Halderman ch094 lecture

© 2011 Pearson Education, Inc.All Rights Reserved

Automotive Technology, Fourth EditionJames Halderman

BRAKE HYDRAULIC SYSTEMS

94

94 BRAKE HYDRAULIC SYSTEMS



ObjectivesObjectives

• The student should be able to:– Prepare for the Brakes (A5) ASE

certification test content area “A” (Hydraulic System Diagnosis and Repair).

– State Pascal’s law. – Describe the function, purpose, and

operation of the master cylinder.– Explain how hydraulic force can be used to

supply high pressures to each individual wheel brake.




ObjectivesObjectives

• The student should be able to:– Describe the process of troubleshooting

master cylinders and related brake hydraulic components.

– Explain how a quick take-up master cylinder works.

– Describe how to bench bleed a master cylinder.




HYDRAULIC HYDRAULIC PRINCIPLESPRINCIPLES




Hydraulic PrinciplesHydraulic Principles

• All braking systems require that a driver’s force is transmitted to the drum or rotor attached to each wheel




Figure 94-1 Hydraulic brake lines transfer the brake effort to each brake assembly attached to all four wheels.





• Noncompressibility of Liquids– Hydraulic systems use liquids to transmit

motion– No matter how much pressure is placed on

a quantity of liquid, its volume remains the same

– This fact enables liquids in a closed system to transmit motion





• Noncompressibility of Liquids– Air trapped in the system can be

compressed– A brake hydraulic system must be free of

air or the brakes will not operate correctly




Figure 94-2 Because liquids cannot be compressed, they are able to transmit motion in a closed system.




Figure 94-3 Hydraulic system must be free of air to operate properly. If air is in the system, the air is compressed when the brake pedal is depressed and the brake fluid does not transmit the force to the wheel brakes.




PASCAL’S LAWPASCAL’S LAW




Pascal's Law Pascal's Law

• Hydraulic principles that permit a brake system to function were discovered by Blaise Pascal (1632–1662)

• Pascal’s law states that “when force is applied to a liquid confined in a container or an enclosure, the pressure is transmitted equal and undiminished in every direction”





• If two out of the three factors are known, then the other one can be calculated by using this formula:– F = P × A (force is equal to the pressure

multiplied by the area)





• If two out of the three factors are known, then the other one can be calculated by using this formula:– P = F ÷ A (pressure is equal to the force

divided by the area)





• If two out of the three factors are known, then the other one can be calculated by using this formula:– A = F ÷ P (area is equal to the force

divided by the pressure)





• If two out of the three factors are known, then the other one can be calculated by using this formula:– Where the capital letters mean:• F = force (lb) (Newtons)• P = pressure in pounds per square inch (kilo

Pascals [kPa])• A = area in sq. in. (cm2)

?




Figure 94-4 A one-pound force exerted on a small piston in a sealed system transfers the pressure to each square inch throughout the system. In this example, the 1-lb force is able to lift a 100-lb weight because it is supported by a piston that is 100 times larger in area than the small piston.




Figure 94-5 The amount of force (F) on the piston is the result of pressure (P) multiplied by the surface area (A). In this example, the driver is applying a force of 150 pounds but through the mechanical advantage of the brake pedal (3.3 to 1 ratio), the force is increased to 500 pounds into the master cylinder.




Figure 94-6 Drum brake illustrating the typical clearance between the brake shoes (friction material) and the rotating brake drum represented as the outermost black circle.




Figure 94-7 The brake pad (friction material) is pressed on both sides of the rotating rotor by the hydraulic pressure of the caliper.




Pascal's LawPascal's Law

• Hydraulic Pressure and Piston Size– If a mechanical force of 100 lb is exerted

by the brake pedal pushrod onto a master cylinder piston with 1 sq. in. of surface area, the equation is as follows:

= 100 PSI100 lb 1 sq. in.





• Equation– If the same force is applied to a master

cylinder piston with twice the area (2 sq. in.) the equation is as follows:

= 50 PSI100 lb 2 sq. in.





• Equation– If the same force is applied to a master

cylinder piston with only half the area (0.5 or 1/2 sq. in.), the equation is as follows:

= 200 PSI100 lb

0.5 sq. in.





• Application Force and Piston Size– Hydraulic pressure is the same throughout

a closed system and acts with equal force on equal areas





• Application Force and Piston Size– The mechanical force (f) at the brake pedal

pushrod is applied to the master cylinder piston area (a) and converted into brake system hydraulic pressure (p)





• Application Force and Piston Size– Hydraulic pressure applied to the wheel

cylinder or brake caliper piston area is converted back into mechanical force that is used to apply the wheel friction assemblies





• Application Force and Piston Size– Because the variables are identical, the

same equation can be rewritten to explain how changes in piston size affect brake application force• P × A = F





• Application Force and Piston Size– In the simple brake system, the pedal and

linkage apply a 100-lb force on a master cylinder piston with an area of 1 sq. in.

