoaccumulate,thissludge(waterandoil)wouldenterthebrakingsystemandcouldcausetroublewithvalvesandotherparts.Inwinter,waterinthesystemmayfreeze,causingthemalfunctionofvalvesorbrakechambers.Reservoirsareequippedwithdrainvalvessothatanymoistureorsludgethatmayhaveaccumulatedcanbedrained.Ifyounoticesludgewhendrainingyoursystem,haveitinspectedbyamechanic.Tominimizetheamountofwatercollection,allreservoirsmustbedraineddaily.Underextremeconditions,reservoirsmayhavetobedrainedmorethanonceaday.Todrainthereservoirsalwaysstartwiththewetreservoironthetractor.Openthedrainvalvefullyandallowallairpressuretoescape,whichwillalsoexhaustthemoisturecollectedinthereservoir.

Somereservoirshavemorethanonecompartmentandeachcompartmenthasitsowndrainvalve,whichmustbedrainedindividually.Brieflyopeningthevalvejusttoallowsomeoftheairtoescapedoesnotdrainthemoisture!Itisnotsafetoassumethatthewetreservoir,orthepresenceofanairdryerisreasontoneglecttheotherreservoirsonthepowerunit,trailersordollies.Theyshouldallbecompletelydraineddailybyopeningthedrainvalvefullyandallowingalloftheairtoescape.Thisshouldbedoneduringthepost–tripinspectionattheendoftheday.Seepost–tripInspectionlaterinthismanualformoreinformation.

Somereservoirsmaybeequippedwithautomaticreservoirdrainvalves(spittervalves).Thesevalveswillautomaticallyexhaustmoisturefromthereservoirwhenrequired,althoughtheyshouldbecheckeddailyanddrainedperiodicallytoensurethemechanismisfunctioningproperly.Anylooseordisconnectedwiresassociatedwiththevalveheatersshouldberepairedimmediately.

Reservoir

Intake air filter

Unload plunger

To reservoir

Inlet valve

Discharge valve

Compressor (Compression stroke)

Piston

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Air Brake Manual • 15

Air Dryer

Anairdryer(3)maybeinstalledbetweenthecompressorandthewetreservoirtohelpremovemoisturefromthecompressedair.Itmaybepartiallyfilledwithahighmoisture–absorbentdesiccantandanoilfilter,oritmaybehollowwithbafflesdesignedtoassistinseparatingthemoisturefromtheair.Bothtypesofairdryersuseairpressuretopurgeorejecttheaccumulatedcontaminantsfromtheirdesiccantbed.Thepurgevalvehasaheaterelement,whichpreventsthemoisturefromfreezingincoldclimateoperation.Thewiringconnectedtotheheatershouldbeinspectedforlooseordisconnectedwires.Theyarealsoequippedwithasafetyvalve.

Control port

Supply port

Cut–off piston

Exhaust

Purge valve

Delivery port

One–way check valve

One–way check valveOrifice

Desiccant bed

Control PortDried Air

Check valve assembly

Delivery Port

Heater element

ExhaustPurge valve

Cut–off piston

Supply Port

ReservoirCompressor

GovernorSump

Air Dryer (Purge cycle)

Desiccant Cartridge

Air Dryer (Drying cycle)

Air Dryer

Control Port

Supply Port

Oil Separator

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Safety Valve

Asafetyvalve(4)protectsreservoirsfrombecomingoverpressurizedandburstingifthegovernormalfunctionedanddidnotplacethecompressorintheunloadingstage.Thevalveconsistsofaspring–loadedballthatwillallowairtoexhaustfromthereservoirintotheatmosphere.Thevalve’spressuresettingisdeterminedbytheforceofthespring.Asafetyvalveisnormallysetat150psi.Ifthepressureinthesystemrisestoapproximately150psi,thepressurewouldforcetheballoffitsseat,allowingthepressuretoexhaustthroughtheexhaustportinthespringcage.Whenreservoirpressureissufficientlyreducedtoapproximately135psi,thespringwillforcetheballbackontoitsseat,sealingoffthereservoirpressure.Notallsafetyvalveshaveamanualreleasefeature.

Ifthesafetyvalvehastorelievepressure,thegovernororcompressorrequiresadjustment,serviceorrepair.Thisshouldbedonebyaqualifiedmechanic.

Foot Valve

Thefoot–operatedvalve(31)isthemeansofapplyingairtooperatethebrakes.Thedistancethetreadleofthefootvalveisdepressedbythedriverdeterminestheairpressurethatwillbeapplied,butthemaximumapplicationwillnotexceedthepressureinthereservoir.Releasingthefootvalvetreadlereleasesthebrakes.

Whenthedriverappliesthebrakes,depressingthetreadlepartway,thefootvalvewillautomaticallymaintaintheapplicationairpressurewithoutthedriverhavingtoadjustthepressureofhisfootonthetreadle.

Releasingthetreadleallowstheapplicationairtobereleasedthroughtheexhaustportsintotheatmosphere.Airtreadlesarespringloaded,producingadifferent“feel”fromhydraulicbrakeapplications.

Brake Chambers, Slack Adjusters and Brake Lining

Brake Chamber and Slack Adjuster (Brakes off)

Treadle

Treadle spring

Supply from reservoir

Exhaust Port

Push rod

Brake chamber

DiaphragmDiaphragm

return spring

Air inlet

Mounting bolts

Clevis and pin

Slack adjuster

Air pressure greater than 150 psi

Safety Valve

Foot Valve

To brake chambers

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Air Brake Manual • 17

Abrakechamber(11)(14)(32)isacircularcontainerdividedinthemiddlebyaflexiblediaphragm.Airpressurepushingagainstthediaphragmcausesittomoveawayfromthepressure,forcingthepushrodoutwardagainsttheslackadjuster.Theforceexertedbythismotiondependsonairpressureanddiaphragmsize.Ifaleakoccursinthediaphragm,airisallowedtoescape,reducingtheeffectivenessofthebrakechamber.Ifthediaphragmiscompletelyruptured,brakesbecomeineffective.

Frontbrakechambers(32)areusuallysmallerthanthoseintherearbecausefrontaxlescarrylessweight.Abrakechamberisusuallymountedontheaxle,nearthewheelthatistobeequippedforbraking.Airpressureisfedthroughaninletport.Theairpushesagainstthediaphragmandthepushrod.Thepushrodisconnectedbyaclevisandpintoacrankarm–typelevercalleda“slackadjuster”.ThisconvertsthepushingmotionofthepushrodfromthebrakechambertoatwistingmotionofthebrakecamshaftandS–cams.Whentheairisexhausted,thereturnspringinthebrakechamberreturnsthediaphragmandpushrodtothereleasedposition.

Asindicatedbyitsname,theslackadjusteradjuststhe“slack”orfreeplayinthelinkagebetweenthepushrodandthebrakeshoes.Thisslackoccursasthebrakeliningswear.Iftheslackadjustersarenotadjustedwithinthelimitations,effectivebrakingisreducedandbrakelagtimeisincreased.Iftoomuchslackdevelops,thediaphragmwilleventually“bottom”inthebrakechamber,andthebrakeswillnotbeeffective.

Illustratedbelowaretwocommontypesofmanualslackadjusters,showingthewormadjustinggear.

Push rod

Brake chamber

Diaphragm Diaphragm return spring

Air inlet

Mounting bolts

Clevis and pin

Slack adjuster

Manual Slack Adjusters

Ball Indent Slack Adjuster Positive Lock Slack Adjuster

Lock screw

Adjusting bolt Worm shaft

Worm gear

Locking collar

Spline Spline

Grease fitting

Adjusting bolt

Brake Chamber and Slack Adjuster (Brakes on)

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18 • Air Brake Manual

Onmanualslackadjusters,theadjustingwormboltisturneduntilthebrakeliningstouchthedrumsandthenbackedoff,normallyN toMaturn.Alockingdevice,whichmaybeaspringloadedcollarovertheheadoftheadjustingbolt,mustbedepressedwhenthewrenchisslippedoverthebolthead.Thisisknownasapositivelockslackadjuster.

Ortheycoulduseaspring–loadedinternalcheckballtolocktheadjustment,anditmustberemovedtomakeanyadjustment.Thisisknownasaballindentslackadjuster.Themoreoftenthedriverchecksthe“slack”,thelesstheprobabilityofbrakefailure.Vehiclesrarely“lose”theirbrakesbecauseofairloss;itisusuallybecausetheyareoutofadjustment.

Itisthedriver’sresponsibilitytoensurethatbrakesareadjustedcorrectly.Asimpleservicebrakeapplicationatlowspeedtocheckbrakeadjustmentisnotadequate.Brakingathighwayspeedcausesbrakedrumexpansionduetoheat,whichinturnrequiresgreaterpushrodtraveltomaintainthesamebrakingforce.Ifabrakeisoutofadjustmenttherewouldnotbeenoughreservestrokeofthepushrodtraveltocompensatefordrumexpansion.Thiswouldcauseabrakefadeandwouldgreatlyextendstoppingdistance.Iftravellingdownahill,thiscouldcausecompletebrakeloss.

Note:DetailedbrakeadjustmentproceduresareoutlinedinSection8.

Pushrod

Air inlet

Slack adjuster

Brake Chamber and Slack Adjuster (Brakes on)

Thrust washer

Clevis

Clevis pin (large)Clevis pin (small)

Actuator rod

Hairpin clip

Boot and strapActuator (adjusting sleeve)

Roller (pin)

Actuator piston

Pressure relief capscrew (pull pawl)

Pawl spring

Adjusting pawl

Worm

Worm seal

Adjusting bolt

Grease groove

Grease fitting

Housing

Worm gear

Automatic Slack Adjuster

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Air Brake Manual • 19

Somesystemshaveautomaticslackadjustersthatadjustautomaticallytocompensateforbrakeliningwear,usuallymaintainingthecorrectclearancebetweenthebrakelininganddrum.Automaticslackadjustersmustbecheckedregularlytoensurethatcorrectadjustmentisbeingmaintained.Therearevariousmakesandmodelsofautomaticslackadjustersinuse.Primarily,theyareeitherstroke–sensingorclearance–sensing.Astroke–sensingadjusterwilladjusttheslackwhenitsensesthesetstrokeisexceeded.Aclearance–sensingadjusterwilladjustwhentheproperclearancebetweenthebrakedrumandbrakeshoeisnotmaintained.Someautomaticslackadjustershavetheabilitytoback–offorincreasetheslackwhenithasoveradjustedthebrake.Ifavehicleisequippedwithautomaticslackadjusters,itshouldnotbetakenforgrantedthatthebrakeswillalwaysbeinadjustment.Thesystemisnotfoolproof.Anumberoffactorscouldresultintheautomaticslackadjusternotmaintainingproperslack.Therecouldbeimproperinstallation,inadequatemaintenance,deformedbrackets,worncambushings,bentpushrods.Evenpoorvisualinspectioncanresultinproblemsunrelatedtoadjusterfunction.Whenconductingapre–triporpost–tripairbrakeinspection,lookforwornordamagedcomponents.Automaticslackadjusterscanmalfunctionandnotkeepthebrakeinadjustment,especiallywhenithasbeeninserviceforalongperiodoftime.Thetwomostcommonproblemsareexcessiveprematurewearandinternalcontamination.Asanautomaticslackadjusteragesinservice,thecomponentswearthatsensewhenanadjustmentisrequired.Theresultismorestrokeisrequiredfortheliningtocontactthebrakedrum,andifnotcheckedthebrakecouldbeoutofadjustment.Ifevenasmallamountofwaterissuckedintoanautomaticslackadjustermechanismitcancause

corrosionor,inwinter,itcanfreezetheinternalsensingcomponentsandinhibitorpreventadjustment.Also,undercertainconditions,anautomaticslackadjusterthatdoesnothavetheabilitytoback–offorincreaseslack,mayoveradjustabrakecausingittodrag.Forexamplethiscouldtakeplacewhenatractor–trailerisnegotiatingalong,curvingdowngrade.Thedrivershould“snub”thebrakes,whichisrepeatedlyapplyingthebrakesmoderatelytomaintainsafecontrolofthevehicle.Howeveritwouldnottakelonginthisseverebrakingconditionforoneormoreofthebrakedrumstooverheatandexpand.Theoverheatingwillphysicallyincreasethebrakedrumsdiameter,andinextremeandprolongedconditionswillleadtolongerpush–rodstrokestoachievethebrakingforcerequired.Theautomaticslackadjusterinterpretsthisasaneedforadjustmentandwilltakeupslack.Whenthebrakedrumcoolsdownandreturnstonormalsizethebrakesareoveradjustedanddragging.Atthattimethedrivershouldstopandcheckthebrakesforadjustment.Anumberoffullbrakeapplicationsabove90psiperdayarerequiredtokeeptheautomaticslackadjustersinadjustment(seeSection8formoreinformation).

Becauseautomaticslackadjustersarenotfoolproof,itisimportanttheoperatorofavehicleequippedwithautomaticslackadjustersbeabletomanuallyadjustthem.Forinformationonmanuallyadjustingtheautomaticslackadjustersonyourvehicleconsultthemanufacturer.

Illustratedisacommontypeofbrakeassemblyusedontruckrearaxlesandtraileraxles.Afrontaxleassemblyhasthebrakechamberandslackadjustermountedonthebacking–platebecauseofthesteeringaction.

Brake drum

Brake chamberPush rod, clevis and pin

Slack adjuster

S–cam

Brake lining

Brake Assembly

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20 • Air Brake Manual

Brakeliningmaterialisattachedtotheshoes.Thematerialuseddependsonthebrakingrequirementsofthevehicle.Brakeliningmustgiveuniformoutputofbrakeeffortwithminimumfadeathightemperatures.

Fadingorreductioninbrakingeffortoccurswhentheheateddrumsexpandawayfromthebrakelinings.Thebrakeliningsalsolosetheireffectivenesswithoverheating.

ThetwistingactionofthebrakecamshaftandS–camforcesthebrakeshoesandliningsagainstthedrums.Thebrakeliningsgenerateheatfromfrictionwiththebrakedrumsurface.

Thethicknessofthedrumsdeterminestheamountofheattheyareabletoabsorbanddissipateintotheatmosphere.Drumswornthinwillbuildupheattooquickly.Dangerouslyundependablebrakeperformancewillresultfromdistorteddrums,weakreturnsprings,improperlining,pooradjustment,orgreaseordirtonthelining.Drumsmustneverbemachinedorwornbeyondthemanufacturer’sspecification.

Wedge Brakes

Thisisanotherexampleofabrakeassemblyusedonsomeairbrake–equippedvehicles.Theactionofthebrakechamberpushrodforcesawedge–shapedpushrodbetweenthebrakeshoerollers.Thisforcesthebrakeshoeliningagainstthebrakedrum.

Thevehiclemaybeequippedwithasingleordualchambersoneachwheel,dependingonthevehicle’ssizeandstyle.

Thesebrakesmaybeequippedwithaself–adjustingmechanismorwithamanual“starwheel”adjuster.Thestarwheeladjustmentismadewiththevehiclejackedup,toinsurethatthebrakeliningsdonotdrag.Manualadjustmentofwedgebrakesisusuallydonebyaqualifiedmechanic.

Brake chamber

Adjusting wheel

Brake shoe roller

Push rod

Shoe return spring

Brake lining

Brake shoe

Wedge Brake – Single Chamber

Brake chamber

Single chamber Dual chamber

Shoe return springs

Brake lining

Adjusting wheel

Brake chambers

Adjusting wheel

Wedge Brakes

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Air Brake Manual • 21

Disc Brakes

Theair–activatedheavytruckdiscbrakeissimilarinprincipletothatusedonpassengervehicles.Airpressureactsonabrakechamberandslackadjuster,activatingthebrakes.Insteadofthecamorwedgeusedinconventionalheavytruckdrumbrakes,a“powerscrew”isused.ApowerscrewworkslikeaC–clamp,sothattheliningpadsexertequalforcetobothsidesofthediscorrotor.Sometypesofdiscbrakeshaveabuilt–inautomaticadjuster.DiscbrakesthatrequiremanualadjustmenthaveadjustmentspecificationsthatdifferfromconventionalS–cambrakingsystems.Alwayscheckthemanufacturer’sspecificationsbeforeadjusting.Discbrakeassembliesmayhaveaspringparkingbrakeunitattachedtotheservicebrakechamber.

Air–Over–Hydraulic Brake Systems

Air–over–hydraulicbrakesystemsweredevelopedformediumweightvehiclesbecause:

• dieselenginesdonothaveasourceforvacuumboostingunlesstheyareequippedwithavacuumpump.

• mediumweightvehiclesdonotrequireafullairbrakesystem.

• itgivestheoptionofpullinganairbrakeequippedtrailer.

Thesesystemscombinethebestfeaturesofanairandhydraulicbrakesystem.Theyusehydraulicbrakesateachwheelwiththeirreliableselfadjustersandlimitedmaintenance.Onthesesystemstheairisusedtoeitheractuatethehydraulicbrakesorboostthehydraulicbrakepressureasexplainedinthefollowing.

