Brake System Service - Denton ISD

CH

AP

TE

R

11

Brake System Service

Chapter ObjectivesAt the conclusion of this chapter you should be able to:

KEY TERMSbench bleeding

brake light switch

brake pedal height

bypassing

dissolved copper

manual bleeding

pressure bleeding

vacuum bleeding

© C

enga

ge L

earn

ing

2014

Copyright 201 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).

Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.

288 Chapter 11 • Brake System Service

Eventually, a vehicle’s brake system will need ser-vice. Brake lines rust through, brake hoses crack and

rupture, and other hydraulic components leak and need to be replaced. This chapter focuses on servicing and repairing the hydraulic system.

Tools and SafetyFollowing proper service procedures includes using the correct tools to perform the job and using the tools cor-rectly. This allows for safe and productive work without injuries or time lost due to poor work practices.

ToolS for HydrAuliC BrAke SySTem ServiCeServicing the hydraulic system requires basic hand tools, such as line wrenches, sockets, and similar tools as well as specialized tools such as brake bleeding equipment. The most common tools you will need are shown in Figure 11-1 through Figure11-6.

• Lineor flare nutwrenches, like those shown inFigure 11-1, are used on brake line fittings. Because of the shape of the wrench, fittings are less likely to strip when loosening and tightening them with a line wrench.

• Aflaringtool,likethatshowninFigure 11-2, is used to make new flares on a brake line. Flaring toolsareavailableforbothSAEandmetriclinesand flares.

• Bleedingthebrakescanbeeasierwithtoolssuchasthe vacuum bleeder shown in Figure 11-3. Shop air is connected to the tool and a vacuum draws air and fluid out of the brake system.

• Apressurebleeder,likethatshowninFigure 11-4, applies pressure to the brake fluid at the master cylinder and forces air and fluid out of the bleeder valves.

• When you aremaking newbrake line, a tubingbender, like that shown in Figure 11-5, is used to bend the brake line without it kinking.

• ThetestershowninFigure 11-6 is used for testing the brake fluid for moisture content.

Tool SAfeTy And SAfe Work PrACTiCeSToolsaredesignedtobeusedinspecificways.Whena tool is used incorrectly or for something other than itsintendedpurpose,accidentscanoccur.Althoughtheservice tools for the hydraulic system do not generally present any significant danger in their typical usage, improper use can lead to personal injury and damage to equipment and vehicles. To reduce the chances of having an accident when you are working on the brake system, follow these tool safety precautions:

• Usethecorrectwrenchesandotherhandtoolsforthe job, and use the tools correctly. Do not use open-end wrenches to break loose tight fasteners, and pull wrenches and ratchets toward your body when you are loosening fasteners.

© C

enga

ge L

earn

ing

2014

fiGure 11-1 Line wrenches, also called tubing or flare nut wrenches, are used on brake line and similar fittings. The extra grip reduces the chance of stripping the hex on the fitting.

© C

enga

ge L

earn

ing

2014

fiGure 11-2 A flaring tool is used to make a flare on the end of a brake line. Flare tools are either SAE or ISO.

© C

enga

ge L

earn

ing

2014

fiGure 11-3 This vacuum bleeder is used when flushing or bleeding the hydraulic system.

39634_ch11_rev02.indd 288 31/01/13 12:02 PM



Chapter 11 • Brake System Service 289

• Ensurethebrakesystemisdepressurizedbeforeyouopen the hydraulic system on vehicles with inte-gral antilock brake systems or systems with high-pressure accumulators.

• Readandfollowalloftheinstructionsofthepres-sure bleeder for setup and use if you use one. Do not overpressurize the bleeder or the brake system.

• Usecautionwhenusingabrakefluidmoisturetes-ter as it heats the fluid. Both the tester and the fluid become very hot.

• Allow the brakes to cool before you attempt toremove or service any components.

© C

enga

ge L

earn

ing

2014

0

40

80

100

120

160

200

Airinlet

Fluid controlvalve

Pressuregauge

Fluid

Air

To adapter

Internaldiaphragm

fiGure 11-4 A pressure bleeder forces brake fluid through the system under pressure to remove any trapped air.

© C

enga

ge L

earn

ing

2014

fiGure 11-5 A tubing bender is used when fabricating new brake lines. Using a bender reduces the chance of kinking the line.

© C

enga

ge L

earn

ing

2014

fiGure 11-6 A moisture tester heats a sample of brake fluid to determine its moisture content.

39634_ch11_rev02.indd 289 31/01/13 12:02 PM




motor vehicle laws. It is important to remember that even though accidents do happen, an accident involving a customer’s car is not going to be tolerated if it is your fault. If it is necessary, have a coworker ride along to help with locating noises so that you are not dividing your attention between driving and diagnosing.

Asastudent,youareprobablynotgoingtobeabletotake extended test-drives off the school’s campus. How-ever, you may be able to test-drive within the school’s parking lots. If so, make sure you observe the speed limit within the parking lot, and pay close attention for vehi-cles pulling in and out of parking spaces while you are test-driving.

Brake Pedals and Hydraulic System Most modern vehicles do not require adjustment to the brake pedal or pushrod unless components such as the vacuum power assist unit are replaced. However, some vehicles permit adjustments of the brake pedal push-rod and pedal free play. Many brake switches will, if replaced, require adjustment once they are installed.

BrAke PedAl inSPeCTion And ServiCeUnlessavehiclehaspoweradjustablepedals,typicallytheonly inspection for the brake pedal assembly is for pedal height, travel, and free play. It is possible that a customer may experience a noise when the brake pedal is applied that is caused by the bushing located on the pedal pivot. The pedal should move freely but without play within the pedal assembly. If noise or play is present, inspect the pedal mounting components as shown in Figure 11-7.

• Usefendercoverstoprotectthevehiclefrombrakefluid.

• Cleanupbrakefluidimmediately;donotallowfluidto remain in contact with painted surfaces, wiring, plastics, or any other parts of the vehicle.

• Donotreusebrakefluidorusebrakefluidfromanopen container.

• Usethecorrecttoolsforthejob.

• Donotreusedamagedparts.

