90463746 Automotive Braking Systems

8/2/2019 90463746 Automotive Braking Systems

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Study Unit

Automotive Braking

Systems


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Of all of the systems on a modern vehicle, the braking system is one of the most important. Thissystem is responsible for taking a very heavy vehicle thats traveling at a high rate of speed to a

complete stop in a very short distance. The lives of the occupants are dependent upon the preciseoperation of this system. Your task, as an automotive technician, is to make sure this system isworking properly now and to ensure that it will continue to work properly in the future. This taskcan be done by thoroughly understanding the operation of the braking systems and by properlyperforming all brake system maintenance.

This study unit will show you the major components of most braking systems and the proper

techniques to use to service these systems. Remember that no text can be a complete substitutefor the vehicle manufacturers service manual. Whenever performing brake system maintenance,you must consult these manuals for specific information, such as fluid types, required tools, torqueratings, and special procedures.

When you complete this study unit, youll be able to

Identify the tools used for brake system repair

Explain how to safely work on a vehicles brake system

Describe how friction is used to slow or stop a vehicle

Describe how the hydraulic system functions and how power boosters operate

Explain the construction and repair of master cylinders

Identify the components of a drum brake system and a disc brake system and explain howthese systems operate and are repaired

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Preview


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AUTOMOTIVEBRAKESYSTEM TOOLSAND SAFETY . . . . . . . . . . . . . 1

ToolsSafety

FRICTION AND HYDRAULIC PRINCIPLESIN BRAKING SYSTEMS . . . . . 12

Typesof Fric tionBasic Fric tion RatingsFric tion Causes Hea tHydraulic Principlesof Braking SystemsBrake Fluid TypesRating Systems for Brake Fluids

MASTERCYLINDERSAND POWERBOOSTERS . . . . . . . . . . . . . . . 26

Master Cylinder Design

Troub leshooting Ma ster Cylinder ProblemsReb uild ing Master CylindersPow er BoostersPower Booster Chec k ValveOther Power Booster SystemsTesting and Repairing Power Booster Systems

Rep lac ing a Power Booster

DISC BRAKESYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

RotorsAdvantages of RotorsOver DrumsCalipers

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Contents


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BRAKESYSTEM VALVEAND BRAKELINES . . . . . . . . . . . . . . . . . 86

Residual Pressure ValvesMetering ValvesProportioning ValvesComb ination ValvesPressure Differentia l Switc hesTroubleshooting Va lve Prob lems

Brake LinesBrake HoseBleed ing the Brakes

SELF-CHECK ANSWERS. . . . . . . . . . . . . . . . . . . . . . . . . . 105

EXAMINATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

vi Contents


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AUTOMOTIVE BRAKE SYSTEM TOOLS AND SAFETY

Tools

As with any other area of vehicle repair, working on brake systems re-quires the use of many different types of tools. Some of these tools arethe standard screwdrivers, pliers, hammers, wrenches, and other such

tools with which youre already familiar. Other tools, however, are oftenused exclusively for vehicle brake system repair and are of little, if any,use on other systems of a vehicle. Its these tools that well discuss here.

Wrenches

There are many types of wrenches that a mechanic uses every day. Themost common wrenches are the open-end, box-end, and combinationwrenches. Youll use these wrenches to remove or install the mastercylinder or power booster bolts, wheel cylinder or caliper mounting

bolts, and control valve mounting bolts. These wrenches, along with6- and 12-point sockets and ratchets, make up the most common handtools used by mechanics.

A different type of wrench is needed for the various tubing fittingsused to transfer brake fluid throughout the system. These wrenchesare called line, tubing, or flare-end wrenches. To avoid confusion as weprogress through this study unit, well call them flare-end wrenches.Typical flare-end wrenches are shown in Figure 1.

1

Automotive Braking Systems


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Another type of wrench youll often use is a brake bleeder wrench, which

is shown in Figure 2. This wrench is a box-end wrench that fits thebleeder screws located on the wheel cylinders and calipers. Thiswrench is used to open the screws so that brake fluid can be pumpedor pulled through the system to flush the system or to remove trappedair. There are two different-size box ends on this wrench.

2 Automotive Braking Systems

FIGURE 1Shown here are two different types of flare-end wrenches. This type of wrench must be usedwhen youre installing or removing brake line fittings.


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In many cases, youll be using a socket or an Allen wrench to removethe bolt that holds the caliper to the spindle of the vehicle. However,

you may also need to use special Torx sockets to remove this bolt.These sockets are shown in Figure 3. Theyre mounted on a standard3

8 drive socket wrench or ratchet. Dont use an Allen wrench to re-move or install this type of retainer. You can easily round out the in-side surfaces, causing the retainer to be destroyed. An Allen wrenchmay feel like it fits perfectly but will reach a point where it slips free

and causes damage.

Measuring Tools

Automotive Braking Systems 3

FIGURE 3Torx-type bolt heads are often used by ma nufacturers to hold the c aliper on the vehicle.

4 A i B ki S


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In some cases, especially where the vehicle owner is complaining ofpulsing brakes, youll need to measure the runout, or the amount therotor surface varies from its normal plane of motion. A dial indicator,

such as the one shown in Figure 5, can be used for this purpose. Thedial indicator is solidly mounted to a bar or other attachment, and itstip is set gently on the rotors braking surface. The rotor is turnedthrough one or more complete revolutions. The amount of change inthe dial reading indicates the runout of the rotor. If the amount ex-ceeds a specified value, the rotor must be changed. Often this value is

as small as 0.002 inch. Most dial indicators are capable of accuratelymeasuring to 0.001 inch.

Another measuring tool thats often used is a large inside micrometer.This tool can be used to check the inside of a drum to see if its warpedor if it exceeds its wear limit.


FIGURE 4A micrometer is often used for measuring the overall thickness of a brake rotor.

A t ti B ki S t 5


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FIGURE5A dial indicatorisoften used to check the

runout of a rotor in a discbrake system.

FIGURE6Thistool isabrake spoon. Itsprimar-ily used to adjust backdrum brakes



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Its often difficult to remove the brake rotor or drum from a vehicle. Adead-blow hammer, such as the one shown in Figure 7, can be used to

remove stuck brake drums or rotors. The dead-blow hammer containslead shot that adds a second striking force to your blow with the hammer.In addition, the nonmarking surface wont damage the surface of thedrum or rotor. Dead-blow hammers come in various weights from thoserated in ounces to those that weigh several pounds. The one shown in thisillustration is 28 ounces and is good for light-duty work. A larger dead-

blow hammer is needed for stubborn brake drums and heavy-duty tasks.

A special tool for removing and installing drum brake springs is shownin Figure 8. The end of the tool with the small round barrel is placedover the center post that holds the brake shoe return springs and thenrotated to lift the springs off the post. The opposite end of the tool isused to apply the springs after the brake shoes have been replaced.


FIGURE 7A dead-blow ha mmer is very useful for removing stuck brake drums.



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FIGURE 8A brake spring tool, like the one shown here, is used to remove and install the springs that holdthe brake shoes against the wheel cylinder in drum brake systems.

FIGURE 9A caliper com-pression tool is used forpushing a piston back

into a ca liper when youreinstalling new brake pads.When using a C-clampinstead of this tool, alwaysremember to protect thepiston with a block ofwood.



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Liftsand Lift Height

When youre working on a vehicles brakes, you dont have to lift thevehicle to extreme heights. If you have a lift at the shop where youreworking, you should have the center of the vehicles front or rear axleat about chest height. However, if youre using jack stands, you shouldonly lift the vehicle high enough to remove and replace the tire. Thevehicle shouldnt be lifted higher than necessary. When using jacks

and jack stands, the vehicle becomes increasingly unstable as you lift ithigher.

Brake Fluid Pressures

You must also be aware of the pressures that are involved in the braking

system. In non-antilock braking systems, brake fluid pressures arereleased when the pedal is released. However, antilock braking systemsoften store high pressures within the system. These high fluid pressuresmust be released before you attempt to disconnect any line fittings.

Burns

Another area to be cautious of is heat. If a vehicle is brought to youwith the complaint that the brakes are dragging, the rear axle is mak-ing noise, or that the car or truck is pulling to one side, the vehicle mayhave a sticking caliper or a broken drum brake return spring. Thedrum or rotor can be extremely hot. Touching any brake componentcan cause a serious burn. To avoid burns, let the system cool by itself

or apply a light stream of compressed air while slowly rotating thewheel before working on the vehicle. Make sure to rotate the wheel sothat the rotor or drum cools evenly to prevent warping.