Air–Actuated Hydraulic Brake System (Air Brake Endorsement Required)

Anair–actuatedsystemusuallyhasthesamecomponentsofastandardairsupplysystemincludingawarningbuzzerandlight,compressor,governor,wetanddryreservoirs,andafootvalvethatcouldbeasingleordualtype.Thesecomponentsarefoundusuallyinthesameplacesasonafullairbrakesystem.Alsothereareoneortwoairactuatedhydraulicpressureconvertersdependingonifthesystemisasingleoradualsystem.Thissystemconsistsofanairchamberorcylinderattachedtoahydraulicmastercylinder.Whenthefootvalveisdepressed,theairpressureactuatesthepushrodfromtheairunitthatpushesagainstthemastercylinderpiston,producinghydraulicpressuredirectedthroughtubingtothewheelcylindersactuatingthefrontandrearaxleservicebrakes.

Disc Brake

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22 • Air Brake Manual

Itisessentialthattheoperatorofsuchavehiclehaveknowledgeofairpressurebuilduptime,governorloadingandunloadingpressure,warningdeviceoperation,andhowtodrainairreservoirsproperly(seeSectionNine;Pre–TripAirBrakeInspection).

Ifanair–actuatedhydraulicbrakesystemwastoloseitsairsupply,thevehiclewouldhavenoservicebrakes.Onlytheparkingbrakewouldbeoperatingasitismechanicalandrequiresnoairpressuretooperate.

Eachvehiclemanufacturermayhavedifferentparkingbrakeapplications,eitherautomaticallywhenairpressureisreducedinthereservoir,ormechanicallybyabrakeontherearofthetransmission,orwiththerearbrakesystem.Sincehydraulicbrakesystemsactuatedbyairpressureareregardedasanairbrakesystem,yourdriver’slicencemusthaveanairbrakeendorsementforyoutooperatevehiclesequippedwithair–activatedhydraulicbrakes.

Astherearemanydifferentsystemsinuse,refertotheoperator’smanual.

Air–Boost Hydraulic Brake System (Air Brake Endorsement not Required)

Anair–boosthydraulicbrakesystemusesairpressuretoassistbrakeforce.Thisissimilartovacuum–assistedbrakesonmostpassengervehicles.Anair–boostsystemusuallyhasthesamecomponentsofastandardairsupplysystemincludingacompressor,governor,wetanddryreservoirs.Thesecomponentsarefoundusuallyinthesameplacesasonafullairbrakesystem.Thebrakepedallinkageoperatesahydraulicmastercylinderthatsendshydraulicpressuretotheboosterunit.Initially,atlowpressurethehydraulicfluidpassesthroughtheboosterandbeginstopressurizethewheelcylindersmovingthebrakeshoesouttothedrums.Theseboosterunitsaresimilarinoperationto“Hypower”or“Hydrovac”vacuumboostersfoundonmostlightandmediumweightvehicles,butairpressureisusedtointensifythehydraulicpressuregeneratedbythemastercylinderratherthanvacuum.Builtintotheboosterunitisahydraulicallyoperatedaircontrolvalve.

Air linesReservoirsCompressor

Foot valve

Hydraulic lines

Air brake chamber

Hydraulic wheel

cylinders

Hydraulic wheel

cylinders

Air lines

Air brake chamber

Hydraulic master cylinder

Hydraulic master cylinder

Air–Actuated Hydraulic Brake System

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Air Brake Manual • 23

Thisiswhereairfromthereservoirisdirected.Asthepressurefromthemastercylinderincreases,theaircontrolsectionintheboosterwillopenandbegintodeliverairpressuretotherearoftheaircylinder.Theaircylinderpushrodtransferspressureonapistoninthehydraulicsectionofthebooster,increasingthehydraulicpressureatthewheelcylinders.

Thedriverhasfullcontrolofthebrakingforceastheaircontrolsectionmodulatestheboostpressureinproportiontothemastercylinderpressure.Ifthevehiclewastolosealloftheairpressurethebrakesystemwouldlosetheairassistboost,howeverthehydraulicsystemwouldcontinuetoworkbutatreducedeffectiveness.Anairbrakeendorsementonadriver’slicenceisnotrequiredtooperateavehiclewiththisbrakesystem.Consulttheoperator’smanualforthevehicleyoudriveformaintenancerequirements.

Air lines

Hydraulic wheel

cylinders

Compressor

Reservoir Hydraulic master cylinder

Brake pedal

Hydraulic lineBooster unit

Air–Boost Hydraulic Brake System

Air lines Booster unit

Hydraulic line

Hydraulic wheel

cylinders

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24 • Air Brake Manual

Section Summary Questions

1. Whatarethefivebasiccomponentsofanairbrakesystem?

2. Atwhatpressureshouldthegovernorcausethecompressortoreturntoits“loading”stage?

3. Atwhatpressurewillthegovernorplacethecompressorinthe“unloading”stage?

4. Howisapluggedairfilterlikelytoaffecttheaircompressor?

5. Whatcausesmoisturetoformintheairbrakesystem?

6. Whenisthecompressorabletoaccomplishmostofitscooling?

7. Howaremostcompressorslubricated?

8. Howoftenshouldthereservoirsbedrained?

9. Isitnecessarytoallowallthepressuretoescapefromthereservoirinordertoremovethemoistureandsludgewhichmayhaveaccumulated?

10. Whatisthemaximumpressureavailableforafullbrakeapplicationatanygiventime?

11. Whatwillresultifthebrakedrumsarewornthinorturnedtoofar?

12. Ifthegovernorvalvefailedto“unload”thecompressor,whatwouldprotectthereservoirsfrombecomingoverpressurizedandbursting?

13. Whatisthepurposeofhavingmorethanonereservoir?

14. Whataretwofunctionsoftheslackadjusters?

15. Doestheamountofslackinthebrakelinkageshaveanyeffectonthebrakingefficiencyofthevehicle?

16. Whatistheadvantageofkeepingthebrakechamberpushrodtraveladjustedwithinlimitations?

17. Whatisthemostcommoncauseoflossofeffectivebrakinginanairbrakesystem?

18. DoautomaticslackadjustersonS–cambrakesrequirechecking?

19. Cantheadjustmentonair–operateddiscbrakesdifferfromS–cambrakes?

20. Whatoccurswhendrumbrakesbecomeoverheated?

21. Whatcausesbrakefade?

22. Whatisthemainfunctionofthefootvalve?

23. Whydoesthe“feel”ofanair–operatedfootvalvedifferfromahydraulicbrakepedal?

24. Onwhatprincipledoesadiscbrakeoperate?

25. Whattypeofair–over–hydraulicbrakesystemrequirestheoperatortoholdanairbrakeendorsement?

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Air Brake Manual • 25

HowtheBasicSystemWorks

Section3

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26 • Air Brake Manual

Basic Air Brake System

Airispumpedbythecompressor(1)tothewetreservoir(5),whichisprotectedfromoverpressurizationbyasafetyvalve(4).Thegovernor(2)controlsthepressureinthereservoirtothebottomofthefootvalve(31).Thedriverpushesthefootvalvetreadledownandairpressureflowstothefrontandrearbrakechambers(32&11).Thebrakechamberpushrodsmovetheslackadjusters.TheslackadjustersrotatetheS–cams,forcingthebrakeshoesagainstthedrums.Thiscausesfrictionthatstopsthewheels.Thedriverreleasesthefootvalvetreadleandtheairinthebrakechambersisallowedtoexhaustthroughthefootvalve,releasingthebrakes.

Thefollowingexplainstheadditionalcomponentsofabasicairbrakesystem.Othervalveswhicharenecessarytoensuresmoothandefficientoperationsarenotincludedinthissimpledrawing.Theywillbediscussedlaterinthemanual.

Note:Anairdryer(3)hasbeenaddedtoreducetheamountofmoistureinthesystem.

One–Way Check Valve

Inthediagrambelow,tworeservoirsareshown(5)(10).Topreventairfromflowingbackwardsinthesystemtowardthecompressor,aone–waycheckvalve(7)isinstalledbetweenthereservoirs.Thisvalveallowstheairtoflowinonedirectiononly.Thevalveisspringloaded.

Pressureattheinletsideovercomesthespringpressureandliftsthecheckvalveball,ordisc,offitsseat.Airpassesthroughthevalvetotheoutlet.Whenpressureattheoutletbecomesgreaterthanattheinlet—togetherwiththespringpressure—thecheckdeviceseats,preventingairfromflowingbackthroughthevalve.

123 4 5 7 9 1032 11

32 31 11

Basic Air Brake System

Ball

Spring

Body

Cap nut

One–Way Check Valve

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Air Brake Manual • 27

Air Pressure Gauge

Vehicleswithanairbrakesystemareequippedwithareservoirairpressuregauge(29).Thisgaugeismountedinthecab,usuallyonthedashboardandindicatestheairpressureintheprimaryandsecondaryordryreservoirs.Thesupplyorwetreservoirdoesnotusuallyhaveanairpressuregauge.Commonoperatingpressuresare80to135psi,dependingonthesystem.Monitoringthegaugewillalertthedrivertoanyunusualchangesinairpressure.

Brake Application Gauge

Anadditionalgaugecanbeinstalledonthedashtoindicatetheapplicationairpressurewhenthebrakesareapplied.Thisgaugecanbepipedtoindicatethepressureofeitherafootorhandapplication.(Handapplicationwillbeexplainedlaterinthemanual.)

Low Pressure Warning Device

Allvehiclesequippedwithanairbrakesystemmusthaveadevicetowarnthedriveriftheairpressureinthesystemdropstoadangerouslevel.Thisdevicecouldbearedwarninglight,abuzzerorawig–wag.Duetooveruseorleaks,thelowpressureindicatorswitch(9)willturnonaredwarninglightonthedashorcauseabuzzertosoundatorbefore60psi.Somevehiclesareequippedwithbothalightandabuzzertowarnthedriverofalowairpressurecondition.

Wig–wagsarenotfoundinmodernvehicleshavingbeenreplacedwitharedwarninglightandbuzzer.Theymaystillbeinuseonoldervehicles.Therearetwotypesofwig–waglowpressurewarningdevicesthatmaybeused.Bothtypeswilldropintothedriver’sviewshouldthesystempressuredropto60psi.Theautomaticwarningdevicewillriseoutofthedriver’sviewwhenthepressureinthesystemrisesabove60psi.Themanualresettypemustbeplacedinthe“outofview”positionmanuallyandwillnotstayinplaceuntilthepressureinthesystemgoesabove60psi.

Whicheverwarningsystemisused,buzzer–lightsorwig–wag,thedrivermuststopthevehicleinasafeplaceimmediatelyandfindthecauseoftheairloss.Theairpressureremaininginthesystem(approximately60psi)isenoughforabrakeapplicationifthedriveractspromptly.

Keepinmindifthevehiclehasspringparkingbrakes,at60psithespringparkingbrakesarepartiallyappliedandyoucannotreleasethemuntilairpressureincreases.Ifyoudonotmoveyourvehicletoasafeplaceyoumayfindyourselfstrandedinthetraveledportionofthehighway.

Stop Light Switch

Anydriverfollowingyourvehiclemustbewarnedwhenreducingspeedorstoppingthevehicle.Thestoplightswitch(25)isanair–operatedelectricswitchthatturnsonthebrakelightsattherearofthevehiclewhenabrakeapplicationisbeingmade.

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Quick Release Valve

Theapplicationofthebrakesinthebasicsystemwasdescribedearlier.Inabasicsystem,whenthedriverreleasesthefootvalve,itwouldbenecessaryfortheairunderpressureinthebrakechamberstoreturntothefootvalvetoreleasethebrakes.Thisreleasingactionwouldbeslowedinlongwheelbasevehiclesbecauseofthelongerlinesbetweenthefootvalveandtherearbrakechambers.Aquickreleasevalve(33)isinstalledtoallowthebrakestoreleasequicklyandfullybydischargingtheapplicationairnearthebrakechambers.Toallowthebrakestoreleasequicklyandfullybydischargingtheapplicationairnearthebrakechambers.

Relay Valve

Thefootvalveisusuallylocatedclosertothefrontwheelsthantotherearwheels.Thelongerthedistancefromthefootvalvetotherearchambers,themoretimeitwilltakebeforetherearbrakesapply.Thisisknownasbrakelag.Tocorrectthisconditiononalongwheelbasevehicle,arelayvalve(13)isinstalledneartherearbrakechambers.Alargerdiameterpipeisconnectedbetweenthemainreservoirandtherelayvalve.Theairlinefromthefootvalvetotherelayvalvenowbecomesa“controlline”.(Theairinthecontrolline“deadends”attherelayvalve.)Whenthefootvalveisdepressed,theairpressureinthecontrollineactsonthetopsectionoftherelayvalve,relayingreservoirairdirectlytotherearbrakechambersthroughthelargerdiameterpipe.Thepressureofthereservoirairdeliveredinthiswaywillbethesameasthecontrolpressuredeliveredbythefootvalve.

Releasingthefootvalveexhauststhecontrolairtotherelayvalve,allowingittocutofftheflowofreservoirairtotherearchambers.Thisinturnexhauststheairinthebrakechambersbythequickreleasefeatureoftherelayvalve.

Manual Front Brake Limiting Valve

Forbettersteeringcontrolwhenbrakingonaslipperyroadsurface,itcanbeanadvantagetoreducethebrakingefforttothefrontwheels.Thiscanbeaccomplishedbyinstallingacontrolvalve(35)inthecab,andafrontbrakelimitingvalve(36)onthefrontaxle.

Thecontrolvalveissetinthe“normal”positionfordryroadsurfacesandthefrontbrakingapplicationairpressureisnormal.Onaslipperyroadsurface,thecontrolvalve(35)issettothe“slipperyroad”position.Inthisposition,thecontrolvalvewillcausethelimitingvalve(36)tooperate.Applyingairpressuretothefrontbrakesisthenreducedto50percentoftheapplicationairpressurebeingdeliveredtotherearbrakechambers.

Delivery ports not shown

Manual Front Brake Limiting Valve

Dash Mounted Control Valve

Service port Lever

Exhaust port Service port

To limiting valve

From limiting

valve

Relay Valve

Quick Release Valve

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Air Brake Manual • 29

Somesystemsareequippedwithanautomaticlimitingvalve(34).Thisvalvewillholdoffbrakeapplicationtothefrontwheelsfrom0to10psi,dependingonhowithasbeenpreset.Betweenthepresetpressureand40psiofbrakeapplication,thereductionis

approximately50percent.Brakeapplicationsbetween40psiand60psiarereducedbylessthan50percent.Brakeapplicationsmorethan60psiarenotreducedandfullapplicationisdirectedtothefrontwheels.

12

3 45

6 7 6 8 932 11

1135

31

32

29

1336

Service port Service port

Delivery port Delivery port

Piston spring

Inlet–exhaust valve springLower piston

assembly

Automatic Front Brake Limiting Valve

Basic Air Brake System with Manual Front Brake Limiting Valve

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30 • Air Brake Manual

Theairbrakesystemdiscussedpreviouslyisforavehiclewithasinglerearaxle.Thediagramillustratesanairbrakesystemforavehicleequippedwithanautomaticfrontbrakelimitingvalve(34),aquickreleasevalve(33)andatandemsetofrearaxles.Bothaxlesofthetandemsetareequippedwithbrakes.

Arelayvalve(13)hastwouses:toprovideaquickerapplicationofairpressuretothetandemrearaxlebrakeswhenabrakeapplicationismade,andtoreleasethebrakesquickerwhenabrakeapplicationisreleased.

Section Summary Questions

1. Howcanthedrivertellhowmuchairpressureisinthemainreservoirs?

2. Whatmustthedriverdowhenalowpressurewarningsystemactivates?

3. Whatisthepurposeofaquickreleasevalve?

4. Whatisthepurposeofarelayvalve?

5. Whatisthepurposeofusingalargerdiameterpipebetweenthereservoirandtherelayvalve?

6. Ifthefrontbrakelimitingvalveisinthe“slipperyroad”position,andthefootvalveisdepressedtomakeabrakeapplicationof30psi,howmuchpressurewillbeappliedinthefrontbrakechambers?

7. Howisthereservoirprotectedfromoverpressurization?

8. Whatstopspressurizedairfromflowingfromthedryreservoirbackintothecompressor?

9. Atwhatpressureshouldthelowpressurewarningdeviceactivate?

10. Howis“brakelag”totherearwheelsminimized?

11. Whenshouldadriverusethefrontbrakelimitingvalve?

34

33 13

Tandem Rear Axles

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Air Brake Manual • 31

SpringParkingBrakes

Section4

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Spring Parking Brake Systems(Single circuit system only)

Theinstallationofspringparkingbrakesandtheirpipingarrangementsintoavehicleairbrakesystemwillvarydependingonthevehiclemake.