• Ensurethebrakesareworkingproperlybeforemov-ing the vehicle.

• Readandfollowallserviceproceduresrelatedtothebrake system service.

determining Brake System ConcernsAsatechnician,youwillberequiredtotest-drivevehi-cles to verify customer concerns and to verify repairs havebeencompletedproperly.Whenitispossible,atest-drive should be performed before and after all repairs. However,commonsenseneedstobeused;ifavehiclehas no brakes, such as from a hydraulic system leak, a test-drive is not needed to confirm the complaint. Once repairs are made and the brake system is functioning, a test-drive should be performed to determine if other problems, such as brake noise or a pulsation, are present.

Many technicians develop a test-drive route that cov-ers different types of driving conditions. This is so the technician can operate the vehicle to check and test the brakes, handling, and performance of the engine, and thedrivetrain.Whenyouaretest-drivingforbrakesys-tem concerns, you will want to be able to check for the following:

• Noiseduringdrivingandbraking.Sometechniciansfind that driving where noise can be reflected, such as behind stores, makes diagnosing some noise com-plaints easier.

• Pulsationduringstopping.Thismayrequiredriv-ing on the highway or other roads with higher speed limits to feel a pulsation.

• Pullingduringstopping.Thisshouldbecheckedona section of smooth, flat road where there is little traffic.

• FalseABSactivation.Thisconcernshouldalsobechecked on a section of smooth, flat road or in a parking lot where there is little traffic.

The main requirements for a test-drive are that it is done safely and that all driving laws are observed. Atnotimeshouldavehiclebeoperatedinanyunsafemanner or in a way that violates speed limits or any other

Brake pedal

mountingbracket

Brakepedal

assembly

Clevis

Pushrod

© C

enga

ge L

earn

ing

2014

fiGure 11-7 An illustration of the brake pedal assembly. Inspect the pedal and bracket for wear and noise as part of a brake inspection.

39634_ch11_rev02.indd 290 31/01/13 12:02 PM




effort gauge, like that shown in Figure 11-9b, may be required. The gauge is used to apply a specific amount of pressure, such as 100 pounds, to the brake pedal when testingpedalheight.Alwaysrefertothemanufacturer’sservice information for the correct procedure for check-ing pedal height. If the height is incorrect, check the rearbrakeadjustment.Excessiveshoe-to-drumclear-ance will cause a low brake pedal. If the vehicle has rear disc brakes with an integral parking brake, lack of use of the parking brake can cause a gap to develop between the pads and rotors, which can cause a low brake pedal. If it is necessary, adjust the rear brakes and recheck thepedalheight.RearbrakeadjustmentisdiscussedinChapters13and15.

Brake pedal free play is the very slight movement of thepedalbeforethebrakepushrodbeginstomove.Pedalfree play is shown in Figure 11-10. Free play on some vehicles is adjusted by loosening a jam nut on the brake pedal pushrod and adjusting rod length.

Brake Light System. Mounted on the brake pedal bracket is the brake light switch.AnexampleisshowninFigure 11-11.Theswitchiseitheropenorclosed.Whenit is open, the circuit is open, and the brake lights are off. Whenitisclosed,aswhenthedriverpressesthebrakepedal, the circuit closes, and the brake lights at the rear of the vehicle should illuminate. The brake light switch is used for more than activating the rear brake lights. It is alsoaninputforthepowertraincontrolmodule(PCM),theantilockbrakesystem(ABS),andthetractioncontrol(TC)system.Brakeswitchactivityisalsomonitoredasan input for the lockup torque converter clutch used in automatictransmissions.Anonfunctioningbrakelightswitch, aside from being a potential driving hazard for

Pedal Height and Free Play. Brake pedal height is measured as the distance from the pedal to the floor with the pedal depressed, and it is measured as shown in Figure 11-8. Some manufacturers may provide pedal height specs from the steering wheel down to the pedal, as shown in Figure 11-9a. In addition, a pedal

Lift floormat

Locknut

Pushrod

Measuringpoint

Pedalheight

Standard pedal height(with carpet removed):

179 mm (7 in.) © C

enga

ge L

earn

ing

2014

fiGure 11-8 This illustrates how to check brake pedal height. It is important to move the floor mat and carpet to get an accurate measurement.

Tape measure

Brake effortpedal gauge

Unappliedbrake pedal ©

Cen

gage

Lea

rnin

g 20

14

fiGure 11-9a Some manufacturers specify checking pedal height from the steering wheel to the pedal.

Brake pedaleffort gauge ©

Cen

gage

Lea

rnin

g 20

14

fiGure 11-9b An example of a brake pedal effort gauge. When testing pedal height, press on the plunger until the specified effort is shown on the gauge and check the pedal height.

39634_ch11_rev02.indd 291 31/01/13 12:02 PM




light circuit is shown in Figure 11-12. If all of the vehi-cle’s brake lights fail to operate, locate and check the brake light circuit’s fuse first. If the brake light fuse is good, then check for power to and from the brake switch, as shown in Figure 11-13. If power is present to the switch but no power is present at the output wire with the brake applied, the switch is open. If there is no power to the switch and the fuse is good, you will need to look for an open in the wiring between the fuse and the switch.

On some vehicles, the brake light switch is an input intothebodycontrolmodule(BCM).WhentheBCMreceivesinputthatbrakepedalispressed,theBCMwillactivate the brake lights. For these systems, you will need to use a scan tool to look at the brake switch data as part of diagnosing a problem with the brake light circuit. This typeoftestingisdiscussedinChapter21.

The brake light switch circuit is usually a hot-at-all-times circuit. This means that battery voltage is present to the switch at all times, even when the key is off and the vehicleisparked.Powertotheswitchistypicallyfroma dedicated fuse. This means the fuse supplies power only to thebrake lightcircuit.Powerpasses throughthe switch when the pedal is pressed a slight amount. If the pedal has to travel excessively before the brake lights illuminate, the switch may need to be adjusted. Many switches thread into the brake pedal bracket and are adjusted by loosening the jam nut and turning the switch until the lights come on within about ¼ inch (6mm)ofpedaltravel.Anexampleofathreadedswitchis shown in Figure 11-14.