Brake Dust and Asbestos




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One of the dangerous materials in some friction linings is asbestos. Formany years asbestos was used in brake linings for reinforcement. As the

friction lining wears, this dust can collect on the brake system compo-nents. Breathing in this dust can cause a disease known as asbestosis. Thedisease may not show itself for 20 years or more. With asbestosis, the as-

bestos lodges in the lungs and causes inflammation of the lung tis-sues. When this inflammation heals, the resultant scar tissue thickensthe lung walls. This thickening causes less oxygen to reach the body,

resulting in a wide array of problems from shortness of breath to lungcancer to death.

In todays large brake shops, certain procedures are followed when-ever a wheel is removed from a vehicle. One such procedure is to use asolvent to wash down the brake areas to collect the brake dust into atank. The tank has a pump that recirculates the fluid, allowing many

such cleanings to occur before the liquid must be changed. This wash-down system is shown in Figure 10.


FIGURE10A wash-down system can beused to rid a brake system of dust that canbe hazardousto your health. (Photographc ourtesy of Clayto n Assoc iates, Inc ., Lakew oo d, NJ,USA, http://www.jclayton.com)
http://www.jclayton.com%29/http://www.jclayton.com%29/http://www.jclayton.com%29/


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When youre disposing of brake dust, such as sweeping the floor afterworking on a vehicles brakes, or removing a vacuum cleaner liner or

filter, the safest method is to lightly mist the item or area with water.The water will keep the dust from floating back into the air.

When the dust is collected it can be placed in a plastic bag and disposedof in the garbage. You should make sure to mark the bag so that some-one else wont open it and cause the dust to be released. Collected

asbestos and brake dust isnt normally considered hazardous wasteunless its released to the air. You should always dispose of thesematerials according to your shops rules and regulations and to anyapplicable state or local regulations.

The air quality in a shop that does brake work is rated according to theparticles of asbestos in the air. These asbestos particles are those that are

longer than 0.0002 inches, as defined by the Occupational Safety and HealthAdministration, or OSHA. If there are fewer than 0.1 fibers of asbestos percubic centimeter of shop air per an eight-hour period, the air quality isacceptable. A shop can maintain these safe levels only by performing

brake repairs using a wash-down or vacuum and dust-collection sys-tem. If there are more than 0.1 particles per cubic centimeter but less than

0.2 particles, the shop is required to monitor employee health and train itsemployees about asbestos and its potentially harmful effects on thehuman body. At levels above 0.2 particles per cubic centimeter, theexposure is at an unacceptable level and measures must be taken toreduce the employees exposure to the fibers.

OSHA requires brake shops to clean the brake system by washing or

using a vacuum system with a HEPA filter before repairing a vehicle.You should also take care when sweeping or cleaning an area where

brake repair has occurred. Instead of sweeping the floor, the floorshould be washed or vacuumed. In addition, if brake shoes must becut to conform to an oversized drum, you should make sure the cut-ting or grinding machine also contains a vacuum system with a HEPA

g y



16/109FRICTION AND HYDRAULIC PRINCIPLES IN BRAKING

Self-Check 1

At the end of each section of Automotive Braking Systems, youll be asked to pause and

check your understanding of what youve just read by completing a Self-Check exer-cise. Writing the answers to these questions will help you to review what youve studiedso far. Please complete Self-Check 1 now.

Fill in the blanks in each of the following statements.

1. A flare-end wrench may also be called a(n) _______ wrench or a(n) _______ wrench.

2. A(n) _______ is used to adjust the star wheel on the rear drum brakes of an older vehicle.

3. A(n) _______ is a common tool used to retract caliper pistons when a specialized tool isntavailable.

4. A(n) _______ measures pad and brake material thickness and is more accurate than a steelrule.

5. The amount the rotor moves away from its normal plane of motion as its rotated is knownas _______ and is measured with a _______.

6. Excessive exposure to _______ found in brake dust can lead to health risks, such as short-ness of breath, lung cancer, and death.

Check your answers with those on page 105.



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Types of Friction

There are basically two different types of friction. These types arestatic and dynamic. Static friction deals with bodies at rest and force isapplied to one or both of the bodies to cause motion between the

bodies. In this case, the bodies have been stationary for some periodof time, and their surfaces have merged. Rough surfaces may havemoved slightly so the peaks in one surface match with the valleys of

the opposite surface. Even very smooth surfaces will experience thissame merging when the peaks and valleys of the surfaces match eachother.

Static friction is a fairly strong force. The energy thats required tomove the objects against static friction is very largemuch larger,in fact, than the forces needed to keep something in motion once ithas begun to move. A vehicles parking brake prevents the stoppedvehicle from rolling downhill, for instance, because of staticfriction.

Dynamic friction is the friction that occurs between moving surfaces.Dynamic friction is much less powerful than static friction. Its dy-

namic friction, however, thats most often involved in vehicle brakingsystems. Often termed kinetic friction, dynamic friction slows or stopsthe vehicle. Static friction, on the other hand, is more often produced

by vehicle parking brakes.

The purpose of a vehicles braking system is to reduce the speed of aheavy car or truck. The braking system performs this function using

frictional force. It takes the energy of the moving vehicle and convertsthat energy to heat.

The amount of friction applied to a brake drum or rotor depends upontwo factors. The first is how much pressure is applied by the system.A i th f i ti l i Th d f t i



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There are many problems with band brakes. First, the amount of fric-tion applied to the drum on the drive shaft is limited. This problem isdue to the small amount of friction material on the band and the lim-ited drum surface area involved in the braking system. Also, the forcerequired on the brake pedal is very large while the pressure of the

FIGURE12One of the old-est brake systems is theband brake system. Thissystem squeezes a brakeband around a rotating

drum that waspart of thetransmission or drive shaft.



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FIGURE 13Here are twonew brake padsremovedfrom and installed in avehicle. You can seethat this is a quality frictionmaterial by itseven blendof particles.



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The following table lists coefficients of friction. Youll see the materialcode letter in the left-hand column and the coefficient of friction in the

right side of the table.

You might think that it would be best to use the highest-rated friction

material possible for all vehicles. While this might be great for brakingcapability, theres always a downside. If you used the highest-ratedfriction material, the rotor or drum would quickly wear down. High-friction pads and shoes tend to rapidly wear out rotors and drums,whereas lower friction linings reduce the amount of wear. A vehicleshould be equipped with the proper level of friction lining, one that

wont prematurely wear down the other parts.

Friction Causes Heat

The energy of motion that the friction material absorbs must be con-

TABLE1Code Lettersand Friction Values

Code Letter Coefficient of Friction

C 0.00 - 0.15

D 0.15 - 0.25

E 0.25 - 0.35

F 0.35 - 0.45

G 0.45 - 0.55

H 0.55 and greater



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you can see heat waves and often smoke coming from the front brakeassemblies. The friction linings used on typical vehicles dont come

close to producing this amount of friction. The braking systems on racecars are only expected to endure five to six hundred miles, while typi-cal vehicles are expected to endure for many thousands of miles, re-quiring long-lasting linings, rotors, and drums in good workingcondition.

Our passenger cars and trucks dont heat up the rotors and drums to ahigh degree under normal operating conditions. However, there aretimes when our vehicles do generate a lot of heat. For instance, if youhave a heavy vehicle going down a long mountain pass, your vehiclecan generate significant heat in your rotors, calipers, and drum brakeassemblies. On long downhill stretches, there are often areas whereyou must pull over and have your brakes checked. If the brakes arent

fully operational, youll be required to let them cool until sufficientbraking friction returns. Excessive heat reduces friction, which canlead to many other problems. The most common problem with heat iscalled brake fade. Brake fade occurs when the surface of the drum or ro-tor is so hot that friction is greatly reduced. You can press harder andharder on the brake pedal, but you end up with less and less friction.

Letting the brakes cool usually helps this situation.

Hydraulic Principles of Braking Systems

The one problem with the band brake system that we briefly discussedis that the vehicle operator couldnt apply very much pressure to the

band that surrounded the drum. Luckily, the vehicles of the past didnttravel at high speeds or frequently travel down long mountain passes.