Springparkingbrakesmaybeinstalledonanairbrake–equippedvehicleforuseasareliableparkingbrakesystem.Intheservicebrakesystem,thebrakesareappliedbyairpressureandretractedbysprings.Inthespringparkingbrakesystem,thebrakesareappliedbyspringpressureandretractedbyairpressure.Thespringparkingbrakechambersareattachedtotheservicebrakechambersandoperatethroughthesamelinkage,thereforetheeffectivenessofthespringparkingbrakedependsontheservicebrakeadjustment.Thespringparkingbrakecontrolvalve(operatedbyasquare,yellowbutton)locatedinthecaballowsthedrivertoexhaustairoutofthespringparkingbrakecircuittoapplythebrakes,orpressurizethecircuittoreleasethem.Somesystemsmayhaveanadditionalvalvecontrolledbyabluebuttonthatappliesonlythetractorspringparkingbrakesandnotthetrailerspringparkingbrakes.Thesystemcanalsoactasanemergencybrake.Lossofairfromthesystemmayautomaticallyapplythebrakes.

Controlvalveswillvary,dependingonthemanufacturerandtypeofpipingarrangements.

Aspring–loadedvalverequiresthatthevalvebepushedintoreleasethespringparkingbrakes.Thisvalvecannotbeleftinthereleasedpositionwithairpressureinthesystembelowapproximately35psi.Anytimethereservoirpressuredropstoapproximately35psi,thisvalvewillcloseautomatically,placingthespringparkingbrakesintofullapplication.Onsomeoldervehiclestheremaybeasingletypeofpush–pullcontrolvalvethatdoesnothaveanautomaticreleasefeature.Toapplythespringparkingbrakes,thevalvemustbeoperatedmanually,eventhoughthereservoirpressurehasbeendepleted.

Duringnormaloperation,airpressurecages(compresses)thespring,holdingitreadyforparkingoremergencybraking.

32 • Air Brake Manual

Mounting Bolts

Spring parking brake

chamber

Service brake chamber

Clevis and pin

Push rod

Diaphragm

Dust cap

Brakes Off

Parking brake spring

Slack adjuster

Diaphragm return spring

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Onthepre–tripairbrakeinspection(Section9),youmustensurethattheparkingbrakespringisnotmanuallycagedoritwillnotexpandandapplythebrake.Thebrakechambersshouldbecheckedforcracksanddamage.Thebrakechambershouldbefittedwithadustcaptoensuredebriswillnotenterthechamber.

Duringnormalservicebrakeoperation,theparkingbrakespringdoesnotexpand.Airpressurekeepsthespringcaged.

However,ittakesapproximately90psiofairpressuretokeepthespringscaged.Ifreservoirairpressureisallowedtodropbelow90psithespringswillstarttomove.Atapproximately70psithebrakeswillbepartiallyappliedmakingitdifficulttocontinuedriving.Iftheairpressuredropsbelow60psithebrakeswillbefullyappliedmakingitimpossibletomovethevehicleuntilairpressureisbuiltuptorecagethespringparkingbrakesorthespringsaremanuallycaged.

Springparkingbrakes(12),addedtothebrakechambersoftherearaxleonthesingleunitvehicle,areillustrated.Acontrolvalve(27)ismountedinthecab.Asupplylineofreservoirairispipedfromthedry

reservoirtothecontrolvalve.Openingthecontrolvalveallowsreservoirairpressuretoflowtothespringparkingbrakechambers,releasingthem.

Service Brakes Applied Brake On

12

12

27

Using a Spring Parking Brake

Mounting Bolts

Clevis and pin

Slack adjuster

Push rod

Diaphragm return spring

Diaphragm

Spring parking brake

chamber

Dust cap

Parking brake spring

Service brake chamber

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Closingthecontrolvalveshutsoffthesupplyofreservoirairpressureandexhauststheexistingpressureinthespringparkingbrakechambers.Thismotionallowsthespringtoexpand,applyingthebrakes.

Caution:Parkingbrakesshouldbeinthereleasepositionbeforemakingaservicebrakeapplication.Afull–brakeapplication,madewhentheparkingbrakesareapplied,cancompoundtheforceexertedontheslackadjustersandlinkageandresultindamageorbrakefailure.Compoundingisthecombinationoftwoforces:theforceappliedbythespringbrakechamberandtheservicebrakechamber.

Springbrakesareprimarilyusedasaparkingbrake,butintheeventoflossofairpressureinthesystem,theycanassistinstoppingthevehicle.Howquicklytheywillstopthevehicledependsonsuchfactorsas:

• theweightandspeedofthevehicle;

• thesteepnessofthegrade;

• thespringforceofthespringbrakesthathavebeeninstalled;and,

• theadjustmentoftheservicebrakes.

Ifthebrakeshaveoverheated,suchasduringmountaindrivingorhardhighwaybraking,caremustbetakenwhenparkingthevehicle.Ifthespringparkingbrakesareappliedwhenthebrakedrumhasexpandedbecauseofextremeheating,whenthebrakedrumstartstocoolandcontract,thepressureexertedbythespringparkingbrakemaycausethebrakedrumtocrackorwarp.Whenparkingavehiclewithoverheatedbrakes,parkonlevelground,stoptheengineandleavethetransmissioninthelowestgearandblockthewheels.Donotsetthespringparkingbrakesuntilyouhaveverifiedthebrakedrumiscooltothetouch.

Mounting Bolts

Clevis and pin

Slack adjuster

Push rod

Diaphragm return spring

Diaphragm

Spring parking brake chamber

Dust cap

Parking brake springService brake chamber

Spring Parking Brakes Applied “Brakes On”

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Air Brake Manual • 35

Mechanical Release (Caging)

Somespringparkingbrakescanbereleasedmechanicallyby“windingthemoff”or“caging”them.Cagingmeansthebrakesarebeingreleased.Thisisachievedwithaboltthatrunsthroughthecentreofthechamberbody,whichisturnedtocompressthespring.Itmaybenecessarytofirstremovealockplateandstudtogainaccesstotheheadofthebolt.Othertypeshaveadustcapthatmustfirstberemovedandaboltinserted.Insomecases,aspecialwrenchisrequired.Instructiononhowto“cage”isusuallyonthebodyoftheparkingbrakechamber.Ifallairislostandthevehiclehastobetowed,theparkingbrakescanbereleasedbycagingthem.Alwaysblockthewheelswhencagingtheparkingbrakespring.

Warning

Springparkingbrakechambersshouldneverbedisassembledwithoutfirstcompressingthespringwithacagingbolt.Thesespringsareunderextremepressureandcouldcauseseriouspersonalinjuryifdisassemblyisattemptedbyanyonenotexperiencedinservicingtheseunits.Disassemblyofaspringbrakechambershouldonlybepreformedbyaqualifiedmechanicortechnician.

Section Summary Questions

1. Whatismeantby“compounding”thebrakes?

2. Whyarespringbrakesareliabletypeofparkingbrake?

3. Howareparkingbrakesheldinthereleasedposition?

4. Whatarethefunctionsofthecab–mountedparkingbrakecontrolvalve?

5. Willparkingbrakesapply“automatically”inallbrakingsystems?

6. Whatisthereasonforreleasingtheparkingbrakesbeforemakingafullbrakeapplicationtest?

7. Whymustyoubecarefulparkingavehiclewithoverheatedbrakes?

8. Howcansometypesofparkingbrakesbereleasedwithouttheuseofairpressure?

9. Whatisthedangerofdisassemblingaspringparkingbrakeunit?

Mounting Bolts

Spring parking brake

chamber

Service brake chamber

Clevis and pin

Slack adjuster

Push rod

DiaphragmDiaphragm return spring

Caging bolt

Parking brake spring

Parking Brake Spring Caged “Brakes Off”

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Notes

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Air Brake Manual • 37 Air Brake Manual • 37

TrailerSystem

Section5

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Uptothispoint,thesystemdiscussedistheairbrakesystemofatruckortractor.Ifatrailerwascoupledtoatruckortractor,thebrakesofthetrailerwouldhavetobeoperatedfromthetruckortractor.

Inthefollowingpagesthepowerunitofacombinationvehiclewillbereferredtoasatractor.

Glad Hands

Thistermreferstothecouplingdeviceusedtoconnectthecontrol(service)andsupply(emergency)linesofthetrailertothetractor.Thesecouplersconnecttogetherandlockinposition.Theyhaverubbergasketsthatpreventairfromescapingattheconnections.

Beforeconnectionismade,couplersshouldbecleanandfreeofdirtandgrit.Whenconnectingthegladhands,startwiththetwogasketstogetherandthecouplersata90°angletoeachother.Aquicksnapdownwardswilljoinandlockthecouplers.“Dead–end”couplersshouldbeusedwheneverthevehicleisdrivenwithoutatrailertopreventwateranddirtfromenteringthecouplerandlines.

Iftheunitisnotequippedwith“dead–end”couplers,thegladhandofthecontrol(service)linecanbelockedtothegladhandofthesupply(emergency)linetokeepwateranddirtfromenteringthelines.

Thecleanertheairsupplyiskept,thelesschanceofbrakeproblems.

Gladhandsandlinesshouldalsobesecuredtopreventthelinesfromchafingagainstvehiclecomponentsorbouncingoffthevehicle.Thiscouldseriouslydamagethegladhandsorlines.

Application Line

Theapplicationlineisreferredtoasacontrol(service)line.Thislineisconnectedtothefootandhandvalve.Whenthedriverdepressesthefootvalvetreadleapplicationairwillbedeliveredtothetractorbrakechambersandtothetrailerbrakechambers.Whenthedriverreleasesthefootvalvetreadle,theapplicationairtothetrailerbrakechambersmustreturntothefootvalvetobeexhaustedtotheatmosphere.

Thedisadvantagesofthissystemare:

• ifthetrailerbrokeawayfromthetractor,thetrailerwouldnothavebrakes.

• ifthecontrol(service)linepartedorruptured,thetrailerbrakeswouldnotbeapplied,andtheapplicationairwouldbelostfromthetractorifthebrakeswereapplied.

• iftheairpressureinthereservoirsislost,therewouldbenowaytoapplythebrakesofthetractororthetrailer.

• thetrailerbrakescannotbeappliedindependentlyfromthetractorandthereisnowaytosetthetrailerbrakeswhencouplingtothetractor.

• theapplicationandreleaseofthetrailerbrakeswouldbeslowerthanthoseofthetractor.

Thesedisadvantagesareovercomebytheadditionofthesupply(emergency)lineandvalvesdiscussedinthefollowingpages.

Theillustrationshowsthepipingofaunitwithbrakesapplied,similartothetandemaxlesofthetractor.Alsowithbrakesapplied,thetrailerhastandemaxlesequippedwithbrakechambers.

Theapplicationlinehasa“T”insertedbetweenthefootvalve(31)andthetractor’srelayvalve(13).Anairlinehasbeenconnectedfromthis“T”tothetrailerbyasetofcouplers(gladhands)(20).

Air line

Rubber seal

Air line

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Air Brake Manual • 39

Thepurposeofthetrailerbrakehandvalve(30)istoallowthedrivertocontrolindependentlytheamountofapplicationairpressuretobedirectedtothetrailerbrakes.Italsoprovidesamethodofapplyingthetrailerbrakeswhencouplingthetrailertothetractor.

Thevalve,alsoallowsthedrivertoapplythetrailerbrakesindependentlyofthetractor.Theamountofapplicationairpressuredelivereddependsontheamountthevalveisopenedbythedriver.(Itcannotexceedthereservoirairpressure.)Somevalvesareequippedwithselfreturninghandles.

13 20“T”

31

Application Line

30

31

Trailer Brake Hand Valve

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Note: Thetrailerbrakehandvalveisnottobeusedforparking,asairmaybleedoffiftheengineisstoppedorthehandvalvemovestothereleasedposition.

Thedriverhasappliedthebrakesbyusingthefootvalve(31).Applicationairisdirectedtothebrakechambersofthetractorandtothetrailerbrakesthroughatwo–waycheckvalve(26).Theshuttlehasshiftedtothelowpressureside,closingoffanyair

Two–Way Check Valves

Thepurposeofatwo–waycheckvalve(26)istodirectairflowintoacommonlinefromeitheroftwosources.Thisvalvewillpermitairtoflowfromthesourcethatissupplyingthehigherapplicationpressures.Theshuttlewillshiftsothatthehigherpressurewillbedirectedtothetrailerthroughthecontrol(service)line.Thisvalveislocatedbetweenthefoot–operatedvalveandthehand–operatedvalve.

flowtowardthehandvalveside.Thehandvalve(30)isintheclosedpositionandequalpressureisbeingappliedtothebrakechambersofthetractorandthebrakechambersofthetrailer.

30 26

31

Air from hand valve

Air from foot valve

To trailer

Shuttle

Foot Valve Application

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Air Brake Manual • 41

Inthisillustrationwiththefootvalve(31)releasedandthehandvalve(30)opened,applicationairisdirectedfromthehandvalvethroughatwo–waycheckvalve(26),tothebrakechambers.Thetwo–waycheckvalveinthisapplicationhasshiftedtothelowpressureside,closingoffanyairflowtowardthefootvalveside.

Anytimeatrailerbrakeapplicationismadeusingthehandvalve,thedrivermaydepressthefootvalvetreadle.Ifthefootvalveapplicationisofahigherpressurethanthatofthehandvalve,thetwo–waycheckvalvewillshifttothelowerpressureside,allowingthehigherpressuretobedirectedtothetractorandtrailerbrakes.Duringafootvalveapplication,ifthedrivermakesahandvalveapplicationofahigherairpressure,thetwo–waycheckvalvewilldirectthehigherhandvalveairpressuretothetrailerbrakes.

Althoughthetrailerbrakesmaybeappliedindependentlybymeansofthehandvalve,themaximumapplicationpressurecanonlybethesameas,orslightlylessthan,reservoirpressure.

Tractor Protection System

Atractorprotectionsystempreventstotallossofairfromthetractorifthetrailerbreaksaway,oriftheconnectingairlinesbetweentractorandtrailerbecomeseparatedorruptured.Thetractorprotectionsystemconsistsoftwovalves:thetractorprotectionvalveandthetrailersupplyvalve.Othernamesforthetrailersupplyvalveare“trailersupplyvalve”and“emergencyvalve.”

Therearetwotypesoftrailersupplyvalves.Themostcommonistheautomatictrailersupplyvalve.Thisisaspring–loadedvalvethatisopenedmanuallyandheldopenbyairsystempressure.Onoldervehiclesyoumayfindamanualtrailersupplyvalve,whichmaybeatoggle–typeswitchorapush/pull–typevalve.

Tounderstandthefunctionofthetrailersupplyvalveandthetractorprotectionvalveinthesystem,itisimportanttounderstandhowtheyoperate.

30 26

31

Hand Valve Application

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Tractor Protection Valve

Atractorprotectionvalve(24)isnormallylocatedneartherearofthetractorcabandhastwoairlinesrunningtoit,onefromthetrailersupplyvalve(28)andtheotherfromthetwo–waycheckvalve(26)fedbythehandorfootvalve.

Italsohastwoairlines:onegoingtothesupply(emergency)line(21),andonegoingtothecontrol(service)line(22).Thetractorprotectionvalveisspring–loadedandrequiresapproximately45psiinthesupply(emergency)linetoopenthevalve.Thisallowscontrol(service)airtopassthroughthecontrol(service)linetothetrailerwhilemakingabrakeapplication.Whenairlinesfromthetractorarecoupledtoatrailer,theopeningorclosingofthetrailersupplyvalveopensorclosesthetractorprotectionvalve.Disconnectingthesupply(emergency)linebetweenthetractorandtrailerwhilethetrailerischargedwillcauseanimmediatelossofpressureinthesupply(emergency)line.Thesuddenlossofairpressureinthelinewillcausethetrailersupplyvalvetoclose,andwithnoairpressureinthelinethetractorprotectionvalvewillclose.Whenthetractorprotectionvalueclosesitwillstoptheairflowtothecontrol(service)line.

Intheeventofacontrol(service)lineruptureordisconnectionbetweenthetractorandtrailernoactionorairlosswilltakeplaceuntilabrakeapplicationismade.Serviceairwillbelostoutofthedisconnectedorrupturedlinecausingthetractor’sairpressuretodrop.Atapproximately45psithetrailersupplyvalvewillclosecausinganemergencyapplicationofthetrailerbrakesandthetractorprotectionvalvetoclose.Thiswillstopthelossofserviceairatthedisconnectedline.

Thetractorprotectionvalvealsoprotectsthetractor’sserviceairfrombeinglostduringnormalbrakeapplicationswhileoperatingthetractorwithoutatrailer.