Adjustable Pedals. Adjustable pedals are anoption on some vehicles, often in combination with power adjustable steering columns and memory seat functions. The pedal assembly, shown in Figure 11-15, is mounted on a motorized carriage that allows for about3inchesoftravelforwardandback.Thisadjust-ment is helpful for shorter drivers who would need to sit too close to the steering wheel—and the air bag—to reach the pedals.

If a vehicle has power adjustable pedals, you may see concerns regarding improper pedal position or a failure of the pedals to adjust position. For adjustable pedal concerns, a scan tool is necessary to access data andstoreddiagnostictroublecodes(DTCs).Beginyourinspection by activating the system and checking that the assembly is able to move forward and backward along the entire range of motion. The pedals should move without binding. If the pedals do not operate, first check the system fuse. If the fuse is intact, con-nectascantooltothedatalinkconnector(DLC),andaccess the menu for the pedal system. First check for

those driving behind the vehicle, may affect more than just the brake lights.

In many vehicles, power from the switch flows to the brakelightsandothercircuits.Anexampleofabrake

(A)Brakeswitch

(B)Pedal

bracket

© C

enga

ge L

earn

ing

2014

fiGure 11-11 An example of one way in which brake light switches can be mounted with the pedal.

Pedal free playto inch

(3 to 6 mm)

Free play atpushrod

inch(1.5 mm)

© C

enga

ge L

earn

ing

2014

161

fiGure 11-10 Free play is measured as the slight amount of movement of the pedal when released.

39634_ch11_rev02.indd 292 31/01/13 12:02 PM




Brn

To pcm

Ppl Wht

(Hazard)

Left tail/stop/turn

light

G300

A B

C

Leftturn

Rightturn

Yel

Lt Blu 18 BR RD Drk Blu

Turn/hazardheadlight switch

Turn/BUfuse10A

Stop/Hazfuse20A

Hot atall times

Hot in run, bulbtest, or start

Fuseblock

Turn/hazardflasher

Brakeswitch

(Front)(Right)(Front)

Right tail/stop/turn

light

A B

C

© C

enga

ge L

earn

ing

2014

fiGure 11-12 In some vehicles, the brake light circuit is completed through the turn signal switch. Other vehicles do not use combination brake/turn signal bulbs and the brake light circuit does not connect through the turn signal switch.

anycurrentandhistoryDTCsrelatedtothepedalsys-tem.IfnoDTCsarepresent,activatethepedalcontrolswitch,andnotetheappropriatedataPIDonthescantool. If the switch input does not change on the data display, inspect the switch and wiring. If the input from the switch is correct, try to control the movement of the pedal assembly through the scan tool. If the pedals

do not respond to active commands, the drive motor or wiring may be at fault.

BrAke fluid inSPeCTinG And TeSTinGUntilthewidespreadadoptionofantilockbrakesystems,brake fluid sometimes remained in the vehicle over the entirelifeofthevehicle.Now,vehiclemanufacturers

39634_ch11_rev02.indd 293 31/01/13 12:02 PM




canbemixedinvehiclesthatspecifytheuseofDOT3.If a vehicle requires DOT 4, use only DOT 4 since it has ahigherboilingtemperaturethanDOT3.Thereservoircap, like that shown in Figure 11-16, usually indicates the recommended fluid.

Whenitisnecessarytoaddbrakefluidtotheres-ervoir, always use new brake fluid from a sealed

recommend periodic flushing of the brake fluid to remove moisture and other contaminants. This prolongs the life of the hydraulic system components and ulti-mately can reduce repair costs.

Checking and Adjusting Brake Fluid Level. AsdiscussedinChapter10,themostcommonbrakefluidsareDOT3andDOT4.Botharechemicallysimilarand

Brake pedalarm pin

Brakes Not Applied Brakes Applied

Master cylinderpushrod

Switch(open contacts)

LampB+ Brake pedal

arm pin

Switch(closed contacts)

LampB+

© C

enga

ge L

earn

ing

2014

fiGure 11-13 This illustrates how to test for power to and through a brake light switch.

© C

enga

ge L

earn

ing

2014

fiGure 11-14 An example of a brake light switch. The threaded section and lock nut are used to position the switch so that the brake lights come on with a small amount of pedal travel.

Adjustablepedal motor

Brakepedal

Acceleratorpedal

Cable

Adjustablepedal module

Adjustable pedal bracket

© C

enga

ge L

earn

ing

2014

fiGure 11-15 If the adjustable pedals do not operate cor-rectly, check for binding in the tracks. Most diagnosis will require a scan tool.

39634_ch11_rev02.indd 294 31/01/13 12:02 PM



Chapter 11 Brake System Service 295

Many vehicles use a brake fluid level sensor in the master cylinder reservoir, shown in Figure 11-19,

instrument panel when the fluid level drops to a cer-

shown in Figure 11-20.If the brake fluid level is low but not enough so that

the warning light is staying on, you should recommend

hydraulic system and worn friction components, specifi-cally worn pads and rotors, will cause a low brake fluid

-tles are shown in Figure 11-17. Before adding brake fluid to the master cylinder reservoir, install a fender

cap to prevent any dirt from getting into the brake

inspect the seal. If the fluid is low, the reservoir seal

seal for damage or swelling as these can indicate fluid contamination. If the cap and seal are good, fill until

-

highlighted is shown in Figure 11-18. If the reservoir caps use accordion seals that have extended out, care-fully push the seals back together before you reinstall the cap.

© C

enga

ge L

earn

ing

2014

FIGURE 11-17 Examples of common brake fluids.

© C

enga

ge L

earn

ing

2014

FIGURE 11-16 Check the brake fluid reservoir cap for the recommended type of brake fluid. Clean the cap and area before removing to prevent dirt from getting into the fluid. ©

Cen

gage

Lea

rnin

g 20

14

FIGURE 11-18 Many reservoirs allow you to check fluid level without opening the cap. Do not open the reservoir unless adding or checking fluid condition as this lets air and moisture into the reservoir.