In order to increase the pressure applied to the friction linings andto send the pressure to all four wheels of the vehicle without the use

f li k th h d li b ki t d l d Thi 18 Automotive Braking Systems


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The plunger in this illustration is sealed so that no water can leak pastit. At this time, a certain pressure exists equally across all surfaces ofthe can. This equal pressure is shown as the series of arrows placed inall directions around the can. What weve done creates pressure in thewater. Water, unlike air, cant be compressed. Therefore, the volume ofwater will remain constant, while the pressure applied by the water

will increase as the amount of weight is increased.

Now, lets add a hose with its end plugged to the bottom of the can.This experiment is shown in Figure 15. Now, will the pressure in thehose be the same as the pressure in the can? Yes! The pressure will bethe same since the fluid will evenly transmit pressure to all of the sur-

FIGURE 14If a weight isplaced on the plunger inthisillustration, the fluid

will come under thepressure of that weight.The fluid will then pressevenly on all surfacesthat it touches.



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FIGURE 15The fluid will provide a n eve n pressure on all surfaces it encounters, even the inside walls of ahose connected to the can as shown here.



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cans added to the system, each with their own plungers but noweights, you would then represent the braking system for each wheel.

The Pressure Equation

In our previous example, we created a very low pressure in the system.The total pressure would be equal to the amount of weight divided bythe area of the piston.

To create friction with a brake pad or shoe, we must apply the pador shoe against a drum or rotor under a large amount of pressure. Ahydraulic system can be used to increase the pressure of a personsfoot upon a brake pedal many times over to increase the pressure ofthe lining on the drum or rotor. To understand how this action occurs,we must look at the pressure formula.

To calculate pressure, use the following equation:

PF

A

where:

P = pressure

F= force

A = area

Using our previous example, if we have a weight of 10 pounds and thepiston area is 10 square inches, then the resultant pressure is equal to1010

or one pound per square inch, or one psi.

Our braking systems require much more than this pressure to operate



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In this system, we have a force of 100 pounds acting on a piston with

an area of one square inch. This action will cause a pressure of 100

1

or

100 psi to be present on the fluid. In normal operation with a powerbooster ahead of the master cylinder, the pressure on the fluid inside amaster cylinder can be much higher.

How does this action help us in a braking system? Lets lookat Figure 18.In this figure, we have connected a pressure-creating cylinder, a mas-

ter cylinder, to a single caliper piston. If we have created 100 psi in themaster cylinder and pressurized the fluid in the entire system, whatwill happen at the two square inch caliper piston?

We can look at the force that will be provided by the hydraulic systemby modifying the pressure formula as follows:

d fF

FIGURE 18In this example, a one-inch piston is applying fluid pressure to a brake system that conta ins atwo-inch piston.



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But, previously we mentioned the Law of the Conservation of Energy.How can an input force of 100 pounds result in an output force of 200pounds? The answer lies in how far the pistons travel. Refer to Figure 19.In this figure, the master cylinder piston travels a total distance of oneinch, while the brake cylinder travels a total distance of1 2 inch. The pres-sure doubled, but the distance the second piston traveled is divided bytwo. All variables are, therefore, equal, and the Law of the Conservation

of Energy holds true.

In a braking system, the distance the master cylinder piston(s) travels israther small since the disc brake pads and the rear drum shoes are locatedvery close to the areas on the rotor or drum where they contact and createfriction. If you have ever driven a vehicle in which the rear shoes werent

adjusted properly and had a large gap between them and the drum, youwould have noticed a large amount of travel in the brake pedal. Thistravel of the brake pedal was necessary to make up for the large amountof travel needed to have the shoes contact the drum. Of course, a goodbrake technician could repair this situation.

C A L I P E R

O N E I N C H

O N E H A L F I N C H

FIGURE19Since we ca nt create energy, we must lose something. This illustration showshow we lose motion.



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3. Hydraulic system mineral oilcolored green

Of these three materials, polyglycol is by far the most commonly usedfluid in todays brake systems. The fluids formal name is actuallypolyglycol-alkylene-glycol-ether. This fluid has been used for manyyears in all types of vehicles. The advantages of polyglycol are that itscompatible with most metals; will slightly swell rubber seals, makingfor a leak-free system; and has a very high boiling point, yet it flowswell in cold temperatures. The main problem with polyglycol is that

it easily attracts and absorbs water. Youd say that polyglycol is veryhygroscopic, meaning that it easily attracts and absorbs water. Thisproblem with brake fluid means that over time the brake fluid canabsorb enough water to cause the water to aid in the corrosion of theinternal pistons, lines, and other metallic components of the brake sys-tem. You must drain and replace the brake fluid on a scheduled basis

to maintain proper brake operation and to limit the repairs that arenecessary on the system. Changing the fluid is known as bleeding thebrakes. The old fluid that leaves the system often has the appearanceof a cola-type soda.

One other problem with polyglycol-based brake fluid is that it can turna painted surface into a damaged surface in a few seconds. The brake

fluid can dissolve the paint and leave it blistered on the surface of thevehicle. Always be careful when draining a master cylinder, changingcomponents, or bleeding the system. Also, you must always use poly-glycol fluid from a tightly sealed container. A loose cap could meanthat the fluid has already absorbed a large quantity of water. When indoubt, use a new container of this type of brake fluid, and try to avoid

buying very large containers unless you perform a large amount ofbrake work.

The second type of brake fluid is silicone, which is an inert brake fluidthats much more expensive than standard polyglycol fluids. Its non-hygroscopic, meaning that it wont readily absorb water. This quality



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fluid. HSMO has a very high boiling point, is an excellent lubricator forthe moving parts of the system, and is nonhygroscopic.

Rating Systems for Brake Fluids

Both the Society of Automotive Engineers, or SAE, and the Depart-ment of Transportation, or DOT, have rating systems for brake fluids.These ratings take into account the boiling points of the brake fluids,when dry and after the fluids have been exposed to moisture. Table 2displays the values used for the DOT-based rating system. This systemis a very popular system of rating brake fluids. In most cases, youll bepurchasing or installing brake fluid thats specified by the DOT ratingsystem.

ERBP stands for Equilibrium Reflux Boiling Point. The dry rating is the

minimum value of temperature at which the brake fluid will begin toboil. The wet values reflect the brake fluid that has absorbed water.Water has a much lower boiling point than brake fluid. Therefore, ifthe brake fluid has absorbed water, the water will lower the boilingpoint of the brake fluid.

TABLE2DOTFluid Temperature Ratings

Fluid DOTRating ERBPDry ERBPWet

DOT3 401F 205C 285F 140C

DOT4 446F 230C 310F 155C

DOT5 500F 260C 356F 180C


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MASTER CYLINDERS AND POWER BOOSTERS

Master Cylinder Design

If the braking system of a vehicle could be compared to the humanbody, the brake systems at the wheels would be the bodys musclesand the master cylinderwould be the heart. Just as the heart pumps

blood to the muscles, the master cylinder is responsible for pumpingfluid to the brake system to make it perform work. Since the mastercylinder is such an important part of the braking system, well coverthis part first in our study of braking systems.

As we progress through the following sections, remember that weresimply discussing pressurized fluid flow. The master cylinder is re-

sponsible for pressurizing fluid and sending it out to the working de-vices at the wheels. There are various devices involved in controllingthis flow to the working devices at the wheels. Some of these controldevices are located in or around the master cylinder; these devices willbe covered in this section. Other control devices located on the fire-wall, or the frame, or at the rear axle, will be covered later in this study

unit.

A typical master cylinder is shown in Figure 20. This master cylinder ismounted to a vacuum power booster. The power booster is used to allowa small force on the brake pedal to create a larger force on the pistonsinside the master cylinder. This force, in turn, relates to a larger brakingforce at the wheels for little effort on the brake pedal. While looking

at the master cylinder and power booster, notice that theres a singlevacuum hose that feeds the power booster. Also, notice there are linesleading out of the master cylinder. These lines are the pressurized linesthat go to the wheel cylinders and calipers.