– This results in a pressure of 100 PSI throughout the hydraulic system





• Application Force and Piston Size– At the front wheels, the 100 PSI is applied

to a brake caliper piston that has an area of 4 sq. in. The equation for this example is as follows:• 100 PSI × 4 sq. in. = 400 lb





• Application Force and Piston Size– The drum brakes at the rear wheels of the

same brake system use wheel cylinders whose pistons have three-quarters (3/4 or 0.75) of an inch of surface area. If the hydraulic system pressure remains 100 PSI, the equation for this example is as follows:• 100 PSI × 0.75 sq. in. = 75 lb




Figure 94-9 Hydraulic pressure is the same throughout a closed system and acts with equal force on equal areas.




Figure 94-10 Differences in brake caliper and wheel cylinder piston area have a major effect on brake application force.





• Piston Size Versus Piston Travel– In disc brakes, the mechanical force

available to apply the brakes is four times greater because of the size difference between the master cylinder and caliper pistons





• Piston Size Versus Piston Travel– Some of the hydraulic energy is converted

into increased mechanical force





• Piston Size Versus Piston Travel– The trade-off is that the larger caliper

piston with the greater force will not move as far as the smaller master cylinder piston

– The amount of hydraulic energy converted into mechanical motion is decreased




Figure 94-11 The increase in application force created by the large brake caliper piston is offset by a decrease in piston travel.





• Hydraulic Principles and Brake Design– Hydraulic relationships play a major part in

determining the sizes of the many pistons within a brake system





• Hydraulic Principles and Brake Design– The sizes must move enough fluid to

operate the wheel cylinder and brake caliper pistons through a wide range of travel, while at the same time they must create enough application force to lock the wheel brakes





• Hydraulic Principles and Brake Design– The sizes chosen should also provide the

driver with good brake pedal “feel” so the brakes are easy to apply in a controlled manner




MASTER CYLINDERSMASTER CYLINDERS




Master CylindersMaster Cylinders

• Purpose and Function– The master cylinder is the heart of the

entire braking system– Brake pedal linkage is used to apply the

force of the driver’s foot into a closed hydraulic system that begins with the master cylinder





• Master Cylinder Reservoirs– Most vehicles built since the early 1980s

have a see-through master cylinder reservoir, which the brake fluid level to be checked without having to remove the top of the reservoir





• Master Cylinder Reservoirs– Capacity is great enough to allow brakes to

become completely worn out and still have enough fluid for safe operation




Figure 94-12 Typical master cylinder showing the reservoir and associated parts. The reservoir diaphragm lays directly on top of the brake fluid, which helps keep air from the surface of the brake fluid because brake fluid easily absorbs moisture from the air.





• Master Cylinder Reservoir Diaphragm– The reservoir is vented so the fluid can

expand and contract without difficulty– Moisture in the air is absorbed into the

brake fluid





• Master Cylinder Reservoir Diaphragm– Master cylinders use a rubber diaphragm or

floating disc to help seal outside air from direct contact with brake fluid

– This diaphragm is vented between the steel cap and diaphragm





• Master Cylinder Reservoir Diaphragm– As the brake fluid level drops due to

normal disc brake pad wear, the rubber diaphragm also lowers to remain like a second skin on top of the brake fluid





• Master Cylinder Reservoir Diaphragm– Whenever adding brake fluid, push the

rubber diaphragm back up into the cover– Normal atmospheric pressure will allow the

diaphragm to return to its normal position on top of the brake fluid





• Master Cylinder Reservoir Diaphragm– Whenever servicing a brake system, be

sure to check that the vent hole is clear on the cover to allow air to get between the cover and the diaphragm




Figure 94-13 Master cylinder with brake fluid level at the “max” (maximum) line.





• Master Cylinder Operation– Brake pedal movement and force are

transferred to the brake fluid and directed to wheel cylinders or calipers




Figure 94-14 The typical brake pedal is supported by a mount and attached to the pushrod by a U-shaped bracket. The pin used to retain the clevis to the brake pedal is usually called a clevis pin.





• Master Cylinder Operation– The master cylinder is separated into two

pressure-building chambers (or circuits)




Figure 94-15 The composite master cylinder is made from two different materials—aluminum for the body and plastic materials for the reservoir and reservoir cover. This type of reservoir feeds both primary (closest to the driver) and secondary chambers, and therefore uses a fluid level switch that activates the red dash warning lamp if the brake fluid level drops.





• Master Cylinder Operation– Both sections contain two holes from the

reservoir– The Society of Automotive Engineers’ (SAE)

term for the forward (tapered) hole is the vent port, and the rearward straight drilled hole is called the replenishing port




Figure 94-16 Note the various names for the vent port (front port) and the replenishing port (rear port). Names vary by vehicle and brake component manufacturer. The names vent port and replenishing port are the terms recommended by the Society of Automotive Engineers (SAE).