© C

enga

ge L

earn

ing

2014

FIGURE 11-19 An example of a brake fluid level sensor.




fluid should turn off the light. If the light remains on after the fluid is refilled, the float may be sticking. Disconnect the sensor connector, and note if the light remains on or goes out. If the light remains on, there may be a problem with the sensor wiring, or another part of the circuit may be grounding the light.

If there is a leak in the hydraulic system, pressure will unbalance the pressure differential valve, shown in Figure 11-22, and the warning light will illuminate. (Once the leak is repaired and the system is bled, the valveshouldresetandthelightwillgoout).Notethatvehicles that use a brake fluid level sensor typically do not use a brake warning light switch as part of the pres-sure differential valve.

AswitchlocatedattheparkingbrakehandleorpedalisalsousedtoturnontheBRAKEwarninglight.Thisis to alert the driver so he or she does not try to drive the vehicle with the parking brake applied. Make sure the parking brake is releasing completely.

IftheBRAKEwarninglightremainsonandthefluidlevel is correct, there are no leaks, and the parking brake is fully releasing, then there may be a short to ground in the light circuit. You will need to locate a wiring dia-gram and test the circuit to locate the fault. This usually involves disconnecting each part of the circuit until the light goes out.

Test Brake Fluid Contamination. AsdiscussedinChapter10,itiscriticalthatonlybrakefluidisaddedtothebrakehydraulicsystem.Petroleum-basedfluids,such as engine oil and power steering fluid, if allowed to enter the brake system, will cause rapid and irreversible damage to the rubber seals and hoses. These parts will

level.Aleakinthehydraulicsystemisusuallynoticeabledue to the very low brake pedal and poor stopping power. Asthebrakepadsandrotorswear,thecaliperpistonsmove farther out of the their bores. This results in more brake fluid sitting in the calipers as the pistons move out-ward. This increases the volume of fluid in the calipers and reduces the amount of fluid in the master cylinder reservoir.

Brake Warning Light. TheredBRAKEwarninglight on the dash should illuminate during key-on bulb check,asshowninFigure11-20.Oncetheenginestarts,the light should go out. If the light remains on, there are three possible reasons: low brake fluid level in the reser-voir, low pressure in the hydraulic system, or the parking brakeisapplied.AnillustrationoftheBRAKEwarn-ing light circuit is shown in Figure 11-21.Asdiscussedpreviously, many vehicles use a fluid level sensor in the master cylinder reservoir. If the fluid level drops to the minimum, the float inside the reservoir closes the circuit and turns on the warning light. Topping off the brake

© C

enga

ge L

earn

ing

2014

fiGure 11-20 An example of the BRAKE warning light.

Ignition switch

Fuse 11Brakewarning

bulb

Ignition switch(closed during crank)

Park brakeswitch

Low brakefluid switch

© C

enga

ge L

earn

ing

2014

fiGure 11-21 The BRAKE warning light can illuminate due to low fluid level, an applied parking brake, or low system pressure. The light should also come on during bulb check when the ignition is turned to the ON or RUN position.

39634_ch11_rev02.indd 296 31/01/13 12:02 PM




componentsareoftenmovingparts,suchasintheABSsolenoid valves, which require very close tolerances to operate properly. If the space between these components iscloggedfromcorrosionorrust,theABSwillnotbeable to function properly or may not work at all.

To test the brake fluid, remove the reservoir cap and install the moisture tester into the fluid. If the reservoir has a screen preventing the tester from going down into the fluid, remove a small amount of fluid, and place it intothecontainerthatcomeswiththetester.Refertothe tester’s operating manual for the specific steps for using the tester. These steps usually require connect-ing the tester to the battery and pressing and holding theTESTbutton.Oncethefluidhasbeentested,notethe results.

If you are using fluid test strips, place the strip with thetestpadsdownintothefluidforonesecond.Removethe strip and check the color of the test pads against the chart with the test strips. If excessive copper is indicated or if the fluid has a low boiling point, it should be flushed out and new fluid installed.

begin to swell and deteriorate, becoming very spongy, distorted, and enlarged, and the brakes will not operate. If this occurs, every component that contains rubber will havetobereplacedandallthesteellinesflushed.Asyoucan imagine, this is a very expensive repair.

Brake fluid also becomes contaminated with moisture. Since brake fluid is hygroscopic, it will absorb moisture over time, lowering the boiling point of the fluid. The amount of moisture the fluid has absorbed can be tested with a brake fluid refractometer, special test strips, or a brake fluid moisture tester, as shown in Figure 11-6 ear-lier in this chapter. Once the moisture level reaches about 3percent,theboilingpointofthefluidisreducedmorethan 100 degrees. This amount of moisture accumulation can occur in just a couple of years, so periodic flushing is important.

Brake fluid can also absorb copper from the brake system.Asthecorrosioninhibitorsinthebrakefluiddiminish, the brake fluid begins to absorb copper from the brake lines. The dissolved copper attaches to steel components, leading to further corrosion. The steel

Piston is normally held centeredby equal pressure at both ends.

Switch trigger extends into grooveand switch is open

Trigger is pushed in to closeswitch and illuminate brake

warning lamp on instrument panelSwitch body

Front brakepressure is

applied here

Rear brakepressure is

applied here

Instrument lamp

A leak in either systemdrops pressure to that

systemThe piston moves

toward the reducedpressure side

© C

enga

ge L

earn

ing

2014

fiGure 11-22 The pressure differential warning light switch will turn on the BRAKE light if there is a loss in pres-sure in the hydraulic system.

39634_ch11_rev02.indd 297 31/01/13 12:02 PM



298 Chapter 11 Brake System Service

rear brake receives pressure, forcing the front brakes to carry the full load of stopping the vehicle. This con-dition will cause the vehicle to nose-dive when stop-ping and will wear the front brake components much faster than normal.

Brake hoses can also fail internally and keep pres-sure applied to a wheel brake after the brakes are

mounting brackets built into the hose, putting pres-sure on the hose and trapping fluid in the hose. If the pressure is not released from a brake caliper, the brake stays applied, which causes the vehicle to pull in the direction of the stuck brake. This also causes very rapid brake pad wear.

Since brake fluid is hygroscopic, over time enough moisture can be absorbed to rust pistons into their bores.