The earliest master cylinders had just one chamber as shown in Figure 21 Automotive Braking Systems 27


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Above the piston is a reservoir thats full of unpressurized brake fluid.A small hole between the reservoir allows fluid to pass from the reser-voir into the tube or cylinder in front of the piston where it can bepressurized by the piston. This flow of fluid into the tube or cylinderis only allowed when the piston is returned to the back of the cylinder

FIGURE 20This is an example of one type of master cylinder.



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clearance at the pad or shoe is small, the push rod and piston travelwill also be small. If the clearance is large, the rod and piston travel

will also be large. Having a large clearance and piston travel resultsin a long pedal travel in the vehicle. Long pedal travel isnt a good con-dition, and the brake system should be repaired to bring pedal travelback to specifications.

The primary seal is responsible for the major work of the master cylinder.The primary seal is also referred to as a cup seal, although some newerd i O i A l l k h i f h i

BRAKE FLUID

RESERVOIR

CAP

VEHICLES

FIREWALL

ACTUATOR OR

PUSH ROD

PISTON

OR SPOO L

PRIMARYSEAL

SPRINGFLUIDOUTLET

SECONDARY

SEAL

FIGURE 21This is a side view of a single ma ster cylinder.



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New cup seals are included in master cylinder rebuilding kits, alongwith many other components. If the service manual requires honing, itsvery important to properly hone a cast-steel master cylinder so that thenew seal will be able to seat in the cylinder bore. If you look down the

bore of a master cylinder and you see pitting or score lines, the mastercylinder should be replaced. The newer aluminum master cylinderscant be honed since theyre normally plated or anodized to preventwearing inside of the master cylinder. Some cast-steel master cylindersalso cant be honed since their bore surface is specially manufactured

FIGURE22The sealsusedin master cylindersare nor-mally cup sealsthat expand

outward to seal when pres-surizing the brake fluid andcollapse backward to allowfree flow of fluid when thepiston travelsback to itsresting position.



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some of the fluid to go back into the reservoir. When the system cools,this same port allows fluid to flow back into the system.

If you remove the cover of the reservoir above the master cylinder and

have someone apply the brakes, you may see the fluid splashing whenthe brake pedal is first applied. These little splashes are caused by fluidbeing pushed up the compensating ports. The fluid flow will be quicklycut off as the primary seal closes off the compensating port as it passesby the port.

FIGURE 23Unlike this single-piston old-fashioned model, a modern master cylinder has two pistons and

two chambers inside the master cylinder.



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seal blowing out, or a line or fitting rupture, all hydraulic pressure

FIGURE 24In the earliest hydraulic brake systems, all of the wheel cylinders were connected together andthen connected to a single outlet at the ma ster cylinder.



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provide pressurized brake fluid to the front wheels if the system is adual-split system. Well look more into how the brake system can besplit into sections later in this study unit.

The two pistons and cylinders allow there to be two separate, yetmechanically linked, pressurization systems that feed brake fluid to therest of the braking system. If one system were to leak or rupture, youd

still have a certain, although less powerful, ability to stop the vehicle. Atthis time, a switch would be enabled to light the dashboard-mountedbrake warning light to tell the vehicle operator of the brake system failure.

Figure 26 shows the internal components of a master cylinder. You canh h h h

FIGURE 25These a re the typical parts of a dual, or tandem, master cylinder.



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FIGURE 26This master cylinder has bee n disassembled to reveal the internal components. As you can see,there are three assemblies inside the master cylinder. The retaining ring that holds the pistons and springs inthe master cylinder isnt shown.



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The main difference between a standard dual master cylinder and aquick-take-up or fast-fill master cylinder is the use of two differentbore sizes in the tube or cylinder. The first piston in the fast-fill and

FIGURE 27The two outlets of a tandem (dua l) ma ster cylinder allow a system to be divided into two differ-

ent braking systems, allowing for greater safety than a single master cylinder and braking system can offer.



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Now, lets apply the brakes as shown in Figure 28B. At this time, thequick-take-up valve at the bottom of the reservoir for the step-bore

side of the master cylinder is closed. This valve is basically just a balland a stiff calibrated spring in a typical check valve configuration. Whenthe pedal pressure is applied, the pistons will begin to travel forward.The front piston closes off the compensating port and is now pressur-izing the fluid in the rear-braking system.

The rear piston is also traveling forward. In this case, however, the vol-

ume of fluid behind the primary cup seal is also being compressed andis being forced forward by piston movement. This forward motion ofthe fluid keeps the cup seal collapsed so that its providing no fluidpressurization. A large amount of fluid is now flowing from behindthe primary cup seal and into the front brake system where its movingthe front brake pads into contact with the rotors. The secondary or rearseal on the piston creates this fluid flow.

The next event is the pressurization of the front brake system. Thebrake system has completely filled with fluid, and the entire system isready for a high-pressure application of brake fluid. Once the pressurein the system reaches 70 to 100 psi, the quick-take-up valve will open.In other words, the ball will lift off of its seat under spring pressure

and allow the fluid in the piston area behind the cup seal to be releasedback into the reservoir. The cup seal at the front of the piston will nowexpand and release high-pressure, but low-volume, brake fluid to thefront brake system as shown in Figure 28C. When the brake pedal isreleased, the fluid will flow back to the reservoir as normal.

Before we leave the subject of master cylinders and braking systems,

theres one other master cylinder in the braking system configurationthat we should discuss. This system is the diagonal-split system. Thissystem is shown in Figure 29.

One of the main problems with the dual-split or front/rear split brak-



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Troubleshooting Master Cylinder Problems

Master cylinders are very durable and, if maintained, can last for theentire life of a vehicle. To properly care for the master cylinder, youshould change DOT 3 and DOT 4 fluids every two years or sooner,depending upon local humidity and extent of use.

Troubleshooting the master cylinder begins with a visual test. Paycareful attention to the vehicle owners description of the perceived

FIGURE 29Ma ny front-wheel drive vehicles now use a diagonal-split braking system, such as the one

shown here.



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With the cover off the reservoir, you can check the quality and quan-tity of brake fluid in the system. A very low or dry reservoir section

means there is trouble somewhere in the system, such as worn brakepads or shoes. A very low condition, as with a dry condition, usuallymeans a leak has occurred and must be repaired. If the leak isnt at theback of the reservoir next to the power booster, then it must be some-where else in the system. To make sure the master cylinder isnt leak-ing, many technicians loosen the bolts that hold the master cylinder tothe power booster and pull it slightly away from the booster while

looking for signs of brake fluid. A light wetting of this area may beconsidered normal, but large quantities of brake fluid arent normal.

One of the most common problems with a master cylinder is the brakepedal fading to the floor. This problem is normally caused by one oftwo situations. One cause is that one or both of the primary seals isdamaged and is allowing fluid to pass by the seal to the rear of thepiston. The second cause is that the side walls of the cylinder can bescored or pitted, allowing the fluid that should be holding the brakesapplied at the wheels to bypass the cup seals and reduce the brakepressure. This situation, in turn, will make the pedal travel down tothe floor.

On some quick-take-up master cylinders, the quick-take-up valve canstick open. If the valve is stuck open, theres excessive downwardtravel of the brake pedal as the small piston and its primary cup sealor O-ring tries to provide a large amount of fluid. In this case, the fluidwill be provided but not until the brake pedal has traveled almost tothe floor.

A second problem with a quick-take-up system is that the quick-take-up valve can become stuck closed. If the valve is stuck closed, thebrake pedal will work normally on the downstroke but will returnvery slowly to the up position.



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As we have seen, there are basically two main types of master cylinders.One type is made of cast steel. The other type is made of aluminum.

The aluminum master cylinder normally has a plastic reservoir.

The first step to rebuilding a master cylinder is to remove the part fromthe vehicle. Using flare-end wrenches, remove the brake lines that attachto the master cylinder. Next, remove the two bolts that hold the mastercylinder to the power booster. With these two bolts removed, pull themaster cylinder from the power booster and then from the vehicle.

Once the master cylinder is free of the vehicle, dump out the reservoirsinto a pan and properly discard the old fluid. Then, clean the mastercylinder in a mild solvent or soap and water. This cleaning allows youto get the majority of the dirt from the outside of the master cylinderand from the inside of the reservoirs.