To the supply (emergency)

glad hand

To the control (service) glad

hand

Tractor Protection System (Open) (Trailer Charged)

Air from foot valve

Air from hand valve

Two–way check valve

(26)

Control (service) line (22)

Supply (emergency)

line (21)

Tractor protection valve (24)

Trailer supply valve (mounted in cab) (28)

Air from reservoir

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Totesttheproperoperationofthetractorprotectionvalve,ensurethetractorspringparkingbrakesareappliedandthewheelsareblocked.Verifythesystemisatfullpressureandthetrailersupplyvalveisopen(trailercharged).Exitthecabanddisconnectthecontrol(service)andensurethatthereisnoairlossfromthecontrol(service)linegladhand.Enterthecab,ensurefullreservoirpressurebetween115psiand135psiandopenthetractorspringparkingbrakecontrolvalve.Shutofftheengineandmakeafullservicebrakeapplication.Airshouldescapefromthecontrol(service)linetonolowerthan20psi,thetractorprotectionvalvewillcloseandtheairlosswillstop.Releasetheservicebrakeapplication.Closethetractorspringparkingbrakecontrolvalveandensurethetrailersupplyvalveisclosed,ifnot,closemanually.Starttheengineandrebuildairpressure.Withairpressureatorabove90psi,makeatrailerservicebrakeapplicationwithtrailerhandvalve.Noairshouldescapefromthedisconnectedcontrol(service)line.Releasethebrakeapplication.Exitthecabandre–connectthecontrol(service)linegladhand.

Thisteststheproperopeningandclosingoperationofthespringportionofthetractorprotectionvalve.Atractorprotectionvalvewithabrokenreturnspringwillnotclosethevalveandwillcauselossofairduringnormalbrakeapplicationsifoperatingthetractorwithoutatrailer.

Trailer Supply Valve

Thetrailersupplyvalve(28)(usuallyaredoctagonalbutton)ismountedinthecabofthetractor.Thedriveropensthevalvebypushingorpullingthebutton,dependingonthetypeused.

Openingthevalvepermitsreservoirairpressuretoflowthrough.Thisairpressureispipedtothetractorprotectionvalveandthentothesupply(emergency)line.

Thevalveisspring–loadedandwillbeheldintheopenpositionwhensufficientpressureisreached.Iftheairpressuredropstoarangebetween45and20psi,thevalvewillcloseautomaticallybyspringpressure.Thedrivercanclosethevalvemanually,applyingthetrailerspringparkingbrakes.

Tractor Protection System (Closed) (Trailer Not Charged)

Air from foot valve

Air from hand valve

Two–way check valve

(26)

Control (service) line (22)

Supply (emergency)

line (21)

Tractor protection valve (24)

Trailer supply valve (mounted in cab) (28)

Air from reservoir

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Automatic Trailer Supply Valve System

Thediagrambelowillustratesairbeingpipedfromthedryreservoirlinetothetrailersupplyvalve(28).Thetractorprotectionvalve(24)isfedbytwolines:onefromthetrailersupplyvalve(28)andonefrom

Inthediagramstheupperlineisthecontrol(service)line(22)andthelowerlineisthesupply(emergency)line(21).

Illustratedisatractorequippedwithatrailersupplyvalve(28)andatractorprotectionvalve(24).

thetwo–waycheckvalve(26).Leadingofffromthetractorprotectionvalvearetwolines,eachwithagladhandcoupler(20).Thesetwolinesarereferredtoasthecontrol(service)line(22)andthesupply(emergency)line(21).

Thetrailerisnotcoupledandthetractorisbeingoperatedalone(“bobtailing”).Thedriverhasnotopenedthetrailersupplyvalve(28)andthehandvalve(30)isclosed.

28 2624

22 21

30 28

31

2624

20

22 21

20

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Inthisillustration,thedriverhasmadeabrakeapplicationwiththefootvalve(31)andapplicationairisbeingdeliveredtothetractorbrakechambers.Thetwo–waycheckvalve(26)hasshiftedtothelow

Ifthedriver,bymistake,appliedthehandvalve(30)withthetrailerdisconnected,theapplicationairdirectedtothetractorprotectionvalvewouldalsobe

pressureside,allowingapplicationairtoreachtheclosedtractorprotectionvalve(24).

Thereisnoairlossfromthetractorthroughthedisconnectedgladhands(20).

dead–ended.Again,noairlosswouldoccurifthetrailersupplyvalve(28)isintheclosedposition.

31

2624

30 28

20

31

2624

30 28

20

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Inthisillustration,thetrailerhasbeencoupledtothetractor,andthecontrol(service)andsupply(emergency)lineshavebeencoupledusinggladhands(20).

Thetrailerunithasareservoir(16)installed.Thisreservoirwillprovideavolumeofairnearthetrailerbrakechambersfornormaloremergencybraking.Thetrailerreservoirisfittedwithadrainvalve(6),thesameasatractorreservoir.

Arelayemergencyvalve(39)ismountedonthetrailerreservoirortothetrailerframenearthebrakechambers.Therelayemergencyvalveservesthreemainfunctionsinthesystem:

1.Itrelaysairfromthetrailerreservoirtothetrailerbrakechambersduringabrakeapplication.Thispartofthevalveoperatesliketherelayvalvepreviouslydiscussed.Italsoprovidesaquickreleaseofthetrailerbrakes.

2.Itdirectstrailerreservoirairpressuretothetrailerbrakes,causinganemergencyapplicationofthebrakes.Thisactionwilloccurautomaticallyintheeventofrupturedorseparatedsupply(emergency)airlinebetweenthetractorandtrailer,orlossofairpressurefromthetractor.Arupturedcontrol(service)linewouldnotcauseanemergencybrakeapplication.Ifabrakeapplicationwasmadetherewouldbearapidlossoftractorreservoirair.Ifthebrakeapplicationwasheldtheairpressurewoulddroplowenoughtocauseanemergencyapplication.Thedriveratanytimemayoperatethecab–mountedtrailersupplyvalve(28)tocauseanemergencyapplicationofthetrailerbrakes.

3.Ithasaone–waycheckvalvewhichstopsairpressureinthetrailerreservoirfromgoingbacktothetractor.

20

166

39

28

Tractor and Trailer Coupled

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Intheillustration,thecompressorhasraisedthereservoirairpressuretomaximum.

Thedriverhasopenedthetrailersupplyvalve(28)toallowthereservoirairpressuretobedirectedthroughthetractorprotectionvalve(24)tothetrailer.Theairpressurepassesthroughtherelayemergencyvalve(39)

Thisandthenextillustrationaretoonlyshowwhichbrakecomponentsandlinesareusedforafootvalveandhandvalveapplication.•Applicationwiththefootvalve(31)inorange.•Applicationwiththehandvalve(30)inbrown.Orangeandbrownareusedtoshowwhereairisappliedanddoesnotrepresentthesourceoftheair.Whenthefootvalveisdepressed,theapplicationair

tothereservoironthetrailer.Pressurewillbuildupinthetrailerreservoirtothesamepressureasthereservoirsonthetractor.Thisisknownas“charging”thetrailersystem.Thetrailersupplyvalveshouldonlybeopenwhenthetractorpressurehasbuiltuptoapproximately90psi,dependingonthevalve.

willbeappliedtothetractorbrakesandthetrailerbrakestogether.Aspreviouslyexplained,thetwo–waycheckvalve(26)hasshifted,andapplicationairisbeingdirectedthroughthetractorprotectionvalve(24)tothecontrol(service)line.Ifthefootvalveisreleasedandthehandvalveisapplied,theshuttleinthetwo–waycheckvalvewouldshiftandapplicationairwouldapplythetrailerbrakesonly.

31

24

30

26

39

2428

Charging the Trailer System

Foot or Hand Valve Brake Application

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Applicationairdirectedfromeitherthefootorhandvalvecausescontrolpressuretomovethroughthecontrol(service)lineandactontherelayemergencyvalve(39).Thiscontrolpressurewillcausetherelayemergencyvalvetodirectairfromthetrailerreservoir(16)tothetrailerbrakechambers(14).Thepressuredirectedbytherelayemergencyvalvetothetrailerbrakechamberswillbethesameasthepressuredirectedtothetractorbrakechambers.Insuchasystem,brakelagtimehasbeenminimizedwiththeadditionofthetrailerreservoirandrelayemergencyvalve.

Releaseofthefootorhandvalvestopstheflowofapplicationair.Therelayportionofthevalvereturnstoitsoriginalposition,stoppingtheflowofairpressure.Theexhaustingportsofthevalveexhausttheairpressurefromthebrakechambers,releasingthebrakes.Inthissystem,thebrakesofboththetractorandtrailercanbereleasedquickly.

Caution:Trailerbrakesmustnotbeusedtoholdaparkedvehiclethatisleftunattended.Lossofpressurewillresultinlossofbrakes.Alwayssettheparkingbrake.

14

39

14

22 21

16

14

39

14

16

30

31

Emergency Application

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Ifatrailerthatisnotequippedwithspringparkingbrakesweretoseparatefromthetractor,thecontrol(service)line(22)andthesupply(emergency)line(21)woulddetachfromthetractor.Thesuddenlossofairpressureinthesupply(emergency)linewilltriggertherelayemergencyvalve(39)whichcausesthetrailerreservoir(16)todumpitsairdirectlytothetrailerbrakechambers(14).Thisplacesthetrailerbrakesintoemergencyapplication.Thecheckvalveintherelay

Supply (Emergency) Line Rupture

Aruptureofthesupply(emergency)line(21)oranuncouplingofthesupply(emergency)linegladhandswilltriggerthesameactionasdescribedabove.

Control (Service) Line RuptureIfthecontrol(service)line(22)isrupturedordisconnected,noactionwilltakeplaceuntilabrakeapplicationismade.Whenitismadebyeitherthefootorhandvalve,theairlossfromthecontrol(service)linewilllowertheairpressureinthetractor’sreservoirsquickly,dependingontheamountofbrakingdemanded.Thisairlosswilleventuallyactivatethetractorprotectionsystemwhichinturnwillexhaustthetrailersupply(emergency)lineandcausethetrailerrelayemergencyvalvetoapplythetrailerbrakes.Itshouldbenotedthatanyproblemthatcauses

emergencyvalvewillclose,preventinganybleedbackofpressurefromthetrailerreservoir.Thelossofpressureinthetrailersupply(emergency)linewillcausethetractorprotectionsystemtosealoffthetractorleavingsufficientairpressuretobrakethetractor.

Thetrailerbrakeswillremainapplieduntileitherthepressureinthetrailerreservoirandlinesisdrainedoff,orthesupply(emergency)lineisrepairedandthesystemisrecharged.

aseveredropinreservoirpressureonthetractorsystem,willcausethelowwarningdevicetosignalthedriver.

Inthefollowingillustration,thecontrol(service)line(22)hasrupturedandthedriverismakingabrakeapplicationwiththefootvalve(31).Thetractorwillhavebrakesappliedbutthetrailerwillhavenobrakes.Ifthebrakeapplicationisheld,theairpressureinthetractorsystemwilllowertoadangerouslylowlevelandthenthetractorprotectionsystemwillplacethetrailerbrakesintoanemergencyapplication.

21

39

16

14

14

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Ruptureofthecompressordischargelinewouldresultinlossofairpressurefromthesupply/wetreservoir(5).Whentheairpressureinthesupply/wetreservoir(5)ofthetractorfallsbelowthewarninglevel,duetoacompressorfailureorexcessiveleakageonthetractor,thewarningdeviceswillstarttooperate.Inthediagram,theone–waycheckvalve(7)haspreventedthereservoirairpressureintheprimary/dryreservoir(8)fromescapingbacktothesupply/wetreservoirandtherupturedline.

Thereissufficientreservoirairpressureintheprimary/dryreservoirforalimitednumberofbrakeapplicationstostopthevehiclebeforethespringparkingbrakesareactivated.(Thiswilldependonhowthespringparkingbrakesarepipedinthesystem.)

22

31

Control (Service) Line Rupture

5 7 8

Loss of Reservoir Air Pressure

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Inthisillustrationthepressurehasbeenloweredtoapproximately45–20psiandthetractorprotectionsystemhasclosedautomatically,placingthetrailerbrakesintoanemergencyposition.Also,thespringparkingbrakessystemhashadtheairpressurereleasedactivatingthespringparkingbrakes.

Manual Trailer Supply Valve

Someoldertractorsmaybeequippedwithadifferenttypeofcab–mountedtrailersupplyvalvewhichmustbeoperatedmanuallybythedriver.Ithastwopositions:normalandemergency.

Thetractorwillbeequippedwithatractorprotectionvalve,andthetrailerunitwitharelayemergencyvalve,asintheprevioussystem.

Thefunctionsofthetrailersupplyvalve,tractorprotectionvalveandtherelayemergencyvalvewillbesimilartothoseexplainedpreviously.However,thereisoneimportantdifference.Intheeventoftractorreservoirairloss,thetrailersupplyvalvemustbeshiftedtotheemergencypositionmanuallytosealoffthetractor.

Thetruckprotectionsystemdescribedisanexampleofatractorequippedwithatypeofcab–mountedtrailersupplyvalve(28)whichwillcloseautomaticallywhentheairpressureinthesupply(emergency)line(21)dropsbelow45–20psi.Thevalvemayalsobeclosedmanually.

Anytimethedrivershiftsthecab–mountedtrailersupplyvalvetotheemergencyposition,andthetrailersystemischarged,thetrailersupplyvalvewillexhaustthesupply(emergency)line,whichcausesthetrailerreservoirtodumpitsairdirectlytothetrailerbrakechambers.

Thetrailerbrakeswillremainappliedonlyaslongasairpressureremainswithinthetrailersystem.Howlongtheairinthesystemwillholdthebrakesapplieddependsonhowairtightthesystemis.Asasafetyprecaution,parkedtrailerswithoutspringparkingbrakesshouldalwayshavethewheelsblockedtopreventapossiblerunaway.Tomoveatrailerthathasbeenparkedwiththebrakesinanemergencyapplication,itisnecessarytochargethesystemtoreleasethetrailerbrakes.

2128

Loss of Reservoir Air Pressure

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Trailer Spring Parking Brakes

Springparkingbrakesarenowcommonlyfoundontrailers.Springparkingbrakesserveasameansofsecuringaparkedtrailerwhetheritisattachedtoatractorornot.Thespringparkingbrakesareappliedwithspringpressureandnotairpressure,sothereisnoriskoftheparkingbrakesreleasingandthetrailermoving.Theyalsoactasanemergencybrakingsystemifthetrailerweretobreakawayfromthetractororifthetractorlostadequateairpressure.

Atrailerwithspringparkingbrakeshasthesecomponents:

• frontandrearservicereservoirs(16)

• trailerspringbrakevalve(18)

• relayvalve(17)(thesameasonatractor—notanemergencyrelayvalveusedontrailerswithoutspringparkingbrakes)

• springparkingbrakechambers(15)(sameasonatractor)

Thetrailerspringbrakevalveisresponsibleforseveralimportantfunctions:

1. Itcontrolstheapplicationandreleaseofthetrailer’sspringbrakes.

2.Itprotectsandisolatesthefrontservicereservoirfromtherearservicereservoir.Thisisimportantasitpreventsanautomaticapplicationofthetrailerspringbrakesifoneofthereservoirsweretoloseairpressure.

3.Itpreventsautomaticspringbrakeapplicationifthetrailersupplylinehasagradualleak.

4.Itwillautomaticallyapplythespringparkingbrakesifsupplypressureisrapidlylost(ie:trailerbreakaway).

22

21

18 17

15

16

Section Summary Questions

1. Whatisthepurposeofatwo–waycheckvalve?

2. Whyshouldthegladhandsbeprotectedwhennotinuse?

3. Howcanadrivercontrolthetrailerbrakesindependently?

4. Shouldthehandvalvebeusedforparking?

5. Whatisthepurposeofthetractorprotectionvalve?

6. Howcanyoutesttheproperfunctionofthetractorprotectionvalve?

7. Whatisthepurposeofthetrailersupplyvalve?

8. Namethreefunctionsoftherelayemergencyvalve.

9. Describethefunctionofthesupply(emergency)line.

10. Describethefunctionofthecontrol(service)line.

11. Whatwilloccurifthesupply(emergency)lineruptures?

12. Whatwilloccurifthecontrol(service)lineruptures?

13. Whatwilloccurifabrakeapplicationismadewitharupturedcontrol(service)line?

14. Ifthefootvalveandthehandvalveareoperatedatthesametime,cantheapplicationairpressurebegreaterthanthereservoirairpressure?

15. Whyisatrailerequippedwithspringparkingbrakes?

16. Whatbrakecomponentsareonatrailerequippedwithspringparkingbrakes?

17. Whatarethefourfunctionsofthetrailerspringbrakevalve?

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DualAirBrakeSystem

Section6

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Basic Dual Air Brake System

Mostairbrakeequippedvehiclesontheroadtodayareusingadualairbrakesystem.Thesystemhasbeendevelopedtoaccommodateamechanicallysecuredparkingbrakethatcanbeappliedintheeventofservicebrakefailure.Italsoaccommodatestheneedforamodulatedbrakingsystemshouldeitheroneofthetwosystemsfail.Itisactuallytwobrakesystemsinone,withmorereservoircapacityresultinginamuchsafersystem.Atfirstglance,thedualsystemmightseemcomplicated,butifyouunderstandthebasicairbrakesystemdescribedsofar,andifthedualsystem

isseparatedintobasicfunctions,itbecomesquitesimple.