HYDRAULIC SYSTEM INSPECTIONAND SERVICE

of three areas: external fluid loss, internal seal bypass-

suspect a hydraulic system problem, as indicated by such signs as a very low, spongy brake pedal, begin your inspection for external leaks from the brake system com-ponents, brake lines, and hoses.

Inspect Brake Lines. -lic system range from the obvious, such as leaks, to the less obvious, such as restricted brake hoses and

in a loss of hydraulic pressure and braking ability, a very low, soft brake pedal, a spongy brake pedal, and fluid loss. The most common causes of external leaks are rusting-through steel brake lines, rupturing brake hoses, and leaking seals in the master cylinder, cali-pers, or wheel cylinders. Figure 11-23 shows a rusting through and leaking brake line. Figure 11-24 shows an example of a leaking brake hose, and Figure 11-25

inadequate hydraulic pressure is restricted or clogged rubber brake hoses. Over time, the interior of the brake hose can deteriorate and begin to come apart, as shown in Figure 11-26the hose, restrict or even block the hose completely. This can keep a wheel brake or both rear brakes from applying with enough pressure, or prevent them from engaging at all if the hose is completely blocked. If a front caliper hose is blocked, only one caliper will apply, causing a serious pull in the direction of the operating caliper when the brakes are applied. If a rear hose is blocked that feeds both rear brakes, neither

© C

enga

ge L

earn

ing

2014

FIGURE 11-23 An example of a rusted and leaking rear brake line.

© C

enga

ge L

earn

ing

2014

FIGURE 11-24 An example of a leaking front brake hose.

© C

enga

ge L

earn

ing

2014

FIGURE 11-25 A leaking wheel cylinder that has contami-nated the linings.



Chapter 11 Brake System Service 299

This can happen to caliper and wheel cylinder pistons, but it occurs most often with wheel cylinders. To check for seized pistons, try to push the pistons back into the cylinder when the drum is off and the brakes are being inspected. If the pistons will not move back into the cyl-inder, the wheel cylinders need to be either rebuilt or replaced.

One quick method to check the hydraulic system is to raise the vehicle on a hoist and to have a helper press the brake pedal, and try to rotate each tire. If a tire or tires rotate with the brakes applied, you should perform a thorough hydraulic and service brake inspection and determine the cause for the brake’s not holding.

Check Master Cylinder for Internal and External Leaks. reliable than ever, the seals can harden over time or can be damaged from contaminants in the brake fluid, and begin

internally, causing conditions ranging from a low, sinking brake pedal to a complete loss of brake pressure.

-mary piston. Depending on how the master cylinder is mounted to the power booster, the leaking fluid may show on the power booster as shown in Figure 11-27,or the fluid may actually leak into the booster. Some master cylinders seal the vacuum in the booster. In this design, fluid leaking from the rear of the master cylinder is pulled into the booster and is not be vis-ible externally. To confirm a leak with this design, you need to unbolt the master cylinder from the booster and inspect the rear of the primary piston for signs of fluid loss.

Seal failure inside the master cylinder can cause the fluid to bypass the pistons, as shown in Figure 11-28.If the vehicle has a sinking, low, and spongy brake pedal, and there is no evidence of any external leaks, fluid may be bypassing inside of the master cylinder. To verify if this is occurring, remove the brake lines

Outer jacketInner liner

Rubberseparator

layer

FittingFabric plies

© C

enga

ge L

earn

ing

2014

BlockageFluid flow Fluid blocked

FIGURE 11-26 Brake hoses can become restricted and block fluid from passing to the wheel brake or keep the fluid from releas-ing pressure in the wheel brake.

© C

enga

ge L

earn

ing

2014

FIGURE 11-27 A leaking master cylinder primary seal allows brake fluid to run out onto the power booster.

Piston

Replenishingport

Ventport

Valley area

Fluid bypasses the piston sealas the brakes are applied

© C

enga

ge L

earn

ing

2014

FIGURE 11-28 Internal seal failure in the master cylinder can allow the fluid to bypass the seal and piston. This will cause a low or sinking pedal and poor stopping performance.

from the master cylinder and plug the outlet ports with threaded plugs. This isolates the master cylinder

brake pedal. If the pedal continues to drop, the internal




types of lines and fittings, shown in Figure 11-30, are not interchangeable.

First remove the damaged section of brake line. If the line runs the length of the vehicle, you need to remove it from several clamps that secure the brake and fuel lines to the body or frame, as shown in Figure 11-31. If you are replacing an entire section of line, from con-nection point to connection point, remove the line and

seals are leaking, and the master cylinder needs to be replaced.

To replace the master cylinder, first install fender cov-ers on the vehicle and place a shop pan under the master cylinder.Removethereservoircapandremovemostofthebrakefluidanddiscardit.Usingthecorrectsizedline-wrenches, loosen the brake line fittings at the mas-tercylinderports.Next,loosenandremovethemountingnuts that secure the master cylinder to the brake booster. Disconnect the fluid level sensor wiring if necessary. Removethefittingsfromthemastercylinderandremovethe cylinder from the vehicle.

Whenyouarereplacingamastercylinder,thereplace-ment should be bench bled before being installed on the vehicle. Bench bleeding removes the air, prefills the master cylinder, and reduces the amount of bleeding that needs to be done on the vehicle. Photo Sequence 7 shows the steps to bench bleed a master cylinder.

Once the master cylinder has been bench bled, rein-stall it on the vehicle. Be sure you have fender cov-ers installed to protect the vehicle. If possible, place a shop pan under the master cylinder to catch any fluid that leaks out during installation. Thread the brake lines into the outlet ports first. This allows for easier alignment and less chance of cross-threading the fit-tings if the master cylinder can be moved as needed to align with the fittings. Once the fittings are started by hand, install the master cylinder over the mount-ing studs on the power assist unit. Tighten the fittings, and torque the mounting nuts to specs. The hydraulic system needs to be bled to remove any air trapped in the system. Bleeding the hydraulic system is discussed later in this chapter.

Replacing Brake Lines and Hoses. AsdiscussedinChapter10,brakelinesandhosesaremanufacturedto withstand the high pressure of the hydraulic system. Whenyouarereplacingbrakelinesandhoses,thecor-rect size line, types of fittings, and replacement parts must be used.