You can then begin disassembly. On some master cylinders, there arepressure switches, fluid level switches, proportioning valves, and residualcheck valves. All these parts can be removed at this time. To reach the re-sidual check valve, you may need to pry up the flare seat in one of themaster cylinders outlets. If you have an aluminum master cylinder, theplastic reservoir can be removed by gently prying up on it using a brake

spoon or small pry bar. It often helps to mount the master cylinder in avise thats equipped with soft jaws. This is especially important with alu-minum master cylinders since you dont want to damage them. The visemust hold the master cylinder only by its mounting flanges. Tighten thevise just enough to hold the master cylinder.

With all of the additional components removed, you can begin gettingthe piston assemblies out of the master cylinder. Turn the master cylin-der with its power booster side up and remove the retaining ring, asshown in Figure 30. To remove this retaining ring or clip, youll haveto lightly depress the piston with a small bar or screwdriver. In the fig-ure, the technician is using a brake spring pliers to depress the piston.



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With the pistons removed, you can look down the bore of the mastercylinder for pitting in the case of a cast-iron cylinder, or wear andscoring in the case of an aluminum cylinder. In either case, the mastercylinder should be replaced.

FIGURE 30To remove the retaining ring at the rear of the master cylinder, you must press down on thepiston as shown here. You can then remove the snap ring or retaining ring.



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Aluminum master cylinder bores should never be honed. Their boresurfaces are anodized and, if damaged, should be replaced.

Once the master cylinder is cleaned and inspected, the old piston sealsshould be removed and replaced with new seals. If there is a residualcheck valve in one or both of the outlets, the piston seals can be in-stalled into the outlets and the new retainers tapped into place with awooden dowel and a hammer. Residual pressure valves will only befound on older drum brake systems where the wheel cylinders dont

have cup expanders. These valves are typically found on pre-1970automobiles and light trucks.

Youre now ready to begin reassembly. Lubricate the master cylinderbore and the seal areas of the front piston with a small amount of brakefluid. Install the residual check valve (again, present only on oldermodels), the spring and spring retainer, and the front piston into the

bore. Follow these steps with the second spring, retainer, and pistonassembly after applying a coating of brake fluid to the seal areas. Thisstep is shown in Figure 31.



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The next step is to replace the retainer by pressing inward on the rearpiston and installing the retaining ring or clip. With the interior section

of the master cylinder complete, you should replace any pressureswitches and proportioning valves that you previously removed. If youremoved the plastic reservoir, you may install and, with clean brakefluid, lubricate new O-rings. Press the reservoir onto the O-rings using astraight downward motion.

The final part of the rebuild procedure is to bench bleed the master

cylinder. You bleed the cylinder in order to remove any trapped airfrom the system before you mount it onto the car.

To bench bleed the master cylinder, mount it into the vise as was donepreviously. Next, fill the reservoir to the top with clean, fresh brakefluid. Now, with the reservoir full of fluid, press in on the rear pistonand allow the fluid to escape the master cylinder outlets until all of the

air has escaped. Slowly press the piston and let the brake fluid flowfrom the master cylinder into a pan. Dont reuse this fluid. You canalso make up short brake lines that screw into the master cylinder out-let ports. You can then bend the lines so that they return to the top ofthe master cylinder, which will allow you to bleed the master cylinderwithout losing the fluid. When youre done bench bleeding the master

cylinder, you can lightly depress the rear piston and install small spac-ers or washers between the retaining ring and the piston. If the spacersare properly placed, the cup seal or O-ring will block the compensat-ing ports, allowing you to remove the fittings and small brake lines orjust to stop bleeding the master cylinder without losing any more fluidfrom the reservoir through the outlet ports. This procedure can be fol-lowed when youre working on other parts of the brake system.

Slightly depress the brake pedal, and little or no fluid will flow froman open brake line. In the case of bench bleeding, make sure to removethe spacers after youve connected the master cylinder to the vehiclesbrake lines and before you bolt it to the power booster. Once the mas-ter cylinder has been installed, youll need to bleed the entire brake



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To display the operation of a power booster, well use a simple example.Suppose that we have two metal bowls pressed together and sealed. A

rubber diaphragm separates the bowls. Both sides of the diaphragmhave a connector to the outside of the bowls. Theres also a rod thatconnects to the center of the rubber diaphragm and passes through asealing grommet. This device is shown in Figure 32.

FIGURE32Thisexample oftwo metal bowlswill be

used to describe the opera-tion of a power booster.

h h h



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Now, lets unbalance the pressures on the opposite sides of the dia-phragm. Lets apply a vacuum to the left side at source 1 and air pres-sure to the right side at source 2. In this case, the force on the outputrod is equal to the pull of the vacuum on the diaphragm and the pushof the air pressure on the opposite side. The rods force can be verylarge since force is equal to pressure times area, F = PA, as weveseen earlier in this text. The diaphragm offers a large surface area soeven a small vacuum multiplied by this area offers quite a bit of force.

In a typical integral vacuum booster, the left side of the booster, or theside closest to the master cylinder, always holds a vacuum. The rightside of the boosters internal diaphragm is switched between vacuumand atmospheric pressure to apply or remove booster power assist.Just as with our metal bowl example, when theres a vacuum on bothsides of the diaphragm the brakes arent being applied and the outputrod remains stationary and doesnt apply any pressure to the mastercylinders pistons. When the right side of the diaphragm inside thepower booster has atmospheric pressure on it, the output rod appliesconsiderable force to the master cylinders pistons. A vacuum powerbooster basically works in the same manner. However, the vacuumpower booster is more complex because of how the vacuum and at-mospheric pressure at the right or passenger compartment side of the

diaphragm is controlled.

Weve disassembled a vacuum power booster to show you the internalcomponents. These components of the braking system are typicallysealed and are simply replaced if they fail. The disassembled boosteris shown in Figure 33.

A power piston assembly is inside the booster. When the brakes arentapplied, the power piston and an internal valve to the power piston areheld back by springs toward the firewall. This setup causes vacuum tobe present on both sides of the diaphragm inside the booster. The inter-nal valve that moves inside the power piston is called a floating control



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d d h d l d th fl ti t l l ill t



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depressed harder or released, the floating control valve will move toopen or close the vacuum or atmosphere control ports to tailor brakingaction to pedal pressure.

Theres a certain amount of pressure thats placed back on the brakepedal to allow the vehicle operator to feel the braking force. A reactiondisc or reaction levers act upon the input push rod from the pedal as-sembly with a counterforce thats proportional to the input force. If thevehicle operator forcefully depresses the brake pedal, as would occur

in an emergency situation, the power piston is moved forward underatmospheric pressure assist until the input push rod is fully extendedinto the power piston and theres a direct connection between the in-put push rod and the output push rod through the opposite surfaces ofthe power piston.

Power Booster Check Valve

A power booster needs a supply of vacuum thats normally deliveredby the intake manifold of a four-stroke gasoline engine. The input tothe master cylinder is by means of a fairly large hose and a one-waycheck valve. These components are shown in Figure 34.

The hose connects to a convenient place on the intake manifold of thevehicle. This location is also normally the vacuum source for manyother of the engines vacuum circuits. The one-way vacuum checkvalve is normally located at the power booster (Figure 34A) but can bepresent in the vacuum line (Figure 34B) or at the intake manifold. Thepurpose of this check valve is to make sure that the greatest amount of

vacuum is always present in the vacuum booster. It performs thisfunction by closing the valve whenever the pressure in the intakemanifold is higher than in the vacuum booster. Conditions such as ac-celeration can cause the pressure in the manifold to decrease belowvacuum booster internal pressure.



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In addition to standard vacuum-operated power boosters there are



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In addition to standard vacuum-operated power boosters, there arealso fluid-powered systems. Where vacuum systems are calledintegral, the hydraulic systems are called multipliers. In one such system,the power steering pump supplies pressurized fluid to a booster as-sembly that connects to the master cylinder. The operator changes theposition of an internal valve that allows a power piston to be extended.This power piston forces an output push rod forward to actuate themaster cylinder. These systems are often termed hydro-boost systems.In some cases, there are even combination vacuum and hydro-boost

systems where the hydro-boost unit mounts on the vacuum boosterand supplies additional power to the output push rod that actuates themaster cylinder. This system is used in some large trucks.