Asitsnamesuggests,thedualsystemistwosystemsorcircuitsinone.Therearedifferentwaysofseparatingthetwopartsofthesystem.Onatwo–axlevehicle,onecircuitoperatestherearaxleandtheothercircuitoperatesthefrontaxle.Ifonecircuithasafailure,theothercircuitisisolatedandwillcontinuetooperate.

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Intheillustration,airispumpedbythecompressor(1)tothesupply/wetreservoir(5)(blue),whichisprotectedfromoverpressurizationbyasafetyvalve(4).Pressurizedairmovesfromthesupply/wetreservoirtotheprimary/dryreservoir(8)(green)andthesecondary/dryreservoir(10)(red)throughone–waycheckvalves(7).Atthispoint,thedualcircuitsstart.Airfromtheprimary/dryreservoirisdirectedtothefootvalve(31).Airisalsodirectedfromthesecondary/dryreservoirtothefootvalve.Thefootvalveissimilartotheonedescribedearlierinthebasicairbrakesystem,butisdividedintotwosections(twofootvalvesinone).Onesectionofthisdualfootvalvecontrolstheprimarycircuitandtheothercontrolsthesecondarycircuit.Whenabrakeapplicationismade,airisdrawnfromtheprimaryreservoirthroughthefootvalveandispassedontotherearbrakechambers.Atthesametime,airisalsodrawnfromthesecondaryreservoir,passesthroughthefootvalveandispassedontothefrontbrakechambers.Ifthereisairlossineithercircuit,theotherwillcontinuetooperateindependently.Unlessairislostinbothcircuits,thevehiclewillcontinuetohavebrakingability.Theprimaryandsecondarycircuitsareequippedwithlowairpressurewarningdevices,whicharetriggeredbythelowairpressureindicatorswitch(9)andreservoirairpressuregauges(29)locatedonthedashofthevehicle.

1 4 5 7 98 10

31

29

Treadle

Supply ports

Exhaust port

Delivery ports

Dual–Circuit Foot Valve

Dual Air Brake System

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Whenspringbrakesareaddedtoadualairbrakesystem,thesametypeofdashcontrolvalvediscussedpreviouslyisused.Blendedairisusedtosupplythespringparkingbrakecontrolvalve(27).Blendedairisairtakenfromtheprimaryandsecondarycircuits

throughatwo–waycheckvalve(26).Withthispipingarrangementthevehiclecanhaveafailureineithercircuitwithoutthespringbrakesapplyingautomatically.Ifairislostinbothcircuits,thespringbrakeswillapply.

27 26

Dual Air Brake System with Spring Parking Brakes

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Springparkingbrakesinthissystemservetwopurposes:first,asaparkingbrake,andsecondasanemergencybrakingsystem.Ifafailureoccursintheprimarycircuit(green),andabrakeapplicationismade,controlairfromthefootvalveisdirectedtoaspringbrakemodulatorvalve(23).Asthereisnosupplyairtomaintainbalanceinthemodulatorvalve,becauseoftheprimarycircuitfailure,themodulatorvalvethenexhaustsairpressurefromthespringparkingbrakecircuit.Theamountofairreleasedisequaltotheamountofairappliedbythefootvalve.Thereleaseofairinthespringparkingbrakecircuitcausesthedriveaxletobrakeusingspringpressure(12).

Whenthebrakesarereleased,supplyairfromthesecondarycircuit(red)returnsthespringparkingbrakestoanoffposition.Brakeapplicationscanberepeateduntilalltheairfromthesecondarycircuitislost.Howeverastheairpressuredropsbelow85psi,thespringparkingbrakeswon’treturntothefulloffposition,infacttheywillstarttodrag.Atapproximately35psi,thespringparkingbrakecontrolvalve(27)onthedashwillexhausttheremainingairinthesecondarycircuit,andthespringparkingbrakesarefullyapplied.Theonlywaythevehiclecanbemovedafterallairislostistorepairthedamagedcircuitandrechargethesystem,orcagethespringparkingbrakesystem.

27 23 12

12

Spring Parking Brakes with Modulator Valve

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Combination Tractor and Trailer with Spring Parking Brakes

Thetrailersystemissuppliedbyblendedtractorairtakenfromtheprimaryandsecondarycircuitsthroughatwo–waycheckvalveaspreviouslydescribed.

Thesystemischargedbyopeningthetrailersupplyvalve(28),allowingairfromthetractortopassthroughthetractorprotectionvalve(24)andthetrailerspringbrakevalve(18)directlyintothetrailerspringparkingbrakechambers(15).Whenairenters,thepressureprotectionpartofthetrailerspringbrakevalveopens,allowingtheairtofillthetrailerreservoirs.Thetrailerspringbrakeswillnotreleaseuntilthereservoirpressureonthetrailerisadequate.

Whenabrakeapplicationismade,blendedcontrolairactsontherelayvalve(17),whichreleasesairfromthetrailerreservoirtothebrakechambers.

Inadualairbrakesystem,ifonecircuitdevelopsaleak,theothercircuitwouldbeprotectedfromairpressurelossbythetwo–waycheckvalve(26).

Ifthetrailerbreaksawayfromthetractor,thecontrol(service)andsupply(emergency)lineswillbepulledapart.Thesuddenlossofairinthesupply(emergency)linewillcausethetrailersupplyvalvetoclose,whichwillcausethetractorprotectionvalvetoclose,preventingairfromescapingoutofeitherbrokenconnection.Theairsupplyinthetractorissealedoffandisavailabletocontrolthetractorbrakes.Atthesameinstant,thesuddenlossofairinthesupply(emergency)linecausesthetrailerspringparkingbrakevalvetoexhausttheairfromthe

trailerspringparkingbrakechambers,applyingthetrailerbrakes.Thetrailerbrakescannotbereleasedundertheseconditionsunlessthelinesarere–coupledandthetrailerreservoirsrecharged.

Ifonlythesupply(emergency)linebreaksbetweentractorandtrailer,thesamesequenceofeventswilloccur.

Abreakorruptureinthecontrol(service)linewillnotaffectthetraileruntilabrakeapplicationismade.Alossofpressureinthetractorsystemwillthenresult,ifpressureisallowedtodroplowenoughitwillcausethesameemergencybrakeapplicationdescribedabove.However,thedriverwillbeabletoreleasethespringparkingbrakesbyreleasingthefootvalve,rebuildingairpressureandopeningthetrailersupplyvalve.

Toapplythespringparkingbrakes,thespringparkingbrakecontrolvalve(27)isclosed,causingalossofairpressureinthelinewhichappliesthespringparkingbrakesasdescribedabove.

Theoldandnewtractorandtrailersystemsarefullyinterchangeable,whethertheyareadualairbrakesystemorbasicairbrakesystem,andwhethertheyaresystemswithorwithoutspringparkingbrakes.

28 18 15

15

17

242627

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Section Summary Questions

1. Whatisthebasicprincipleofthedualairbrakesystem?

2. Whatvalveisusedtoprotecttheprimarycircuitfromthesecondarycircuit?

3. Inadualairbrakesystem,willthevehiclecontinuetohavebrakingabilityifonecircuitfails?

4. Isthereadifferencebetweenthefootvalveusedinabasicairbrakesystemandthefootvalveusedinthedualairbrakesystem?

5. Nametwofunctionsofthespringparkingbrakesinadualairbrakesystem.

6. Describethefunctionsofthespringbrakemodulatorvalve.

7. Ifthetrailerbreaksawayfromthetractoronadualairbrakesystem,whatappliesthebrakesonthetrailer?

8. Whatisblendedair?

9. Canatrailerwithabasicairbrakesystembetowedbyatractorwithadualairbrakesystem?

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Notes

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ElectronicControlledBrakingand

TractionSystems

Section7

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WheelAxle

62 • Air Brake Manual

Anti–Lock Brake System (ABS)

Ananti–lockbrakesystemisanelectronicsystemthatmonitorswheelspeedatalltimesandcontrolswheelspeedwhilebraking.Ifitdetectsawheellocking–upduringabrakeapplication,thesystemreleasesbrakepressuretothatwheelonly.Thiskeepsthewheelfromskiddingandincreasesvehiclestabilityandcontrolduringemergencystopsandinadversesituationssuchasonwetoriceslickedroads,throughcurvesorduringlanechanges.TheairbrakesystemremainsthesamewiththeadditionoftheABScomponents.

AnABSbasicallyconsistsof:

• Anelectroniccontrolunit(ECU)

• Awheelsensorandtoothwheel

• ABSvalves

TheECUisthebrainofthesystem.Thewheelsensorsarelocatedonaminimumoftwo,oruptoallsixsetsofwheels,dependingonthesystem.ThewheelsensorsconstantlysendinformationtotheECU.Whenabrakeapplicationismadeandthesystemdetectsawheellocking–up,theECUsendsamessagetotheABSvalvestoreleasebrakepressureonthatparticularwheelpreventinglockup.Usuallythereisawarninglightlocatedonthedashtoinformthedriverofthesystemoperation.Everydrivershouldhavefullunderstandingofthewarninglightoperationforthevehicletheydrive.

WhendrivingavehiclewithABSapplythebrakesnormally.WhentheABSstartsworking,donotreleasethepressureyouhaveappliedtothebrakepedal.AvoidpumpingthebrakepedalastheABSautomaticallyappliesandreleasesthebrakesuptofivetimesasecondwhichismuchfasterthanyoucanpumpthebrakepedal.ForoptimumABSoperation,thedrivershouldmakeabrakeapplicationandhold,allowingtheABStocontrolbrakingpressureateachwheel.

Whenusinganenginebrakeandyouencounteraslipperyroadsurface,theABSwilldetectthewheellockupandautomaticallyturnofftheenginebrakeuntiltractionisregained,thenresumeenginebraking.

Forinformationspecifictothevehicleyoudrive,refertotheoperator’smanual.

TrailersmayalsobeequippedwithABSandthesystemwilloperatemuchthesameasonthetractor.However,trailerswithABSwillhaveasystemwarninglighttypicallymountedonthetrailer’sleftfrontcorner,visibleintheleftsidemirror.Onsomesystemstheymayhaveanextrawarninglightonthedashofthetractor.

TractorsandtrailerswithorwithoutABScanbeconnectedtooneanotherwithoutaffectingtheoperationoftheairbrakesystem.

Tooth Wheel and Sensor Block

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Four Sensor–Four Modulator Valve Anti–Lock Brake System

E.C.U.

Relay block

Tooth wheel and sensor block

Tooth wheel and

sensor block

Modulator valves

Modulator valves

Brake chambers

Brake chambers

Brake pedalBrake

chambers

Dash lamp

Tooth wheel and sensor block

Tooth wheel and sensor block

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Automatic Traction Control (ATC)

Automatictractioncontrolisanelectronicsystemthatmonitorswheelspinonaccelerationandusescontrolledbrakingtogaintraction.Thissystemreducesthepotentialofpoweringintoajackknifecausedbywheeloverspinduringacceleration,alongwithimprovingtheabilityofthedrivertomanoeuvrethevehicleonslipperysurfacesthroughcurvesandlanechanges.ATCisanoptiononlyavailableonvehiclesequippedwithABS.

Thesystemusestwofunctions:differentialbrakingandelectronicenginecontrol.

DifferentialbrakingoccurswhenadrivewheelbeginstospincausingtheATCtoautomaticallyapplythebrakeonthatwheelandtransferpowertotheotherdrivingwheels.

Differentialbrakingisactivatedwhenthesystemdetectsawheelslippingbecausethewheelsareoperatingondifferentsurfacessuchasonewheelondrypavementandtheotheronice.

ElectronicenginecontrolisactivatedautomaticallybytheATCifallthewheelsbegintoslip.Enginepowerisreducedtoattainoptimumtire–to–roadtractionwithoutdriverintervention.Whenavehicleisbeingoperatedwiththecruisecontrolsetandwheelslipisdetected,theATCwillautomaticallycancelthecruisecontrolsetting.Thiswillaidthedriverinmaintainingcontrol.Cruisecontrolshouldnotbeusedonslipperysurfaces.

Awarninglightislocatedonthedashtoinformthedriverwhenwheelslipisdetected.TheATCsystemusesmanyofthesamesensingandcontrolcomponentsastheanti–lockbrakesystem.Forinformationspecifictothevehicleyoudrive,refertotheoperator’smanual.

Section Summary Questions

1. Whatisthepurposeoftheanti–lockbrakesystem?

2. Whatthreecomponentsdoesananti–lockbrakesystemconsistof?

3. Howshouldyouapplythebrakeswhenbrakingonwetoricyroadsinavehicleequippedwithanti–lockbrakes?

4. Canatrailerwithanti–lockbrakesbeconnectedtoatractorwithoutanti–lockbrakesystem?

5. Whatisthepurposeoftheautomatictractioncontrolsystem?

6. Whattwofunctionsdoesthesystemusetocontroltraction?

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BrakeAdjustmentandIn–ServiceCheck

Section8

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Brake AdjustmentOnvehiclesequippedwithhydraulicbrakesitispossibletopumpthebrakepedaltocompensateforbrakesthatareoutofadjustment.Thisisnotpossiblewithavehicleequippedwithanairbrakesystemwithmanualslackadjusters.Whenthebrakesareadjustedbyslackadjusters,theshoesaremovedtowardthedrum.Thisbringsthemasclosetothedrumsaspossibleminimizingtheamountoffreetravelwhenthebrakesareapplied.Italsoreducesthevolumeofairusedtoapplythebrakes.

Brakeadjustment(pushrodtravel)mustbecheckedaspartofthepre–tripairbrakeinspection(Section9).

Yourcompanymayhaveamaintenancecrewtokeepvehiclessafelyrunning.Butonepersonaloneisultimatelyresponsibletoensurethatthebrakesareoperatingproperlybeforethevehicleismoved.Thatpersonisthedriver.

Brake Adjustment–It’s CriticalThemostimportantpartofanybrakingsystemisthefoundationbrakes.Ifthesearenotincorrectworkingorderandproperlyadjusted,thebestdesignedairbrakesystemwon’tbeabletosafelystopthevehicle.

Unfortunately,brakeadjustmentisoftenneglected.Therealityisthatbrakefailuresandrunawaysthatresultincrashesarealmostnevercausedbyanairsystemfailure,butbytheabsenceofroutinebrakemaintenance,orbythedriverfailingtocheckbrakesonadailybasis.

Dailypre–andpost–tripinspectionsaremandatory.Thedrivermaybeheldresponsibleifthebrakesareincorrectlyadjustedornotworkingproperly.

Brake Adjustment–It’s the LawFederalandprovinciallawsrequirethatmanualandautomaticslackadjustersbecheckeddaily,duringthepre–tripinspection.Itisuptoyou,theprofessionaldriver,toensureyourvehiclehassafe,correctlyadjustedbrakes.Youthedriver,canbefinedifthebrakesonthevehicleyouaredrivingarefoundtobeoutofadjustment.

You’realsorequiredbylawtocheckyourbrakesbeforedrivingdownsteepgradesthatarepostedwithregulatorysigns.

Youcan’talwayscrawlunderneathyourvehicletomeasureairbrakechamberpushrodtravel,buttherearedevicesavailabletohelpyouvisuallycheckforbrakeadjustment.

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DefinitionSlackadjustmentmeansadjustingthebrakestokeepairchamberpushrodtravelwithintolerancetoensurethatthebrakesareeffective.

Pushrodtravelmeansthedistancethepushrodextendsfromtheairbrakechamberwhenthebrakesareapplied.

S–Cam Brake–Released

A�typical�s–cam�brake.��The�brake�is�shown�in�the�released�position.

ThemostcommontypeofairbrakechamberusedontrucksandtrailersistheType30.Thesechambershaveamaximumavailablestrokeof2Minches.AlongstrokeType30isdesignatedType30LS,andhasamaximumavailablestrokeofthreeinches.

S-Cam Brake–Applied

A�typical�s–cam�brake.��The�brake�is�shown�in�the�applied�position.

Inthisdiagram,thebrakeshavebeenappliedandthebrakeliningshavecontactedthebrakedrum.ThebrakechamberhasstrokedlessthanMofitsmaximumstroke,indicatingthatthebrakeisincorrectadjustment.

Eachbrakeonanaxleshouldbeadjustedtohaveasimilaramountofpushrodtravel—thatis,thepushrodtraveloftheleftbrakeshouldbesimilartothepushrodtraveloftherightbrakeontheaxle.

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S–Cam Brake–Incorrectly Adjusted and Cold Brake Drum

Check�air�brake�chamber��pushrod�travel�at�least�daily.

Inthisdiagram,thebrakesareappliedandtheliningshavecontactedthebrakedrum.

This�brake�chamber�pushrod�has�excessive�travel,��but�may�work�as�long�as�the�brake�drum�is�cool.��But�it�won’t�stay�cool�for�long!

NotethatthebrakechamberhasstrokedmorethanMofitsmaximum,whichmeansthatthisairbrakechamberpushrodhasexcessivetravelandthebrakesareinneedofadjustment.