Arustingandleakingbrakelinemustbereplaced.Donot attempt to repair a leaking steel line with a compres-sion fitting. The pressure in the brake system can eas-ily rupture any type of patch job or compression fitting, resulting in brake failure.

If you are replacing a section of brake line, like the rear brake line shown in Figure 11-29, start by determining the size and type of line that is needed. Brake line is sold in pieces that can range from as small as 6 inches long to 6 feet long. Brake line is also sized by its diameter and by the type of flare used. Brake line canbeeitherSAEorISO,whichmeansitismeasuredinfractionsofaninchorinmillimeters.SAElinehasadouble flare, and ISO line uses a bubble flare. The two

© C

enga

ge L

earn

ing

2014

fiGure 11-29 A leaking steel brake line.

35°

32°

30°

45°42°

Inverted Double Flare

ISO Flare © C

enga

ge L

earn

ing

2014

fiGure 11-30 Examples of the SAE double flare and ISO bubble flare. The two types of flares and lines are not inter-changeable.

39634_ch11_rev02.indd 300 31/01/13 12:03 PM



© C

enga

ge L

earn

ing

2014

PS7-1 Mount the master cylinder in a vice and fill with the correct brake fluid. Let the fluid begin to drip from the ports before continuing.

© C

enga

ge L

earn

ing

2014

PS7-2 Install the bench bleeding kit adapters into the ports, and install a hose on each port fitting.

© C

enga

ge L

earn

ing

2014

PS7-3 Place the ends of the hoses into the reservoir.

© C

enga

ge L

earn

ing

2014

PS7-4 Top off the brake fluid as neces-sary.

© C

enga

ge L

earn

ing

2014

PS7-5 Slowly push the pistons into the master cylinder bore and release.

© C

enga

ge L

earn

ing

2014

PS7-6 Each time the pistons are pumped, air and fluid should pass from the hoses into the reservoir.

© C

enga

ge L

earn

ing

2014

PS7-7 Continue to bleed until air bub-bles no longer are present in the fluid.

© C

enga

ge L

earn

ing

2014

PS7-8 Remove the adapters and lines. Install plugs into the ports to prevent fluid from dripping out during installation.

© C

enga

ge L

earn

ing

2014

PS7-9 Place the master cylinder into posi-tion on the brake booster studs. Remove the port plugs and install the brake lines.

© C

enga

ge L

earn

ing

2014

PS7-10 Tighten the brake line fittings using a line wrench. Bleed the rest of the hydraulic system as needed to remove any air in the system.

PHOTO SEQUENCE 7BENCH BLEEDING A MASTER CYLINDER




before you try to install the hose. Install the hose by threading the fittings together by hand. Once it is seated, tighten it by using line wrenches. Be sure you reconnect all brackets and clips that hold the hose in place during reinstallation. These brackets and clips are necessary to keep the hose from being damaged by contacting mov-ing parts and tires.

compare the fittings of the new line to the old line, and ensure the fittings have the correct type of flare and threads. To make removal of severely rusted lines eas-ier, cut the line just behind the fittings where the line is tobedisconnected.Useasockettoremovethefittingwhile holding the other section of line in place with a wrench.

If you are replacing a section of line between con-nection points, you will need to cut the new line to length and flare the ends of the lines to install a union. This requires using a flaring tool, as shown in Figure 11-2atthebeginningofthischapter.Thistoolisusedtoflare the end of the lines so that the flared end matches exactly with the fittings to provide a leak-free connec-tion. Photo Sequence 8 shows how to create a double flare.

Once the new line is ready to install, carefully begin to thread the fitting into the connection. Start the fitting by hand, and continue to tighten it until the fitting is seated. You should not have to use a wrench to start the fitting. If it is difficult to thread into the connection, remove the fitting and inspect the threads for dirt or damage. Be carefulnottoovertightenthefitting;thiscancracktheflare or strip the threads, resulting in a leak.

When replacingbrakehoses,youneed to removethe hose from its bracket where it connects to the steel line. Hoses are usually held in place with a clip, shown in Figure 11-32.Usealinewrenchwhenyouremovethe fittings, and hold the hose with a second wrench, as shown in Figure 11-33. This helps prevent twisting and damaging the lines and hoses and reduces the chances ofstrippingthefittings.Comparethenewandoldhosesbefore you install the new ones. Make sure the new hose matches the old hose at the fittings and brackets. Most front brake hoses are specific to the left and right sides of the vehicle, so make sure you have the correct parts

© C

enga

ge L

earn

ing

2014

fiGure 11-31 When replacing a brake line, remove the clamps that hold the line to the frame or body. Reinstall these clamps with the new line.

© C

enga

ge L

earn

ing

2014

fiGure 11-32 Brake hoses are often bolted and clipped to the body, strut, or other components to prevent contact with moving parts, such as the wheels and tires. Be sure to reinstall all mounting bolts and clips when installing a new hose.

Brakepipe

Brake hose

Flare-nutwrench

Tocaliper ©

Cen

gage

Lea

rnin

g 20

14

fiGure 11-33 Use a line wrench and a second wrench to steady the line or hose when removing. This helps prevent damage to the fittings and line.

39634_ch11_rev02.indd 302 31/01/13 12:03 PM



© C

enga

ge L

earn

ing

2014

PS8-1 Determine the diameter of the brake line to be flared and have all fittings and lines ready.

© C

enga

ge L

earn

ing

2014

PS8-2 Measure the length of line needed.

© C

enga

ge L

earn

ing

2014

PS8-3 Using a tubing cutter, cut the line to length. Do not use a hacksaw or cutting pliers as these will damage the line.

© C

enga

ge L

earn

ing

2014

PS8-4 Deburr the end of the line and ensure that the opening is round and not damaged by the tubing cutter. You may need to ream the opening slightly to remove any metal that may obstruct the opening. Most tubing cut-ters have a reaming tool built into the cutter.

© C

enga

ge L

earn

ing

2014

PS8-5 Install the fitting onto the line and ensure that it is going in the right direction.