One of the newer systems today uses an electric motor thats mountedbeneath the master cylinder. The purpose of this motor is to drive avane pump that provides a very high-pressure brake fluid to a storage

device called an accumulator. The accumulator stores this high-pressurebrake fluid and uses it to push a power piston forward when the brakepedal triggers a valve. The power piston moves a push rod into themaster cylinder to engage the brakes. When the pedal is released, adischarge valve opens and a large amount of brake fluid is dumpedback into the reservoir. The electric pump can then be restarted and thesystem repressurized rapidly. This is often called an Electro-Hydraulic,or E-H, system and is coming into more widespread use in antilockbraking systems. Well discuss E-H systems in another study unit.

Testing and Repairing Power Booster Systems

Most manufacturers suggest that the integral power booster unit be re-placed rather than rebuilt. Replacement is also suggested for mosthydro-boost and E-H systems. However, there are a number of teststhat can be performed to make sure the power booster itself has failedand that the problem isnt outside the booster.

Using a vacuum gauge at the inlet of the vacuum booster you can



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Using a vacuum gauge at the inlet of the vacuum booster, you cancheck these problems. The vacuum gauge should show a minimumreading of 15 inches of mercury, 15 in. Hg, or 50 kPa. If the reading is

below this point, the power booster wont operate properly, and youllhave to repair the problem before performing other tests.

If theres sufficient vacuum, listen carefully for a leak in the booster.Start the vehicle, and let it run for a few minutes. Shut off the engineand listen near the hose, check valve, and grommet for a vacuum leak.

A leak will sound like a faint hiss. While the power booster is stillcharged with vacuum, go to the inside of the vehicle and apply the

brakes. You should get at least two applications of the brake pedalunder power assist before the internal vacuum is equalized. Less thantwo applications usually indicates a leak in the system. While the

booster is in this depleted condition, start the engine while applyinga downward pressure on the brake pedal. If the floating control valveis operating properly, the pedal should drop slightly toward the floor.The pedal should also feel easier to apply. If these conditions dontexist after the vehicle is started and vacuum reaches the booster, youshould replace the booster.

Replacing a Power Booster

The replacement of a power booster isnt a difficult task. The first stepis to disconnect the master cylinder from the power booster by remov-ing its two retaining bolts. If the master cylinder isnt being replaced,you can usually gently pull it away from the power booster and secure

it to the vehicle using plastic wire wraps or wire. With the master cyl-inder removed, you can remove the vacuum line to the booster and thelinkage that connects the pedal to the input push rod. When thesecomponents are disconnected from the integral power booster, you canremove the four nuts that hold the booster to the firewall and removeth b t f th hi l I th t li d

Hydro-boost units are normally tested by measuring hydraulic pres-



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Hydro boost units are normally tested by measuring hydraulic pressure at the booster unit. You must open the system and insert specialT fittings into the lines.

You can then measure hydraulic system pressure and compare it tothose readings in the service manual. Normally, the power source forthe system is the power steering pump. If youre having pump pressureor volume problems, youll also be having power steering problems.

Some of the most important tests you can make are fluid level tests atthe power steering pump, and a visual inspection of all of the hosesand pipes in the system. Leaks must be repaired since they cause fluidloss as well as pressure loss. A pressure leak is easy to spot; fluid willbe all over the engine compartment.

Typically, components that are faulty are changed rather than repaired.

In addition, if a component is changed, the system can simply betopped off with fluid. The system can then be run to eliminate air asits self-bleeding.

Now, take a few moments to review what youve learned by complet-ing Self-Check 3.



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DISC BRAKE SYSTEMSMany of the vehicles of the 1950s and 1960s used four wheel-drum

brakes. These systems used single- or dual-master cylinders. In mostcars of that time, all four wheels held plain drums. Performance carsused finned drums

Self-Check 3

Fill in the blanks in each of the following statements.

1. The section of the master cylinder that holds the unpressurized brake fluid is called the_______.

2. Master cylinder bodies are normally made of _______ or _______.

3. The port inside the master cylinder that allows fluid to return to the area above the mastercylinder is called the _______ port.

4. The piston in the front of a fast-fill master cylinder is used for the _______ brakes.

5. The front side of a power booster, or the side toward the master cylinder, is always kept ata _______ condition.

6. The internal valve that switches the vacuum and atmospheric pressure inside the booster iscalled a _______ control valve.



FIGURE35Thisillustrationh id i f


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Rotors

The rotor section of the disc brake system is attached to the axle orspindle. The rotor is either driven or allowed to freely rotate. The rotor

often has threaded studs on which the wheel is mounted and bolted.Some rotors are simple plates that slip over the threaded studs, whichare a part of the hub assembly. Rotors are made of steel thats machinedto very close tolerances. There are single and double rotors. Doublerotors normally have cooling fins between the rotor sections. In some

showsyou a side view ofatypical disc brake system.

caliper to cool more efficiently than a drum brake. Cooling prevents



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p y g pbrake fade. Next, a disc brake wont be as affected by dirt or moistureas a drum brake since the pads are located very close to the rotor; thepads constantly clean the rotor. Disc brakes allow the vehicle to stop ina straight line, even after one side of the vehicle has gone through apuddle. The centrifugal force, the force pushing out from the center, ofa spinning rotor also throws off any impurities that land on the rotor,keeping dirt out of the braking system.

Heat is typically an enemy of a drum brake system. Heat expands thedrum, which causes a greater space between the shoes and the drum.If a disc brake rotor heats and expands, it only presses harder into the

brake pads. This isnt to say that too much heat cant hurt a disc brakesystem. Too much heat at the pads can cause the pads to lose friction.Excessive heat can actually alter the material of the pads and renderthem inefficient. If this occurs, the pads must be replaced.

The main disadvantage to disc brakes is that they can be noisy. Thisproblem can often be remedied by a spray- or brush-on material thatsplaced at the back of the pads, or by using antirattle hardware. An-other disadvantage is that disc brakes often coat the outside hubcapsor exposed wheels with dark or reddish-brown brake dust that must

be periodically cleaned.

Still, disc brakes are the brakes of choice by all modern car and lighttruck manufacturers. Somewhere between 60 and 80 percent of thestopping force of a vehicle is concentrated on the front wheels and thefront braking system of a vehicle. Since most vehicles are now front-engine, front-wheel drive vehicles with a major proportion of the

weight at the front of the vehicle, you can understand why disc, ratherthan drum, brakes are selected. Disc brakes can be used on the front ofa vehicle to provide the braking forces needed to easily stop todaysvehicles within the distances specified by the government. Front drumbrakes wouldnt meet these specifications without the use of high-



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In this system, the caliper is mounted to the spindle by means of pins,bolts, or other such retainers. The caliper is a simple device that con-tains a pressure chamber and a piston that is sealed by means of O-rings or special seals and an exterior boot. When brake fluid pressureis applied, the piston is pushed out of the caliper. Once the inboard, or

FIGURE 36This is a disc brake system thats on a front-wheel drive a utomobile.



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to the spindle via the retaining bolts but can still translate by sliding

along the bushings.The pistons used in brake calipers are typically made of steel or alumi-num. Some systems have used phenolic pistons. The caliper is normallymade of cast iron but can also be made of various alloys. A weatherboot protects the area between the piston and the caliper from dirt and

FIGURE 37A caliper actually contains few components. There are the piston, the seal, the caliper housing,the bushings, a nd retaining bolts.



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There are also two other types of calipers in use today. These types arethe sliding and the fixed calipers. Sliding calipers are typically used onlight trucks. These calipers fit into slots that are machined into thespindle anchors.

FIGURE 38This disc brake is at the rear of a midsize front-wheel drive vehicle .

The Friction Material



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Earlier in this study unit, we displayed a table of friction material rat-ings and their code letters. If you look at a new set of pads, one of thesecode letters will be displayed on the side of the friction material. Youllnote that premium friction materials will have a higher rating than

budget materials.

There are a wide variety of friction materials from which you can

choose. The best choice is premium-grade linings since theyre care-fully constructed of better materials. On inexpensive pads, you canoften see chunks of nonblended materials, and youll see quick wearof the material or the rotor.

There are many different types of friction materials used as brake

linings. For example, the oldest type of lining is called the organiclining. This lining uses ingredients such as carbon and aluminum;organic and inorganic friction materials such as rubber and talc; asbestosfibers; and a phenolic resin as a binder. Normally the bag or box theselinings come in will have a large letter A or a on it to symbolize thatasbestos in included in the product.