Whenthebrakedrumiscoolandwithnormalbrakeapplicationpressure,thebrakewillseemtobeeffective,soitiseasytobelulledintoafalsesenseofsecurity.

Airbrakeequippedvehiclesrarely“lose”theirbrakes.Rather,brakefailureoccursbecausethedriverhasfailedtocheckandcorrectlyadjustthebrakes.

S–Cam Brake–Incorrectly Adjusted and Hot Brake Drum

Now�that�the�brake�drum�is�hot,�the�brake�linings�will�no�longer��contact�the�drum.�The�brake�chamber�has�bottomed�out.

Thisdiagramshowsthesamebrakeafterafewbrakeapplications.

Castironbrakedrumsexpandwhenheated,causingtheairchambertostrokefurtherandfurtherasthetemperaturerises.

Ifanunforeseeneventrequiredthedrivertomakeasuddenstop,thebrakechambercouldbottomout,andbrakingpowerwouldbegreatlyreduced.

Onlongdowngrades,theexpansionofhotbrakedrumscancauseatotallossofbrakingandresultinarunaway.

Evenincorrectlyadjustedbrakesmayseemtoworkwhenbrakesarelightlyapplied.

Thiscouldresultinbrakefadeundermoderatetoheavybraking,oronadownhill.

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Checking and Adjusting S–Cam Brakes–Manual Slack AdjustersDriversseldomhavetheluxuryofhavingtheuseofapitandhydraulicjackssothatbrakeadjustmentcanbedonewithwheelsofftheground.However,brakescanstillbeadjustedveryaccuratelywiththewheelsonthegroundusingoneofthefollowingmethods.

Besurethatthevehicleissafelyparkedwiththewheelsblocked.Theairsystemshouldbeatfullpressureandthespringparkingbrakesreleased.Turntheengineoffsothatyoucanlistenforairleaks.

Therearetwomethodsofcheckingforcorrectadjustment,butthemeasurementsthatindicatetheneedforadjustmentaredifferent.

Pry Method of Free Stroke Measurement

Thispictureshowsthecommonmethodofcheckingthefreestrokeofthepushrod:

Using�a�brake�tool�to�measure�air�brake��chamber�pushrod�free�stroke.

Makesureyouhavetheproperequipmenttocheckandadjustthebrakes.Youwillneed:

•Awrench

•Amarkertomeasurepushrodtravel

•Sturdyclothingsuitableforcrawlingunderthevehicle

•Aflashlight

Youshouldalsoweareyeprotectionandabumpcap.

Tobegin,makesuresystemairpressureisatleast90–100psiandallparkingbrakesarereleased.

1. Makeamarkonthepushrodwhereitexitsthebrakechamber.

Note:Itissometimesdifficulttomarkthepushrodatthislocation.Alternatively,markthecentreoftheclevispinontheslackadjusterarm.

2. Pullthepushrodoutfromthebrakechamber,usingatoolforleverage.

3. MeasurethedistancebetweenthechalkmarkandthefaceofthebrakechamberMtoPofaninchoffreestrokeisagoodrangeforatype30standardstrokebrakechamber.IfthisdistanceismorethanPofaninch,thebrakesneedadjustment.

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Applied Stroke Method (service brake application)Theappliedstroke(brakeapplication)methodrequirestwopeople—onetoapplythebrakesandonetomeasuretravel.

Ifyouhaveadevicetoapplyandholdtheservicebrakeson,youcanusethebrakeapplicationmethodwithoutneedinganotherperson.

Checking�the�pushrod�stroke��by�applying�the�brakes.

Brake Adjustment IndicatorsNewairbrakechamberpushrodshaveamarking(usuallyred)toindicatewhenbrakeadjustmentmustbedoneimmediately.Ifthepushrodtravelbecomesexcessive,themarkingwillshow.

Don’twaituntiltheredmarkingisexposedbeforeadjustingthebrakes.

If�the�red�marking�on�the�pushrod�(at�the�chamber�face)�is�visible,��it�indicates�that�the�brake�is�dangerously�out�of�adjustment��and�it�needs�immediate�attention.

Theappliedstrokemethodisthepreferredmethodandisalsoamethodrecommendedbycommercialfleetmaintenancesupervisors.

1. Withthebrakesreleased,makeamarkwherethepushrodexitsthebrakechamber.

2. Withtheengineoff,makeaseriesofbrakeapplicationstoreducethereservoirpressuretobetween90to100psi.

3. Applyandholdafullbrakeapplication(90to100psi).

4. Measurethedistancebetweenthemarkandthefaceofthebrakechamber.Thedifferencebetweenmeasurementsiscalledthechamberappliedstroke.

Whilecheckingairbrakeslookforotherbrakecomponentwearandexcessiveplayincluding:

•S–cambushingwear

•S–camsplinewear

•Slackadjusterplay(worngear)

•Clevispinwear

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Brake Adjustment–Manual Slack Adjuster

WithatypicalType30standardstrokebrakechamber,youmustadjustthebrakesifpushrodtravelis:

•morethanPofaninchusingtheprymethodoffreestrokemeasurement,or

•morethan1 1/@inchesusingthebrakeapplication—appliedstrokemeasurementmethod.

Notethatthesearemaximummeasurements.Youshouldadjustthebrakesifyourmeasurementsapproachtheselimits.

AdjustmentMostmanualslackadjustershaveaspring–loadedlockingsleevethatmustbepressedinandheldsotheadjustingboltcanbeturned.

Dependingupontheorientationoftheslackadjuster,thecorrectdirectiontoturntheadjustingboltmaybeclockwiseorcounterclockwise.Watchtoensurethatyouaretighteningandnotlooseningtheslackadjustment.

Withawrenchofthepropersize,usuallya9/!^inch,depressthelockingsleeveandturntheadjustingboltwhilewatchingtheendofthecamshaft.Thecamshaftwillrotateslightlyastheboltisbeingturned.Ifyouareturningintherightdirection,thecamwillrotateinthesamedirectionthatitwouldifthebrakeswerebeingapplied,asshownbythearrowonthecamendinthediagrambelow.

The�push�rod�and�the�slack�adjuster�arm�should�never�move�away�from�the�brake�chamber�while�turning�the�adjusting�bolt.

Ifwhenturningtheadjustingbolt,theslackadjusterispullingthepushrodoutofthechamber,stop.Theadjustingboltisbeingturnedinthewrongdirection.

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Oncetheproperdirectionisestablished,continueturninguntilsolidresistanceismet.Thisindicatesthatthebrakeliningshavecontactedthebrakedrum.

Ifthebrakehasnodustshields,orifyoucanseethebrakeshoesandliningsthroughaninspectionslot,youcanvisuallyverifythattheliningshavecontactedthedrum.

Backingofftheadjustingboltabout1/ #ofaturnshouldestablishcorrectrunningclearancebetweenthelininganddrum.Besurethelockingsleevere–engagestheboltsothattheadjustmentwillnotbackoff.

Itiscommon,especiallyontandemaxleunits,fortheadjustingboltsononeaxletoadjustinonedirection,whiletheotheraxlerequiresanoppositeturn.Itisnotuncommontofindthatthebrakesononeortheotheraxlehavebeenmistakenlybackedoff,creatingaserioussafetyhazard.

Afteradjusting,verifythatthereissufficientclearancebyagainpullingorpryingontheslackadjusterarm,andre–measuringthefreestroke,rememberthatMinchisideal.

Adjusting S–Cam Brakes with Automatic Slack Adjusters—Why—When—How

Automaticslackadjustersareentirelydifferentthanthemanualtypes.

Onceproperlyinstalled,automaticslackadjustersshouldnotneedmanualadjustment.Ifanautomaticslackadjusterisfoundtostrokebeyondthemaximumallowed,thisusuallyindicatesthattherearebrakeproblemsthatneedtoberepairedbyqualifiedbrakeservicepersonnel.

Automaticslackadjustersstillneedtoberegularlycheckedtoensurethatcorrectadjustmentisbeingmaintained.

Pushrodstrokeswithautomaticslackadjustersareusuallyslightlylongerthanwithwell–adjustedmanualslackadjusters.Whileamanualadjustmentmaytemporarilyshortenthestroke,theautomaticslackwillsoonre–settoitsdesignedstroke.

Manufacturersrecommendthatautomaticslackadjustersbecheckedbyamechanicateachchassislubricationinterval,orfrom40,000to80,000km.

However,theNationalSafetyCodeofCanada,aswellasfederalandstatelawsintheU.S.requireadailycheckofbrakeadjustmentaspartofapre-tripinspection.Also,incertainmountainousareasofNorthAmerica,trucksarerequiredtostopandcheckbrakesbeforeproceedingdownlonggrades.Thecheckingisrequiredregardlessofwhethermanualorautomaticslackadjustersareused.

Priortomanuallycheckingslackadjusterstroke,ensuretractorspringparkingbrakecontrolvalveandtrailersupplyvalveareopen.Ensurereservoirairpressureisabove90psiandmakeafullbrakeapplicationandrelease.Repeatthisprocess6times—thesebrakeapplicationswillbeadequatetocauseanautomaticslackadjustertoadjust.Alightbrakeapplicationwillnotcausetheadjustment.

Ifaftermaking6fullbrakeapplicationswithreservoirpressureabove90psi,apushrodstrokeisexcessive,theautomaticslackadjusterhaseitherfailed,beenincorrectlyinstalled,orthereisaproblemwithinthefoundationbrake.Manualadjustmentatthistimeisonlyastop–gapmeasuresothevehiclecanbedriventoaservicedepotforinspectionandpossiblereplacement.

Mostmanufacturersofautomaticslackadjustersspecifythatpushrodstrokebecheckedbymakinga90to100psiapplication.Ifyouhavenoapplicationpressuregauge,turntheengineoffandpumpthereservoirpressuredowntobetween90and100psi–thenmakeafullapplication.

Automaticslackadjustersshouldonlybeadjustedorrepairedbyaqualifiedmechanicexceptinanemergency.Followthemanufacturer’sinstructionsincarryingouranyadjustment.

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Emergency Manual Brake Adjustment—Automatic Slack Adjusters

Ifthevehiclemustbedriventoaservicedepot,anemergencymanualadjustmentmaybedone.

THESE ARE EMERGENCY PROCEDURES ONLY. A REPAIR OR REPLACEMENT MUST BE MADE AS SOON AS POSSIBLE.

Besurethatthevehicleissafelyparkedwiththewheelsblocked.Theairsystemshouldbeatfullpressureandthespringparkingbrakesreleased.

WithatypicalType30standardstrokebrakechamber,youmustadjustthebrakesifpushrodmeasurementsare:

•morethanPofaninchusingtheprymethodoffreestrokemeasurement,or

•morethantwoinchesusingthebrakeapplication—appliedstrokemethod.

Four�types�of�automatic�slack�adjusters

Notethatthesearemaximummeasurements.Youshouldadjustthebrakesifyourmeasurementsapproachtheselimits.

Ifyoumakeanemergencyadjustmentofanautomaticslackadjuster,besuretorecorditonyourdailypost–tripinspectionreport.

Itisstronglyrecommendedthatyouobtaintheservicedatabooksforthemakeofautomaticslackadjustersthatyouareusing.Theycontainalltheinformationneededtokeepthemoperatingproperly,includinglubricationrequirements,initialsetup,inspectionandtesting.Theyareavailablewheretruckpartsaresold.

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Slack Adjusters with Hexagonal Adjusting Bolts

Iftheslackadjusterhasahexagonal(six-sided)adjustingbolt,thebrakesmaybesetupbyturningtheadjustingboltinaclockwisedirectionuntiltheliningcontactsthedrum.BackingofftheadjustingboltbyMaturnshouldrestorerunningclearance.Backingoffmaytakeconsiderableforceandmaybeaccompaniedbyaratchetingsoundandfeel.Thisisnormal.

These�two�types�of�automatic�slack�adjusters�both�have�hexagonal�adjusting�bolts.

Slack Adjusters with Square Adjusting Bolts

Iftheslackadjusterhasasquareadjustingboltlocatedatthebottomendofthebody,donotattemptadjustinguntilaspring–loadedpawlthatmesheswithinternalteethisdisengaged.TheseunitshaveaPinchhexagonal(hex)caplocatedontheslackadjusterbody.Someofthesehexcapsareequippedwitharound“button”thatcanbepriedupapproximately1/ #@ofaninchandheld,usingascrewdriver.Onunitsnotequippedwiththebutton,thehexcap,springandpawlmustberemoved.Withthespringandpawldisengaged,anadjustmentcanbemade.

Thesebrakesmustbesetupwithacounter–clockwiseturnuntiltheliningcontactsthedrum.TurningtheadjustingboltMaturnclockwisewillrestorerunningclearance.Releasethebuttonorre–installthespringandpawliftheywereremoved.

This�type�of�slack�adjuster�has�a�square�adjusting�bolt.

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Air Brake Manual • 75

Disc Brake Adjustment

Thereareanumberofmakesandmodelsofairdiscbrakes,eachwithadifferentadjustmentprocedure.Itisthereforerecommendedthatyouconsultthemanufacturer’smanualforadjustmentorservice.

Wedge Brake Adjustment

Manualadjusters:

• Hoistorjackwheelsofftheground.

• Removedustcoverfromadjustingslotsattwoplacesoneachbrake.Ontwinchamberunitstheadjustingslotsareinthebackingplatebelowtheforwardchamberandabovetherearchamber.Ifstarwheeladjustingboltsarenotfoundatthesepositions,thebrakehasbeenassembledonthewrongsideofthevehicle.Thesinglechamberunitshaveadjustingslotsinthebackingplateoneachsideofthechamber.

• Adjustingboltshaveright–handthreads.Turningthewheelbyhand,useanadjustingspoontoturnthestarwheeluntiltheshoedevelopsaheavydragonthedrum.Thenbackoffthebolttoaverylightdragonthedrum.

• Repeatfortheothershoeonthatbrakeassembly.

• Replacethedustcoversintheadjustingslots.

• Repeattheabovestepsontheotherbrakeassemblies.

There is no reliable substitute for physically checking brake adjustment.

After a Brake Adjustment

Atthefirststopafteradjustingthebrakes,checkeachbrakedrumorrotorfortemperature.Anextrahotbrakedrumindicatesthatyouhaveoveradjustedit.

Onlywhenallbrakesareproperlyadjustedwillthesystembeabletoabsorbtherequiredamountofenergytobringtheloadedvehicletoasafestopunderextremeconditions.Rememberthatpoorlyadjustedbrakesmayappeartobeworkingatslowspeedsonlightairapplication.Theonlywaytobesureyourvehiclebrakesareproperlyadjustedistophysicallycheckthepushrodtravelateachwheel.

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In–Service Checks

Insomeareassignsarepostedinadvanceofsteeporlongdowngrades.Thesesignsindicatethatthedrivermuststopthevehicleinthepulloutareaandinspectthebrakingsystembeforeproceeding.Checkthat:

• thecompressorismaintainingfullreservoirairpressure;

• pushrodtraveliswithinlimitationsonallchambers;

• therearenoairleaks;

• thegladhandsandthelinesaresecure;

• thedrums,bearingsandtiresarenotoverheating;

• thetrailersupplyvalveisoperatingproperly.

Thisshouldbeusedasanin–servicecheckonlyandnotmistakenasadailypre–tripairbrakeinspection.

Thedrivermustbeawareoftheconditionofthevehicle’sbrakingsystematalltimes.Thiscanbedoneby:

• watchingthepressuregauges;

• hearingthewarningsignals;and

• feelingthebrakingresponseofthevehicle.

Bythismethodofobservation,thedrivershouldbeabletonoticeanydefectsdevelopinginthebrakingsystemandbeawarethatserviceoradjustmentisrequired.

Maintenance and Servicing of the Air Brake System

Ensuringthatthevehicleisinproperconditiontodriveistheresponsibilityofeveryoneinvolved.

Preventivemaintenancechecksaredesignedtopreventmechanicalfailuresthatcouldresultfromneglect.Thesecheckscutrepaircostsbydetectingminormechanicaldefectswhichwouldotherwisedevelopintomajorbreakdowns.

Preventivemaintenancemaybeassignedtoamaintenancecrewortothedriver.Howeverthedrivermustensurethatthebrakingsystemisinsafeoperatingconditionbeforethevehiclemoves.

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Air Brake Manual • 77

Air Brake Adjustment Myths

There�is�some�misinformation�about�air�brakes�that�you�may�hear.�Most�of�this�is�not�critical,�but�the�following�three�myths�could�be�dangerous,�if�believed.

Myth#1:Brakeadjustmentcanbecheckedfromthecabbymakingafullbrakeapplicationandcheckingforaninitialpressuredropofbetween8–12psi.Theassumptionisthatasthebrakechambersstrokefurtherandfurther,thatmoreairvolumewillberequired,andthisshouldshowuponthereservoirgauges.

Fact #1:Modern trucks have very large air reservoirs, and even if all the brakes had excess pushrod travel, the pressure drop would not reach the 8–12 psi range. Also, most truck reservoir gauges do not have markings fine enough to accurately estimate such pressure changes.