© C

enga

ge L

earn

ing

2014

PS8-6 Place the line into the clamping fixture. Check the directions for the flar-ing tool to determine how much line to extend out of the fixture.

©Ce

ngag

eLe

arni

ng20

14

PS8-7 Once properly positioned, tighten the clamps to secure the line in the fixture. Do not overtighten the clamps as this will break the pins in the clamp. However, failure to tighten the clamps enough will allow the line to push through the clamp while flaring the end.

© C

enga

ge L

earn

ing

2014

PS8-8 Install the adapter for the size of line being flared.

© C

enga

ge L

earn

ing

2014

PS8-9 Install the anvil over the clamping fixture and against the adapter.

© C

enga

ge L

earn

ing

2014

PS8-10 Tighten the anvil until the adapter is flush with the clamping fixture. Remove the anvil and adapter.

© C

enga

ge L

earn

ing

2014

PS8-11 Place the anvil into the opening of the brake line and tighten to roll the flared metal into the opening.

© C

enga

ge L

earn

ing

2014

PS8-12 Remove the anvil and inspect the flare.

PHOTO SEQUENCE 8FLARING A BRAKE LINE




4. Leavethebleederopenuntiltheflowoffluidstops,then close the bleeder screw.

5. Have your assistant pump the pedal several times until the pedal feel returns to normal.

6. Open the bleeder again. Once the fluid flow stops, close thebleeder.Repeat thisprocessuntilallairbubbles are gone and only clean fluid flows from the bleeder.

7. Checkthefluidlevelatthemastercylinder.Donotletthe fluid level get too low or air may be pulled into the system. If this occurs you will need to repeat bleeding the system to remove the air.

8. Repeatateachwheel,movingfromtherightreartoleft rear, then right front and finally the left front. If the vehicle has a diagonally split system, the bleeding sequenceisusuallyRR,LF,LR,RF.Whencomplete,all air should be removed from the system and only new, clean fluid is present.

Anotherwaytobleedandflushthebrakesissimi-lar to the previous procedure, but instead of opening and closing the bleeder only to let fluid out, the bottle used to catch the fluid is used to also hold a reservoir of fluid.

1. Removeasmuchoftheoldfluidfromthemastercyl-inder reservoir as possible using a syringe or suction tool. Fill the master cylinder reservoir with new fluid. Reinstallthereservoircap(s).

2. Install a hose over the bleeder, and place the other end of the hose into the bottle. Fill the bottle half full with clean new fluid. Open the bleeder screw at the right rear wheel.

3. Pumpthebrakepedalrepeatedlyuntilthefluidenter-ing the bottle no longer has any air bubbles.

HydrAuliC SySTem fluSHinG And BleedinGFlushing the brake system removes all of the old fluid and replaces it with new. This is done as a mainte-nance service to prolong the life of the hydraulic brake components. Bleeding the brakes removes air trapped in the system, whether from a leak or from repairs. If the system is opened for bleeding, many technicians perform a flush at the same time.

Reasons to Flush the Brake System. Asdis-cussed in Chapter 10, brake fluid is hygroscopic,meaning it readily absorbs moisture. Because of this property, brake fluid should be flushed periodically to prevent rust and corrosion buildup within the system. Carandtruckmanufacturersoftenrecommendthebrakesystem be flushed every two years and whenever brake servicesareperformed.Rustorcorrosionofthehydrau-liccomponentscancausetheABSsystemtomalfunc-tion, cause the pistons to bind or seize in calipers and wheel cylinders, and increase the rate of deterioration of metalcomponents.Additionally,sincewaterhasamuchlower boiling point than brake fluid, moisture trapped in the system can cause brake fade or a loss of braking if the fluid boils.

Bleeding the Brake System. Bleeding the brakes removes air from the system and removes old fluid. Bleeding and flushing are really two operations but are often performed as one since both remove old fluid and any air trapped in the system. There are several meth-ods for bleeding the brake system, including manual and pressurized procedures.

Manual bleeding usually requires two people, one to pump the brakes, and the other to open and close the bleeder screws. Many vehicles have specific bleeding procedures that need to be followed, so refer to the cor-rect service information before bleeding the brake sys-tem. Some vehicles can be bled by using the following general procedure:

1. Removeasmuchoftheoldfluidfromthemastercyl-inder reservoir as possible using a syringe or suction tool. Fill the master cylinder with new brake fluid. Reinstallthereservoircap(s).

2. Start at the right rear wheel and place a hose over the bleeder screw into a bottle or other container to catch the old fluid, as shown in Figure 11-34.

3. Have your assistant pump the brake pedal several times and hold the pedal down. Open the bleeder usingabox-endwrench.Whenthebleederisopened,the pedal will drop. Do not allow the pedal to go com-pletely to the floor.

© C

enga

ge L

earn

ing

2014

Backingplate

Glass jar

BleederscrewBleeder

hose

fiGure 11-34 This illustrates manual bleeding to remove air from the hydraulic system. Using a container of clean brake fluid allows one person to bleed the brakes.

39634_ch11_rev02.indd 304 31/01/13 12:03 PM




1. Removeasmuchoftheoldfluidfromthemastercyl-inder reservoir as possible using a syringe or suction tool.Fillthemastercylinderwithnewfluid.Leavethereservoir cap loose.

2. Open the right rear bleeder screw, and install the hose of the vacuum bleeder over the bleeder screw.

3. Attachashopairhosetothebleederairfittingandpullthetrigger.Astheairflowsthroughthevacuumbleeder, brake fluid is pulled from the system and is collected in the tool’s reservoir.

4. Closethebleederandrefill themastercylinderasneeded. Then proceed to the left rear wheel and repeat.Bleedallfourwheelbrakes.Whenbleedingiscomplete, only new fluid should remain in the system and all air will be removed.

Anothermethodofbleedingandflushingusesavac-uumevacuationtool,liketheoneshowninFigure11-3at the beginning of the chapter. This tool is used to pull the fluid through the brake system to remove air and to flushouttheoldfluid.Anillustrationofhowvacuum bleeding works is shown in Figure 11-35.

Shop airsource

Airflow

Leave cap off.Atmospheric pressure helps

push fluid out.