Now, theres also a nonasbestos friction material that substitutes syn-thetic materials for the asbestos. These materials typically use aramidfibers instead of asbestos and are often called synthetic or nonasbestospads.

Theres also a series of friction materials called semimetallic pads. Thefriction material of these pads uses many different types of metallicpowders along with ceramic powders, graphite or carbon powders,and phenolic resins. These pads offer long wear and good friction.

Rear Disc Brake Systems

brake parking brakes of this type are somewhat weaker comparedto drum parking brakes A second type of rear disc brake parking



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to drum parking brakes. A second type of rear disc brake parkingbrake system uses a small area of the rotor near the axle as a drum

and mounts parking brake drum shoes inside this area. The disc brakesoperate from the master cylinder while the drum brakes operate fromthe parking brake lever.

Inspecting Disc Brake Systems

The inspection of a disc brake system should include at least threesteps. The first step is a visual inspection of the brake lines to the cali-pers. Check for wear or abrasion, and look for signs of cracks andleaks. You may need to slightly flex the rubber hose to inspect it forcracks. These cracks will normally appear at the point on the hosewhere the bend is the sharpest.

The second inspection is of the rotor. The rotor surfaces should besmooth and free of scratches or other defects. The rotor shown inFigure 39 has manufacturing defects that have caused the back surfaceof the rotor to break away. This rotor must be replaced.

FIGURE39Thisrotor isbadly damaged. Poor steelwasused in the manufac-turing of the rotor.

In many cases, youll find that the vehicle operator has driven the vehi-cle with the pads worn down to an unacceptable level If the vehicle



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cle with the pads worn down to an unacceptable level. If the vehicleis driven with worn brake pads, the rotor can be scored or grooved.In this case, the rotor can be measured and if its thick enough, it can

be turned on a brake disc lathe and returned to service. There are twobasic types of disc-brake lathes. One is a bench- or stand-mounted unitthat requires the rotor be removed from the vehicle. The other latheactually attaches to the vehicle and can cut or resurface the rotor onthe vehicle. Make sure the rotor is thick enough to allow it to be cut yet

remain above the minimum thickness limit. Using a micrometer, youcan check the thickness of a rotor at the center of the swept area in twoor more places.

The final step is to check the brake pads themselves. The brake padsshould never be less than 1 8" for bonded linings, and there should be atleast 116" of brake material above the rivets on a pad with riveted fric-tion material. Often youll see a wear groove in the pad material thatcan be used as an indicator. Usually, the cost of new brake pads issmall enough that, if theyre worn halfway or more, you shouldreplace them. Although theres an inspection hole in the top of mostcalipers, the best way to inspect the pads is to remove the caliperfrom the rotor and carefully look at the pads. You should also look at

the pad for physical damage and cracked or tapered friction linings.Tapered linings are often caused by weak or worn mounting hardware.Its a good idea to always change the mounting hardware wheneveryou change brake pads. This hardware can come with the replacement

brake pads or can be purchased separately.

Besides the wear dimensions, there are a number of other methods of

determining brake pad wear. In some systems, theres a small piece ofmetal, known as a wear indicator, attached to the brake pad. This compo-nent is shown in Figure 40. When the pad wears down enough, thismetal bar will rub against the rotor, causing a loud squeal each timethe brakes are applied There are also electric contacts buried within



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FIGURE 40One of the most common indications of wear on a rotor is a sma ll section of metal that norma llyrides well away from the surface of the rotor when the pa ds are new. When the pad material wea rs enough,the metal strike will hit the rotor, resulting in a loud squeal each time the brakes are applied.



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To remove the caliper, you remove these two Torx bolts. Once thesetwo bolts are removed, you can simply pull the caliper from the twoanchor points. With the caliper removed, you can pry the two brake

d f th li i ll d i d ll t f

FIGURE 41In this illustration, you can see the two mounting bolts that hold the caliper to the anchors.



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The backsides of the new disc brake pads are shown in Figure 43. Notehow there are antirattle springs on both the inboard and the outboardpads. These springs will help you retain the pads in the caliper during

FIGURE 42With the bolts loosened, the caliper can be lifted off the spindle. The pads can then be removedfrom the c aliper. If you need to set the caliper down for any reason, dont hang it by the brake hose. In-

stead, tie it up with a wire or place it in a secure location as shown here.



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Before we install the new pads, we must retract the piston into thecaliper. On non-antilock vehicles, you can simply use a C-clamp orpiston-retracting tool to apply light pressure to the piston to retractit back into the caliper. On newer antilock braking systems and on

hi l ith l i li it d id t th bl d

FIGURE 43This photograph of the back of the brake shoes shows the antirattle springs that fit intothe piston and across the outside of that caliper.



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The next step in the repair of this disc brake system is to remove thedisc or rotor. In the case of this front-wheel drive vehicle, the rotor willeasily come off of the hub with a gentle tug. The wheels lug nuts areused to hold the wheel and the rotor on the front hub.

FIGURE 44This illustration shows the pads clamped to the outside of the caliper and seated into the piston.

In Figure 45, the rotors have shallow grooves across the surface. Notonly was this rotor grooved, but it also had some runout, resulting in a



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pulsating pedal and steering wheel movement upon heavy braking.

Light grooves can usually be removed by turning the rotor on a brakelathe or an on-vehicle lathe. Heavier grooves usually mean that the ro-tor will need to be replaced since turning the rotor to the point wherethe grooves are gone will result in an undersized rotor.



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On rear-wheel drive vehicles, the front disc brake rotor is more diffi-cult to remove and replace since the rotor is suspended between twotapered roller thrust bearings. Usually, the bearings can be cleaned,repacked with grease, and reused.

FIGURE 46Here is a close-up of the defective rotor in Figure 39. Note that the metal has pulled out of therotor, leaving a hole. You might also see chips and cracks form on some rotors.

Remove the inner and outer bearings, clean and inspect them,and repack them with grease if they can be reused.



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Place the inner bearing onto the rotor, being sure to use a newgrease seal.

Install the new rotor with the new outer races onto the spindle.

Install the outer bearing, washer, and castle nut on the spindle.

Tighten the castle nut lightly and spin the rotor.

Retighten the castle nut.

Loosen the castle nut until the first hole in the spindle lines upwith the castle nut and insert a new cotter pin through the nut

and spindle. Bend the cotter pin to make sure the nut cant comeloose.

Clean the rotor surface of any grease or fingerprints.

Install the caliper and the wheel.

Test drive the vehicle.

Disc brake pads can also fail. For example, a pad can become so hot itscomposition can change and its braking force can be reduced signifi-cantly. Usually, the pad will have cracks on its surface such as the oneshown in Figure 47.

One common form of irregular pad wear is taperwear. Taper wearis usually caused by more pressure affecting the front of the frictionmaterial compared to the back of the friction material. Some specialtymotorcycle and racing calipers now use multiple piston calipers and



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FIGURE 47This brake pa d suffered from overheating and the friction material cracked. Of course, thesepads were replaced.

FIGURE48One cause oftapered pad wear iswhen

the mounting hardware isworn and the caliper isnolonger held straight against

Completing the Brake Repair Job



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A completed repair job is shown in Figure 49. The new rotor has beenmounted on the hub, and new brake pads have been placed in thecaliper. The caliper bolts have been tightened to the manufacturersspecifications. You can also see a light layer of brake grease that has beenplaced on the anchor points where the pads and the caliper will rub.



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DRUM BRAKE SYSTEMS

For many years, the drum brake system was used on all four wheels ofcars and trucks. On many of todays cars and light trucks, drum brakes

are still used at the rear wheels of the vehicle where less braking force isrequired than at the front brakes. The drum brake system uses a dual-piston hydraulic cylinder to push outward on two brake shoes to createbraking force. These shoes are, for the most part, stationary and pressagainst a rotating drum. A drum brake system is shown in Figure 50

Self-Check 4

Indicate whether each of the following statements is True or False.