Myth#2:Withthebrakesapplied,a90-degreeanglebetweenthecentreoftheslackadjusterarmandthechamberpushrodisagoodindicationthatthebrakeisincorrectadjustment.

Fact #2: The 90-degree angle is more dependent on the length of the chamber pushrod than on brake adjustment. Also, to prevent interference between the slack adjuster and suspension parts, some manufacturers will vary the angle up to plus or minus 10 degrees.

Myth#3:Aclockwiseturnoftheadjustingboltonamanualslackadjusterwillsetupthebrakes.

Fact #3: Depending on the orientation of the slack adjuster on the brake assembly, the correct direction to set up the brakes may be clockwise, or counter-clockwise. On vehicles with tandem axles, it is common to find that one axle sets up with a clockwise turn, while the other requires a reverse direction.

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Section Summary Questions

1. Whyisitsocriticaltocheckslackadjustment?

2. Whoisultimatelyresponsibleforthebrakesonavehicle?

3. Whowillbefinedforoperatingavehiclewithbrakesimproperlyadjusted?

4. Whatarethedangersofoperatingavehiclewherethepushrodtravelisbarelywithintolerancewhenthebrakedrumsarecold?

5. Whatitemsshouldyoucarrytomeasureandadjustslackadjusters?

6. Whatisthefirstthingyoushoulddowhenpreparingtomeasureslackadjustment?

7. Howmuchpressureshouldyouapplytothefootvalvewhenmeasuringslackadjustmentusingtheappliedstrokemethod?

8. Howdoyouknowyouareturningtheadjustingboltinthecorrectdirectionwhenadjustingaslackadjuster?

9. Whatisanadvantageofautomaticslackadjusters?

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Air Brake Manual • 79

Pre–TripandPost–TripAirBrakeInspection

Section9

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Vehicle InspectionAnairbrakeinspectionmustbecarriedoutoneveryairbrakeequippedvehiclebeforeyoustartdrivingitfortheday.

EverydriverofacommercialmotorvehiclemustcarryoutaninspectionandlogitasoutlinedinThe�Highway�Traffic�Actanditsregulations.Acommercialmotorvehicleis:

a. amotorvehiclewithaGVWRof4,500kgormorethatisapublicservicevehicleorcommercialtruck;

b. abuswithaseatingcapacityof11ormore,includingthedriver,thatisusedfortransportationofpersonsorpropertyonahighwayforgainorcompensation;or

c. aschoolbus.

Theinspectionwilltakeplacebeforethevehicle’sfirsttripoftheday,andifatriplastsmorethanoneday,beforethetripbeginsonthefirstday,andnolaterthanthefirstreststoponthesecondandeverysubsequentday.

Regardlessofthemaintenancepoliciesofcompaniesofvehicleowners,alldriversmustbecapableofdeterminingiftheirvehicleisinsafeoperatingconditionasoutlinedinThe�Highway�Traffic�ActanditsRegulations.

Apersonwhocarriesoutavehiclepre–tripinspectionmustmakeatripinspectionreportinlegiblewritingthatcontainsthefollowing:

1. Thelicenceplatenumberorunitnumberofthevehicleandanytrailertowedbyit.

2. Thedateofinspection.

3. Alistofanysafetydefects.

4. Ifnoitemsaredefectiveorrequiredtobereportedonthevehicleoranytrailertowedbyit,astatementtothateffect.

5. Thenameandsignatureofthepersonwhocarriedouttheinspection.

Ifadriverbelievesorsuspectsthatthereisasafetydefecttheymustreportthatbelieforsuspiciontothecarrier.Ifthedefectposesanunreasonableriskitmustbereportedimmediately.Anyothercasemustbereportedinatimelymanner.

Everycommercialvehicledrivermusthaveacurrenttripinspectionreportintheirpossessionforthevehicle?

Checkingavehiclebeforestartingout,asrequiredbylaw,willoftenpreventcostlydelaysaswellasreducethechancesofanaccidentresultingfromamechanicalfailure.

Air Brake Practical TestWhenupgradingyourdriver’slicencefromsingletocombinationvehicleswithairbrakes,youwillberequiredtosatisfactorilydemonstratethecompletepre–tripairbrakeinspectionforcombinationvehicles.

Thepre–tripairbrakeinspectionforcombinationunitsshouldbecompletedwithin30minutesand20minutesforsingleunits.Failuretocompletethetestwithintheabovetimelimitmayresultinthetestbeingdiscontinued.

Theairreservoirsmustbedrainedorfanneddowntozeropressurepriortothepre–tripinspectiontosimulatethattheyweredrainedduringtheposttripinspection.

Youarerequiredtobringallmaterial/toolstocompletetheairbraketest(i.e.wrench,chalk,watch,etc.)Itisrecommendedthatyouwearsafetyglassesandabumpcapwhenperformingapre–tripinspection.

Practicalexaminationwillbeconductedineitherthemetricorimperialsystemsofmeasurement,whichevercoincideswiththeequipmentbeingoperated.

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Pre–Trip Inspection—Combination UnitTheairbrakepre–tripinspectionwhenperformedcorrectlywillidentifyanymalfunctioningcomponentsorproblemswiththesystem.Failuretocompleteathoroughpre–tripinspectionoftheairbrakesystemcouldresultinavehiclebeingoperatedonthehighwaywithafaultybrakesystem,whichcouldpotentiallybedeadly.Completingathroughpre–tripinspectionwillallowthedrivertooperatethevehiclewithreducedchanceofbreakdownorsystemmalfunction.Itisthedriver’sresponsibilitytoensuretheairbrakesystemissafeandfunctioningproperlybeforemovingthevehicle.

Vehicle Secure

A.1 Ensureparkingbrakeisapplied(trailersupplyvalveandtractorspringparkingbrakecontrolvalveareclosed),wheelsareblockedandparkedonlevelground.

A.2 Checkreservoirairpressureisat0psiandthecompressorissecure,nooilorcoolantleaksanddrivebeltsforwearandtension.

A.3 Visuallyinspectallbrakesystemairhoses,gladhands,chambers,pushrods,slackadjusters,drumsfordamageandaremountedproperlyandsecure.

Compressor and Warning Devices Check

A.4 Starttheengineandcheckthatthegaugesarefunctioningproperlyandlowairpressurewarningdevicesareoperating.

A.5 Buildairpressureathighidle,ensurereservoirairpressurebuildsfrom50to90psiwithinthreeminutes.

A.6 Lowairpressurewarningdevicesmustoperatetoaminimumof60psi.

A.7 At90psiorabove,openthetractorspringparkingbrakecontrolvalveandthetrailersupplyvalve,releasingthespringparkingbrakes.Applyandreleasethetrailerservicebrakeswiththetrailerhandvalvewhilelisteningforairtoexhaustatthetrailer.Thiswillindicateifthebrakesarereleasingandthatthegladhandsarenotcrossed.

A.8 Continuebuildingairpressureandcheckthatthegovernorplacesthecompressorintheunloadingstagebetween115psiand135psi,(theairpressuregaugeswillstoprising).

A.9 Fantheservicebrakepedaluntiltheairpressuredropsto90psiandnolowerthan80psi.Stopfanningtoseeifthegovernorhasplacedthecompressorintheloadingstage(thereadingontheairpressuregaugeshouldincrease).

A.10 Continuefanningtheservicebrakepedal.Lowairpressurewarningdevicesmustoperateataminimumof60psiandthetrailersupplyvalvemustcloseataminimumof20psi.

Note: Tractor spring parking brake control may–or may not–also close.

Tractor and Trailer Emergency Systems Check

A.11 Ensurethetrailersupplyvalveandtractorspringparkingbrakecontrolvalveareclosed,applyingthespringparkingbrakes.Exitthecabanddisconnectthesupply(emergency)linegladhands.Noairshouldescapefromeitherofthegladhands.

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A.12 Enterthecabandensurethereservoirairpressureisbetween115psito135psi,shut–offtheengine.Openthetrailersupplyvalve.Airlossfromdisconnectedgladhandwilleitherstopimmediatelyorbleeddowntonolowerthan20psiwhenthetrailersupplyvalveclosesandtheairlossstops.

A.13 Ensuretrailersupplyvalveandtractorspringparkingbrakecontrolvalveareclosed,applyingthespringparkingbrakes.Startenginetorebuildtofullreservoirairpressurebetween115psiand135psi.Exitthecabandre–connectthesupply(emergency)linegladhands.

Tractor Protection Valve Check

A.14 Enterthecabandatorabove90psi,openthetrailersupplyvalve.Exitthecabanddisconnectthecontrol(service)linegladhands.Noairshouldescapefromeitherofthegladhands.

A.15 Enterthecab,ensurefullreservoirairpressurebetween115psiand135psi,openthetractorspringparkingbrakecontrolvalve.Shutofftheengine.

A.16 Makeafullservicebrakeapplicationandholdit.Airshouldescapefromthecontrol(service)linetonolowerthan20psi,thetractorprotectionvalvewillcloseandtheairlosswillstop.Releasetheservicebrakeapplication.

A.17 Closethetractorspringparkingbrakecontrolvalveandensurethetrailersupplyvalveisclosed,ifnot,closeitmanually.Starttheengineandrebuildairpressure.Atorabove90psi,makeatrailerservicebrakeapplicationwithtrailerhandvalve.Noairshouldescapefromthedisconnectedcontrol(service)line.Releasethehandvalveapplication.

A.18 Exitthecabandre–connectthecontrol(service)linegladhands.

A.19 Enterthecabandatorabove90psi,openthetractorspringparkingbrakecontrolvalveandtrailersupplyvalve,continuebuildingairpressuretobetween115psito135psi.Onceatfullpressure,shutofftheengine.

Air Leak Check

A.20 Makeafullservicebrakeapplicationwiththefootvalveandholditforoneminute.Airpressuremustnotdropmorethan4psi.

A.21 Listenforaudibleairleaks.Releasetheservicebrakeapplication,starttheengineandrebuildtofullsystemairpressurebetween115psiand135psi.

Brake Adjustment—Combination UnitNote: For checking if your brakes are in adjustment you will require a second person to apply and hold the service brakes on or a device to apply and hold the service brakes on.

A.22 Ensurethetractorspringparkingbrakecontrolvalveandtrailersupplyvalveareopen.(Forvehiclesequippedwithautomaticslackadjusters, makeafullservicebrakeapplicationandrelease.Repeatthisprocess6times,ensurereservoirairpressureisabove90psiforeachfullbrakeapplication).

A.23 Ensurereservoirairpressureisabove90psi,shutofftheengine.(Foraddedsecurity,placemanualtransmissioninlowgear.)

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Brake Adjustment Verification for Automatic or Manual Slack AdjustersForsafetyreasonsinspectthetractorortrailerbrakesseparately.Thiswillallowthespringparkingbraketobeappliedonthebrakesthatarenotbeinginspectedpreventingthevehiclefrommoving.Measurementsforslackadjustmentarefortype30standardstrokebrakechambers.

A.24 Exitthecabandmakeachalkmarkonthepushrodwhereitentersthebrakechamber.Repeatforallchambers.

A.25 Enterthecabandensurereservoirpressureisbetween90psiand100psi.Withabrakepedalassistingdeviceorasecondperson,makea90psito100psiservicebrakeapplicationandhold.

A.26 Returntothebrakechambersandmeasurethepushrodtravelbycheckingthedistancefromthechambertothechalkmarkonthepushrod.Ensurethepushrodtraveliscorrectforthesizeandtypeofbrakechamberbasedonmanufacturer’sspecifications.Also,brakechamberswhichsharethesameaxlemusthaveapushrodtravelmeasurementwithin1/$inchofeachother.

Note:For automatic slack adjusters, the slack adjuster must be replaced if measurements are not within the above specifications.

Adjustment for Manual Slack Adjusters OnlyFordriver’swith“S”airbrakeendorsement

M.1 Enterthecabandremovethebrakepedalassistingdevicereleasingtheservicebrakes.Ensurereservoirpressureisatorabove90psi.

M.2 Returntotheslackadjusterrequiringadjustmentandlocatetheadjustmentbolt.Withtheproperwrench,disengagetheadjustmentboltlockingmechanism.Turntheadjustmentboltuntilthebrakeliningcomesintocontactwiththebrakedrum.Ifthiscannotbeconfirmedbyvisuallyseeingtheliningandbrakedrumduetothedustcover,observethedirectionoftheS–camshaftwhileturningtheadjustmentbolt.TheS–camshaftshouldturninthesamedirectionasitwouldwhenabrakeapplicationisbeingmade.

M.3 Aftertighteningtheadjustmentbolt,turntheadjustmentboltintheoppositedirection1/$toMaturn.Removethewrenchandensurethelockingdevicereturnstothe“locked”position.Failuretoengagethelockingdevicecouldresultintheadjustmentboltturningandcausingthebraketobecomeoutofadjustmentwiththeapplicationofthebrakes.Completethesestepsforallslackadjusters.

M.4 Enterthecab;ensurereservoirairpressureisabove90psiandmakea90psito100psiservicebrakeapplicationandhold.Useabrakepedalassistingdeviceorasecondpersontoholdtheapplication.

M.5 Returntothebrakechambersandmeasurethepushrodtraveltoensuretraveliswithin1Minchesorwithinmanufacturer’sspecifications.Alsobrakeswhichsharethesameaxlemusthaveapushrodtravelmeasurementwithin1/$inchofeachother.

Note: If one slack adjuster requires adjustment then all slack adjusters must be adjusted.

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Brake Response Tests

A.27 Ensurethetractorandtrailerspringparkingbrakesareappliedandremovethewheelblocks.

A.28 Enterthecabandstarttheengine.Openthetractorspringparkingbrakecontrolvalveandplacethevehicleinalowforwardgear.Gentlytugagainsttheappliedtrailerspringparkingbrakes.Theymustholdthevehicle.

A.29 Closethetractorspringparkingbrakecontrolvalveandopenthetrailersupplyvalve.Gentlytugagainsttheappliedtractorspringparkingbrakes.Theymustholdthevehicle.

A.30 Openthetractorspringparkingbrakecontrolvalveandapplythetrailerservicebrakesusingthetrailerhandvalve.Gentlytugagainstthetrailerservicebrakes.Theymustholdthevehicle.

A.31 Releasethetrailerservicebrakeapplication,movethevehicleaheadslowlyandmakeaservicebrakeapplicationwiththefootvalvetocheckbrakeresponse.

Post–Trip Inspection—Combination UnitAttheendofthefinaltripofthedayyoumustinspectyourvehicleforanydamageofproblemsthatmayhavedevelopedontheroadandcompleteatripinspectionreport.Thisreportmustnoteanydefectsfoundduringthepost–tripinspection.Thepost–tripinspectioncanbeaddedtothereportyoucompletedduringthepre–tripinspectionatthestartofthatday.

Theairbrakesystempost–tripinspectionshouldincludethefollowing:

• Parkthevehicleonlevelground,applythespringparkingbrakes,shutofftheengineandleaveitingear.

• Visuallyinspectallbrakesystemairhoses,chambers,pushrods,slackadjusters,drumsfordamageandaremountedproperlyandsecure.

• Openthedrainvalvesonallairreservoirsandallowallairpressuretoescapethenclosethedrainvalves.

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Pre–Trip Inspection—Single Unit(Not for air–over–hydraulic brake systems.)

Theairbrakepre–tripinspectionwhenperformedcorrectlywillidentifyanymalfunctioningcomponentsorproblemswiththesystem.Failuretocompleteathoroughpre–tripinspectionoftheairbrakesystemcouldresultinavehiclebeingoperatedonthehighwaywithafaultybrakesystem,whichcouldpotentiallybedeadly.Completingathroughpre–tripinspectionwillallowthedrivertooperatethevehiclewithreducedchanceofbreakdownorsystemmalfunction.Itisthedriver’sresponsibilitytoensuretheairbrakesystemissafeandfunctioningproperlybeforemovingthevehicle.

Vehicle Secure

B.1 Ensurethespringparkingbrakecontrolvalveisclosedapplyingthespringparkingbrakes,wheelsareblockedandparkedonlevelground.

B.2 Checkreservoirairpressureisat0psiandthecompressorissecure,nooilorcoolantleaksanddrivebeltsforwearandtension.

B.3 Visuallyinspectallbrakesystemairhoses,chambers,pushrods,slackadjusters,drumsfordamageandaremountedproperlyandsecure.

Compressor and Warning Devices Check

B.4 Starttheengineandobservethegaugesarefunctioningproperlyandlowairpressurewarningdevicesareoperating.

B.5 Buildairpressureathighidle,ensurereservoirairpressurebuildsfrom50to90psiwithinthreeminutes.

B.6 Lowairpressurewarningdevicesmustoperatetoaminimumof60psi.

B.7 Continuebuildingairpressureandcheckthatthegovernorplacesthecompressorintheunloadingstagebetween115psiand135psi.(theairpressuregaugeswillstoprising.)Openthespringparkingbrakecontrolvalve,releasingthespringparkingbrakes.