Airflow Airflow

Airflow creates low pressure at

this point.

Interior underlow pressure

Fluidflow

© C

enga

ge L

earn

ing

2014

fiGure 11-35 A vacuum bleeder uses shop air to pull a vacuum at the bleeder valve. This tool allows the brakes to be bled by one person.

39634_ch11_rev02.indd 305 31/01/13 12:03 PM




Anotherwaytoflushandbleedthebrakesystemisbyusing a pressure bleedingtool.Pressurebleeding,asthename implies, uses pressure to force new fluid through thebrakesystem.Aspecialadapterisinstalledoverthemaster cylinder reservoir, shown in Figure 11-37. Many different adapters are available to fit various master cyl-inder arrangements. The pressure tank is then filled with brake fluid, and an air hose is connected to the tank, as shown in Figure 11-38.Whenableederscrewisopened,the pressure on the system forces air and fluid out. This method requires only one person to bleed the brake sys-tem, so it is convenient in situations where a helper is not available.

4. Continuetobleeduntilallairisremovedandcleanbrakefluidisflowingfromthebleeder.Remembertocheck the fluid level in the master cylinder often to prevent air from being pulled into the system.

5. Whencomplete,closethebleederscrewandmovetothe left rear wheel.

Whenusingavacuumbleeder,youwill likelyseesmall air bubbles or foam being pulled into the bleeder even after all of the air has been removed from the hydraulic system. This is because the vacuum pulled through the bleeder valve can pull air from around the threads of the valve, causing the fluid to foam. This is shown in Figure 11-36. Once you are finished with the vacuum bleeder, manually bleed each wheel once to ensure that no air remains trapped in the system.

Fluid fromwheel circuit

Air drawnpast threads

Vacuum drawsfoamy fluidinto bleedcanister.

Bleed screw

Fluid mixedwith air

© C

enga

ge L

earn

ing

2014

fiGure 11-36 Very small bubbles often develop around the bleeder screw when using a vacuum bleeder. ©

Cen

gage

Lea

rnin

g 20

14

fiGure 11-37 An example of a pressure bleeder connec-tion at the master cylinder. Many different types of adapters are available to fit different types of caps.

Adapter

Rear brake

Bleeder hose

Glass jarPressure bleeder

tank

Master cylinderValve

© C

enga

ge L

earn

ing

2014

fiGure 11-38 This illustrates how a pressure bleeder is used to bleed the hydraulic system.

39634_ch11_rev02.indd 306 31/01/13 12:03 PM




Bleeding ABS Systems. The procedures listed previouslyapplytomostnon-ABSvehiclesandmayalsobeusedonmanyABS-equippedcarsandtrucks.BeforeattemptingtobleedthebrakesystemonanABS-equipped vehicle, refer to the manufacturer’s service pro-cedures.SomeABSsystems,especiallythosethathaveintegralABSsystems,requirespecialbleedingproce-dures that must be followed to prevent vehicle damage and even personal injury.

Some vehicles require the use of a scan tool con-nected to the on-board computer system to activate a specific hydraulic bleeding mode before the system can be bled. Figure 11-39 shows one example of this scan tool procedure. Once the system bleed is started, fol-low the onscreen commands to bleed the air from the system.

© C

enga

ge L

earn

ing

2014

fiGure 11-39 Some vehicles require using a scan tool to activate the ABS to bleed the hydraulic system.

SuMMARY

If the brake pedal height is incorrect, check the rear brake adjustment.

Inspect adjustable pedals by activating the system, and check that the assembly can move forward and backward along the entire range of motion.

Many vehicle manufacturers recommend periodic flushing of the brake fluid to remove moisture and other contaminants.

Cleanaroundthereservoircaptopreventanydirtfromgetting into the brake fluid when the cap is removed.

Oncethemoisture levelreachesabout3percent, theboiling point of the brake fluid is reduced more than 100 degrees.

Brakefluidcanabsorbcopperfromthebrakelines.Con-taminated fluid should be flushed out and replaced.

Leakswillresultinalossofhydraulicpressureandbrak-ing ability, a very low, soft brake pedal, a spongy brake pedal, and fluid loss.

Amastercylindercanleakeitherexternallyorinternally.

Bench bleeding removes air, prefills the master cylinder, and reduces the amount of bleeding that needs to be done on the vehicle.

Bleeding the brakes removes air from the system and removes the old fluid.

Some vehicles will require the use of a scan tool to acti-vate a specific hydraulic bleeding mode.

REVIEW quESTIonS

1. The is also used as an input for engine,

transmission, and cruise control operation.

2. Since brake fluid is , it easily absorbs moisture.

3. the hydraulic system removes any trapped air.

4. Many manufacturers recommend periodic of the brake fluid to remove

moisture and contaminants.

5. Some vehicles require the use of a to bleed the brake system.

6. TechnicianAsayslowbrakepedalheightcanbecaused by improper rear drum brake adjustment. Technician B says a low, sinking brake pedal can becausedbywornbrakeshoesandpads.Whoiscorrect?

a. TechnicianA c.BothAandB

b. Technician B d.NeitherAnorB

39634_ch11_rev02.indd 307 31/01/13 12:04 PM



7. Allofthefollowingarebrakebleedingmethodsexcept:

a. Vacuum c. Manual

b. Pressure d.Reverse

8. TheredBRAKEwarninglightisilluminatedonalow-mileage, late-model vehicle with four-wheel discbrakes.TechnicianAsaystheremaybealeakin the hydraulic system. Technician B says low fluid level in the master cylinder reservoir may be thecause.Whoiscorrect?



9. TechnicianAsayspressurebleedingrequiresusinga special tool connected to the shop compressed air system.TechnicianBsaysbleedinganABSsystemmayrequireascantool.Whoiscorrect?



10. Allofthefollowingarecommonhydraulicsystemconcerns except:

a. Restrictedbrakehose

b. Brake line rust through

c. Seized master cylinder pistons

d. Master cylinder bypassing


39634_ch11_rev02.indd 308 31/01/13 12:04 PM



Brake System Service - Denton ISD

Documents

Transcript of Brake System Service - Denton ISD