_____ 1. Centrifugal force causes impurities to be thrown from a spinning rotor.

_____ 2. Todays calipers typically use two pistons in each caliper.

_____ 3. A weather boot protects the area between the piston and the caliper from dirt andwater.

_____ 4. The most powerful type of disc brake system is the fixed caliper system.

_____ 5. A warped rotor usually results in a steering wheel that locks when the brakes areapplied.

_____ 6. All taper wear is due to loose mounting hardware.




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The dual-servo, or self-energized, drum brake system is shown in Figure 51B.In this system, the force applied to the primary or forward shoe helpspress it into the drum. However, since this shoe isnt permanently at-tached or restrained at the bottom of the drum brake assembly, it canb d t l f th t ili h Thi dditi l f

FIGURE 50This is a brake drum mounted on the rea r axle of a rea r-wheel drive vehicle.


FIGURE51There are twobasic kindsof drum brakesystems, the nonservo(Figure 51A) and the servo


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(Figure 51A) and the servo

(Figure 51B) brake systems.



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The hydraulic brake component of this system is the wheel cylinder.Brake fluid is sent under pressure to the wheel cylinder when the

brakes are applied. Inside the wheel cylinder, two pistons oppose eachth Th l i t l i ith it d hi h

FIGURE52This illustration showsthe configuration of the rear drum brakeson a modern vehicle. Before

work isdone on these brakes, they must be cleaned.



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When pressurized fluid enters the brake or wheel cylinder, the pres-

sure expands the cup seals, which, in turn, press on the pistons. Thepistons transfer this force to the push rods and then to the brake shoes.When the brake fluid pressure is released, the return springs in thebrake assembly are responsible for returning the pistons and the restof the brake assembly back into their retreated position.

FIGURE 53This is a typical whee l cylinder used to sprea d the brake shoes on a drum brake system.

The bottom section of the brake assembly contains the adjuster thatkeeps the brake shoes close to the inside diameter of the brake drum.This adjuster is a star wheel that has many points. The star wheel is



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mounted on a fine-thread screw. This screw turns into and out of a col-lar that fits into one of the shoes. The opposite side of the star wheelscrew is a post that fits into a second nonthreaded collar. This collarmounts into the leading or primary shoe. When there are new brakeshoes and a new drum in the drum brake assembly, the star wheel as-sembly is fully screwed into the threaded collar. As the friction linings

and the inside of the drum wear, the adjuster is turned to spread thebottom section of the shoes. This keeps the shoes in a position thatsclose to the drum surface through their useful life. The adjuster mecha-nism is usually triggered automatically when the brakes are appliedand the vehicle is moving backward. Some adjusters use cables thatare attached to the secondary brake shoe as shown in Figure 54. Othersystems use levers and other such devices that rely on the motion of

the secondary shoe to transmit motion to a lever that moves the staradjuster. If the star adjuster is tight enough, the lever will skip over thestar wheel and not cause the brakes to bind. Some vehicles rely on thesetting of the parking brake to create the motion that moves the staradjuster.

The star wheel should be replaced whenever you see that it has roundededges. Also, when replacing brake shoes, make sure to apply a lightcoating of high-temperature or brake grease in the threads of the star



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wheel screw and at the washers that separate the star wheel from theend collars. This procedure will allow free operation of the star wheel.

One additional component of the rear drum brake assembly is the strutrod, which fits between the brake shoes near the top of the assembly.In addition, theres a stop post on which the tops of the brake shoes rest.Theres also a guide plate that covers the shoes and allows them tomove off the post yet remain captured in that area so that theyll returnto the post once the brake pedal is released.

Drum Brake Maintenance

All forms of inspection and repair require the drum be removed fromthe axle. Often this drum is tightly bonded to the axle hub and is diffi-cult to remove. However, there are certain procedures you can use tomake this task easier.

First, always remember that there will be brake dust and possiblyasbestos dust inside the brake drum. For the safest repair, always usea vacuum system or a wash down system to remove the dust beforeworking on the internal components of the drum brake system.

Once the vehicle is safely supported and the tire and wheel are re-moved, you can begin to remove the drum. The first step is to removethe small plug that covers the adjustment hole at the rear of the back-

ing plate. This hole is shown in Figure 55.

In most cases, the plug is made of rubber and can be easily pried fromthe backing plate by using a small screwdriver. In other cases, thebacking plate will have knock-outs that must be broken loose to gain



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FIGURE 55In order to remove the drum, youll have to remove the plug and gain access to the star wheelinside the drum. This hole is shown here.


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The next place to inspect the system is at the top post where the top ofthe shoes, the wheel cylinder, the springs, and the adjuster cable are lo-cated. This area is shown in Figure 58. In this case, the springs and the



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cable are in good condition. Both shoes are also seated on the post. Ifthe leading or trailing shoe isnt seated on the post, look for a misad-

justed parking brake or a stuck parking brake cable as the problem.You must also inspect the wheel cylinder for leaks. To do this inspec-tion, you must slightly pull the boot to inspect behind the boot on eachside of the wheel cylinder. In some cases, however, the leakage will be

apparent because youll see brake fluid running down the backingplate and on the shoes and other components of the system. Any signof dampness is cause for alarm, and its source should be identified andrepaired.

Dampness can also come from a leaking seal on the axle of a rear-wheeldrive vehicle. This axle grease can get on the brake friction surfaces,

creating a wheel lockup condition. This problem must be repairedbefore the brake system can be serviced.

The next area to inspect is the star wheel adjustment assembly. This as-sembly is shown in Figure 59. The star wheel assembly, along with themechanism that turns it, is responsible for keeping the brake shoes in a



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position thats close to the brake drum surface. In this system, if the trail-ing shoe moves an excessive amount, it will move the lever against thestar wheel and cause it to ratchet forward one increment, bringing theshoes closer to the drum. The lever keeps the star wheel from movingbackward. In some other systems, the lever only moves the star wheelforward. Theres a spring that holds the star wheel in position. This isthe spring that must be raised with the screwdriver when youre usingthe spoon to back off on the brake shoe star wheel adjuster.

The final inspection points are at the parking brake mechanism andthe adjustment mechanism. Look for signs of abuse or damage, andmake sure all levers can pivot properly. The parking brake lever that

h l h h ld b l h l l h h k



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presses out on the trailing shoe should be slightly loose when the park-ing brake isnt applied.

In Figure 60, you can also see one of the two shoe retainers. If the up-per shoe retainer washer is pressed inward and rotated 90 degrees, theretainer will come off of the nail and free that brake shoe. The nail canalso be cut, since it will normally be replaced with the other brakehardware when the shoes are replaced.

After the top two springs are removed, the two small retainers can beremoved from the bottom section of the shoe. A special tool is availableto remove the outer retainer. You can also use a pair of slip-joint pliers.

Oft ll h t h ld th b k f th il t th t id f th b k



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Often, youll have to hold the back of the nail at the outside of the back-ing plate while you push in on the retainer. With the two retainers,youll next need to remove the adjustment and parking brake linkages.With these items removed, you can spread the top of the shoes and re-move them along with the bottom star wheel adjusting assembly.

With the brake shoes and all of their retainers removed, you should be

careful not to let the pistons come out of the wheel cylinder. A smallC-clamp can be used to hold the pistons in the cylinder.

Next, you should perform a careful inspection and cleaning of the back-ing plate. Place a small amount of high-temperature or brake grease atthe points where the shoes contact the backing plate on the shoe ledges.

If the wheel cylinder is leaking, it will normally be replaced as a unit.You should be aware that most modern wheel cylinders use a specialbore that cant be honed. This bore is created at the time of manufac-turing by pushing a very hard ball bearing through the bore of thewheel cylinder. This bends over the grain of the metal and creates avery hard and smooth surface. If you were to hone the bore, you

would actually soften and destroy its surface grain structure.

Some older wheel cylinders can be honed, using a tool such as the oneshown in Figure 61. This tool is placed in an electric drill or drill pressand the wheel cylinder is moved in and out quickly over the turningstone to create the crosshatch pattern on the bore of the wheel cylinder.The wheel cylinder can then be rebuilt using a parts kit.



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The drum brake system can then be rebuilt by using new shoes andnew hardware. The first step is to place a new star wheel adjusteracross the bottom of the shoes. The top of the shoes can then be spread

d l d th l h b If th i t t d b t th h

FIGURE 62These a re the internal parts of a wheel cylinder. Note that this wheel cylinder uses the outerwindings of the center spring as cup expanders.

The next step is to install the cable for the adjuster, if present, followedby the two large springs. These springs can be difficult to install.

90463746 Automotive Braking Systems

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Transcript of 90463746 Automotive Braking Systems