B.8 Fantheservicebrakepedaluntiltheairpressuredropsto90psiandnolowerthan80psi.Stopfanningtoseeifthegovernorhasplacedthecompressorintheloadingstage(thereadingontheairpressuregaugeshouldincrease.)

B.9 Continuefanningtheservicebrakepedal.Lowairpressurewarningdevicesmustoperateataminimumof60psi.Buildreservoirairpressureatfastidletofullairpressurebetween115psiand135psi.

B.10 Ensurethespringparkingbrakesarereleasedbyopeningthespringparkingbrakecontrolvalveandthenshutofftheengine.

Air Leak Check

B.11 Makeafullservicebrakeapplicationwiththefootvalveandholditforoneminute.Airpressuremustnotdropmorethan3psi.

B.12 Listenforaudibleairleaks.Releasetheservicebrakeapplication,starttheengineandrebuildtofullsystemairpressurebetween115psiand135psi.

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Brake Adjustment—Single Unit

(Buseswithlowgroundclearancemustbeadjustedbyaqualifiedmechanic)

Fortestingpurposes,applicantsoperatingbuseswithlowgroundclearancewillnotbeallowedtomanuallyadjustedthebrakesonthebus.

Note: For checking if your brakes are in adjustment you will require a second person to apply and hold the service brakes on or a device to apply and hold the service brakes on. Measurements for slack adjustment are for type 30 standard stroke brake chambers.

B.13 Ensurethespringparkingbrakecontrolvalveisopen.(Forvehiclesequippedwithautomaticslackadjusters,makeafullbrakeapplicationandrelease.Repeatthisprocess6times,ensurereservoirairpressureisabove90psiforeachfullbrakeapplication).

B.14 Ensurereservoirairpressureisabove90psiandshutofftheengine.(Foraddedsecurity,placemanualtransmissioninlowgear.)

Brake Adjustment Verification for Automatic or Manual Slack Adjusters

B.15 Exitthecabandmakeachalkmarkonthepushrodwhereitentersthebrakechamber.Repeatforallchambers.

B.16 Enterthecabandensurereservoirpressureisbetween90psiand100psi.Withabrakepedalassistingdeviceorasecondperson,makea90to100psiservicebrakeapplicationandhold.

B.17 Returntothebrakechambersandmeasurethepushrodtravelbycheckingthedistancefromthechambertothechalkmarkonthepushrod.Ensurethepushrodtraveliswithintwoinchesforautomaticslackadjusters,andwithin1Minchesformanualslackadjustersorwithinmanufacturer’sspecifications.Also,brakechamberswhichsharethesameaxlemusthaveapushrodtravelmeasurementwithin1/$inchofeachother.

Note: For automatic slack adjusters, the slack adjuster must be replaced if measurements are not within the above specifications.

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Adjustment for Manual Slack Adjusters OnlyFordriver’swith“S”brakeendorsement

M.1 Enterthecabandremovethebrakepedalassistingdevicereleasingtheservicebrakes.Ensurereservoirpressureisatorabove90psi.

M.2 Returntotheslackadjustersrequiringadjustmentandlocatetheadjustmentbolt.Withtheproperwrench,disengagetheadjustmentboltlockingmechanism.Turntheadjustmentboltuntilthebrakeliningcomesintocontactwiththebrakedrum.Ifthiscannotbeconfirmedbyvisuallyseeingtheliningandbrakedrumduetothedustcover,observethedirectionoftheS–camshaftwhileturningtheadjustmentbolt.TheS–camshaftshouldturninthesamedirectionasitwouldwhenabrakeapplicationisbeingmade.

M.3 Aftertighteningtheadjustmentbolt,turntheadjustmentboltintheoppositedirection1/$toMaturn.Removethewrenchandensurethelockingdevicereturnstothe“locked”position.Failuretoengagethelockingdevicecouldresultintheadjustmentboltturningandcausingthebraketobecomeoutofadjustmentwiththeapplicationofthebrakes.Completethesestepsforallslackadjusters.

M.4 Enterthecab;ensurereservoirairpressureisabove90psiandmakea90psito100psiservicebrakeapplicationandhold.Useabrakepedalassistingdeviceorasecondpersontoholdtheapplication.

M.5 Returntothebrakechambersandmeasurethepushrodtraveltoensuretraveliswithin1Minchesorwithinmanufacturer’sspecifications.Also,brakeswhichsharethesameaxlemusthaveapushrodtravelmeasurementwithinNinchofeachother.

Note: If one slack adjuster requires adjustment then all slack adjusters must be adjusted.

Brake Response Test

B.18 Ensurethespringparkingbrakesareappliedandremovethewheelblocks.

B.19 Enterthecabandstarttheengine.Withthespringparkingbrakecontrolvalveclosed.Placethevehicleinalowforwardgear.Gentlytugagainstthespringparkingbrakes.Theymustholdthevehicle.

B.20 Openthespringparkingbrakecontrolvalve,movethevehicleaheadslowlyandmakeaservicebrakeapplicationwiththefootvalvetocheckbrakeresponse.

Post–Trip Inspection—Single UnitAttheendofthefinaltripofthedayyoumustinspectyourvehicleforanydamageofproblemsthatmayhavedevelopedontheroadandcompleteatripinspectionreport.Thisreportmustnoteanydefectsfoundduringthepost–tripinspection.Thepost–tripinspectioncanbeaddedtothereportyoucompletedduringthepre–tripinspectionatthestartofthatday.

Theairbrakesystempost–tripinspectionshouldincludethefollowing:

• Parkthevehicleonlevelground,applythespringparkingbrakes,shutofftheengineandleaveitingear.

• Visuallyinspectallbrakesystemairhoses,chambers,pushrods,slackadjusters,drumsfordamageandaremountedproperlyandsecure.

• Openthedrainvalvesonallairreservoirsandallowallairpressuretoescapethenclosethedrainvalves.

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Pre–Trip Inspection—Air–Over–Hydraulic (Air Actuated) Brake SystemTheair–over–hydraulicpre–tripinspectionwhenperformedcorrectlywillidentifyanymalfunctioningcomponentsorproblemswiththesystem.Failuretocompleteathoroughpre–tripinspectionoftheairbrakesystemcouldresultinavehiclebeingoperatedonthehighwaywithafaultybrakesystem,whichcouldpotentiallybedeadly.Completingathoroughpre–tripinspectionwillallowthedrivertooperatethevehiclewithreducedchanceofbreakdownorsystemmalfunction.Itisthedriver’sresponsibilitytoensurethebrakesystemissafeandfunctioningproperlybeforemovingthevehicle.

Vehicle Secure

D.1 Ensuretheparkingbrakeisapplied,thewheelsareblockedandparkedonlevelground.

D.2 Checkreservoirpressureisat0psi(0kPa),thecompressorissecureandithasnooilorcoolantleaksandcheckthecompressordrivebeltsforwearandtension.Checkthehydraulicfluidreservoirlevelandthatthereisnohydraulicfluidleaks.

D.3 Visuallyinspectallwheelcylindersandlinesforhydraulicfluidleaks,drumsfordamage.

Compressor and Warning Devices Check

D.4 Starttheengineandcheckthatthegaugesarefunctioningproperlyandlowairpressurewarningdevicesareoperating.

D.5 Buildairpressureathighidle,ensurereservoirairpressurebuildsfrom50psi(344kPa)to90psi(621kPa)withinthreeminutes.

D.6 Lowairpressurewarningdevicesmustoperatetoaminimumof60psi(414kPa).

D.7 Continuebuildingairpressureandcheckthatthegovernorplacesthecompressorintheunloadingstagebetween115psi(793kPa)and135psi(931kPa).(theairpressuregaugewillstoprising).Openthespringparkingbrakecontrolvalve,releasingthespringparkingbrakes(ifequipped).

D.8 Fantheservicebrakepedaluntilthepressuredropsto90psi(621kPa)andnolowerthan80psi(552kPa).Stopfanningtoseeifthegovernorhasplacedthecompressorintheloadingstage(thereadingontheairpressuregaugeshouldincrease.)

D.9 Continuefanningtheservicebrakepedal.Lowairpressurewarningdevicesmustoperateataminimumof60psi(414kPa).Buildreservoirairpressureatfastidletofullpressurebetween115psi(793kPa)and135psi(931kPa).

D.10 Releasethespringparkingbrakesifequipped.Shutofftheengine.

Air Leak Check

D.11 Makeafullservicebrakeapplicationwiththefootvalveandholditforoneminute.Airpressuremustnotdropmorethat3psi(21kPa).

D.12 Listenforaudibleairleaks.Releasetheservicebrakeapplication,starttheengineandrebuildtofullsystempressurebetween115psi(793kPa)and135psi(931kPa).

Brake Response Test

D.13 Ensuretheparkingbrakeisappliedandremovethewheelblocks.

D.14 Applytheparkingbrakeandplacethevehicleinalowforwardgearandgentlytugagainsttheparkingbrake.Itmustholdthevehicle.

D.15 Releasetheparkingbrake,movethevehicleaheadslowlyandmakeaservicebrakeapplicationwiththefootvalvetocheckbrakeresponse.

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Metric Conversion TableAconversiontothemetricsystemtookplaceinCanadaonSeptember1,1977,andsomecommonlyusedconversionsarelistedbelow.Numbershavebeenroundedandthereforearenotpreciseequivalents.

kPa to psi

5 0.72

10 1.45

15 2.17

20 2.90

25 3.62

30 4.35

35 5.07

40 5.80

45 6.52

50 7.25

60 8.70

70 10.15

80 11.60

90 13.05

100 14.50

150 21.75

200 29.00

250 36.29

300 43.51

310 44.96

350 50.76

400 58.01

415 60.19

450 65.26

500 72.51

550 79.77

585 84.84

600 87.02

650 94.27

700 101.52

725 105.15

750 108.77

800 116.03

850 123.28

900 130.53

950 137.78

1000 145.03

1050 152.29

psi to kPa

1 6.89

2 13.78

3 20.68

4 27.57

5 34.47

6 41.36

7 48.26

8 55.15

9 62.05

10 68.94

15 103.42

20 137.89

25 172.36

30 206.84

35 241.31

40 275.78

45 310.26

50 344.73

55 379.20

60 413.68

65 448.15

70 482.62

75 517.10

80 551.57

85 586.04

90 620.52

95 654.99

100 689.47

105 723.94

110 758.41

115 792.89

120 827.36

125 861.83

130 896.31

135 930.78

140 965.25

145 999.73

150 1034.20

Post–Trip Inspection—Air–Over–Hydraulic (Air Actuated) Brake System Attheendofthefinaltripofthedayyoumustinspectyourvehicleforanydamageorproblemsthatmayhavedevelopedontheroadandcompleteatripinspectionreport.Thisreportmustnoteanydefectsfoundduringthepost–tripinspection.Thepost–tripinspectioncanbeaddedtothereportyoucompletedduringthepre–tripinspectionatthestartofthatday.

Theair–over–hydraulicbrakesystempost–tripinspectionshouldincludethefollowing

• Parkthevehicleonlevelground,applythespringparkingbrakes,shutofftheengineandleaveitingear.

• Openthedrainvalvesonallairreservoirs,allowallairpressuretoescapeandthendrainvalves.

Section Summary Questions

1. Whatisthemaximumtimepermittedforthecompressortobuildfrom50to90psi?

2. Whatisthemaximumairpressurelosspermittedonafullbrakeapplicationwiththemotorshutoffforcombinationandsingleunits?

3. Howcantheholdingpowerofthetrailerbrakesbetested?

4. Whatisthefinalbraketestthatshouldbemadebeforethevehicleisputintoservice?

5. IsabrakeadjustmentpartoftheairbraketestgivenbyManitobaPublicInsurance?

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Metric Conversion Table

1 inch = 2.54 cm

1 foot = 30.48 cm

1 mile = 1.61 km

Length

Volume

1 pint = 0.57 l

1 quart = 1.13 l

1 gallon = 4.55 l

Speed

1 mph = 1.61 km/h

10 mph = 15 km/h

15 mph = 25 km/h

20 mph = 30 km/h

30 mph = 50 km/h

40 mph = 60 km/h

50 mph = 80 km/h

60 mph = 100 km/h

8’ 6” = 2.60 m

10’ = 3.00 m

12’ 6” = 3.80 m

13’ 6” = 4.15 m

14’ = 4.20 m

35’ = 11.00 m

38’ = 11.50 m

40’ = 12.50 m

45’ = 14.00 m

60’ = 18.50 m

65’ = 20.00 m

100’ = 30.00 m

500’ = 150.00 m

Converted Linear Measurements

Converted Weight

10,000 lbs = 4 500 kg

12,000 lbs = 5 500 kg

20,000 lbs = 9 100 kg

24,000 lbs = 11 000 kg

Abbreviations

mm = millimetre

cm = centimetre

l = litre

km/h = kilometre per hour

m = metre

kg = kilogram

kPa = kilopascal

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Pre-trip Inspection Tear Out SheetYou may use this sheet for your test. Do not add to, or write on this form. If altered it cannot be used on the test. The Air Brake Manual has specific inspection information that must be studied prior to the test.

Key points:• You need to perform the steps required for the

inspection of various air brake components and tell the examiner what you’re checking for. Never assume the examiner has enough information and delete items. When the examiner is satisfied in your knowledge of the area you may be asked to move along.

• The pre-trip inspection should be done in a systematic manner to conserve time. The air brake pre-trip inspection should take no more than 30 minutes to complete.

Inspection PointsVehicle Secure

• Check all air brake components outside

Compressor and Warning Devices Check

• Confirm zero air pressure

• Pressure build up

• Fan down air pressure

Tractor and Trailer Emergency Systems Check

• Disconnect the supply (emergency) line

Tractor Protection Value Check

• Disconnect the control (service) line

Air Leak Check

Brake Adjustment Verification

Adjustment for Manual Slack Adjusters Only (if Needed)

Brake Response Tests

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Organ and Tissue DonationEvery year in Canada, thousands of people are waiting for organ and tissue transplants. You can make the diff erence in the lives of many people by deciding to become a donor. Everyone has the potential to be an organ and tissue donor. The health of the donor, not the age, is the most important thing considered. Generally, organ donors are healthy people who have suff ered an irreversible brain injury, as in a motor vehicle accident or a brain hemorrhage. Organs and tissues that can be donated include: kidneys, heart, lungs, liver, pancreas, small bowel, corneas, solid bones and joints, skin and heart valves.

Sign a Donor Card Today and Discuss Your Wishes with Your Family!Signing a donor card is one way to indicate your wishes to be an organ and tissue donor. However, donor cards are often not available at the time of the donor’s death. Therefore, it is important that you discuss your wishes with your family. Other people you may want to inform of this wish are your clergy and family doctor. A Living Will is another place where you can record your wishes. If you have named a proxy in your Living Will, the proxy has precedence over the legal next-of-kin to give consent for organ and tissue donation if you have discussed your wishes with your proxy.

Drivers under the age of 18 years require the signature of a parent or guardian. Simply cut out the card on the following page, complete the information, and keep it with your Manitoba Health Registration Certifi cate or Driver’s Licence so it’s visible for law enforcement and hospital offi cials. Your donor card is a legal document that allows you to give any or all tissues and organs after death.

For further information and extra donor cards, contact:

Transplant Manitoba – Gift of Life Programwww.transplantmanitoba.ca204-787-1897

Tissue Bank Manitoba204-940-1750

The Lions Eye Bank of Manitoba and Northwest Ontario Inc.204-788-8419

DONOR CARDDirection under The Human Tissue Gift Act The Human Tissue Gift Act T C.C.S.M. c. H180

I,

direct the use, after my death (please check ✓) of:❑ any needed organs or parts of my body; or❑ the following specified organs or parts of my body, namely:

for the following purposes:❑ transplant and other therapeutic purposes;❑ medical education; ❑ scientific research.

Donor Signature Date

Co-signature of parent or guardian Date where donor is under 18 years of age.

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94 • Air Brake Manual

If you wish to donate organ(s) or tissue after death, complete theattached donor card and KEEP IT WITH YOUR MANITOBA HEALTHREGISTRATION CERTIFICATE OR DRIVER’S LICENCE SO IT IS VISIBLEFOR LAW ENFORCEMENT AND HOSPITAL OFFICIALS.

IT IS EXTREMELY IMPORTANT TO DISCUSS YOUR DECISION WITHYOUR FAMILY TO ENSURE YOUR WISHES ARE FULFILLED.YOUR FAMILY TO ENSURE YOUR WISHES ARE FULFILLED.Y

For more information on organ and tissue donation, please contact one of the Human Tissue Gift Agencies listed below:

Transplant Manitoba-Gift of Life Program 204-787-1897

Tissue Bank Manitoba 204-940-1750

Lions Eye Bank of Manitoba and Northwest Ontario Inc. 204-788-8419

(R-09/09)

ORGAN AND TISSUE DONATION

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$10.00DVL0025/0337–16–B3 (R–2010)

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For further information, please see our website: www.mpi.mb.ca