• Master Cylinder Operation– The vent port might also be called the

high-pressure port or the compensating port





• Master Cylinder Operation– The replenishing port is the low-pressure

rearward, larger diameter hole and might also be called the bypass port, filler port, or breather port





• Master Cylinder Operation– The function of the master cylinder can be

explained from the at-rest, applied, and released positions• At-rest position

– The primary (forward facing) sealing cups are between the compensating port hole and the inlet port hole







– If the temperature of the brake fluid rises and the fluid expands, it is free to expand up into the reservoir through the vent port (compensating port)







– If the fluid was trapped, the pressure of the brake fluid would increase with temperature, causing the brakes to self-apply




Figure 94-17 The vent ports must remain open to allow brake fluid to expand when heated by the friction material and transferred to the caliper and/or wheel cylinder. As the brake fluid increases in temperature, it expands causing the brakes to self-apply which is prevented by the open vent ports.






explained from the at-rest, applied, and released positions• Applied position

–When the brake pedal is depressed, the pedal linkage forces the pushrod and primary piston down the bore of the master cylinder




Figure 94-18 As the brake pedal is depressed, the pushrod moves the primary piston forward, closing off the vent port. As soon as the port is blocked, pressure builds in front of the primary sealing cup, which pushes on the secondary piston. The secondary piston also moves forward, blocking the secondary vent port and building pressure in front of the primary sealing cup.







– The purpose of the replenishing port is to keep the volume behind the primary piston filled with brake fluid from the reservoir as the piston moves forward during a brake application







– This prevents a vacuum from forming behind the piston







– The secondary piston is moved forward as pressure is exerted by the primary piston







– If the primary piston cannot build pressure, a mechanical extension rod on the front of the primary piston will touch the secondary piston and move it forward, as the primary piston is pushed forward by the pushrod and brake pedal







– This causes brake fluid pressure to be applied to both outlets of the master cylinder






explained from the at-rest, applied, and released positions• Released position

– Releasing the brake pedal removes the pressure on the pushrod and master cylinder pistons







– The spring in front of the master cylinder piston expands, pushing the pistons rearward

– At the same time, pressure is released and exerted on the master cylinder pistons, forcing them rearward







– As the piston is pushed back, the lips of the seal fold forward allowing fluid to quickly move past the piston




Figure 94-20 When the brake pedal is released, the master cylinder piston moves rearward. Some of the brake fluid is pushed back up through the replenishing port, but most of the fluid flows past the sealing cup. Therefore, when the driver pumps the brake pedal, the additional fluid in front of the pressure-building sealing cup is available quickly.







– Once the primary seal passes the vent port, the remaining hydraulic pressure forces any excess fluid into the reservoir




MASTER CYLINDER MASTER CYLINDER DESIGNSDESIGNS




Master Cylinder Designs Master Cylinder Designs

• Dual Split Master Cylinders– Dual split master cylinders use two

separate pressure-building sections (one operates the front brakes and the other operates the rear brakes on vehicles equipped with a front/rear-split system)





• Dual Split Master Cylinders– The nose end of the master cylinder is the

closed end toward the front of the vehicle





• Dual Split Master Cylinders– The open end is often called the pushrod

end– The nose end is considered the more

reliable of the two sections





• NOTE: On vehicles equipped with front and rear split master cylinders, the front brakes may or may not be operated from the front chamber. General Motors typically uses the front (nose end) chamber for the front brakes and the rear (pushrod end) for the rear brakes.





• NOTE: Many other makes and models of vehicles use the rear chamber for the front brakes. If in doubt, consult the factory service information for the exact vehicle being serviced.





• Dual Split Master Cylinders– The primary piston extension rod will

mechanically contact and push on the secondary piston to permit operation in the event of a hydraulic failure of the rear section





• Dual Split Master Cylinders– If there is a failure of the front section

hydraulic system, the primary piston operates normally and exerts pressure on the secondary piston

– The secondary piston, however, will not be able to build pressure because of the leak in the system




Figure 94-21 Rear-wheel-drive vehicles use a dual split master cylinder.




Figure 94-22 The primary outlet is the outlet closest to the pushrod end of the master cylinder and the secondary outlet is closest to the nose end of the master cylinder.




Figure 94-23 In the event of a primary system failure, no hydraulic pressure is available to push the second piston forward. As a result, the primary piston extension rod contacts the secondary piston and pushes on the secondary piston mechanically rather than hydraulically. The loss of pressure in the primary system is usually noticed by the driver by a lower-than-normal brake pedal and the lighting of the red brake warning lamp.




Master Cylinder DesignsMaster Cylinder Designs

• Diagonal Split Master Cylinders– The left front brake and the right rear

brake are on one circuit, and the right front with the left rear is another circuit of the master cylinder





• Diagonal Split Master Cylinders– If one circuit fails, the other circuit can

reasonably stop the vehicle because each circuit has one front brake





• Diagonal Split Master Cylinders– To prevent the one front brake from

causing the vehicle to pull toward one side during braking, the front suspension is designed with negative scrub radius geometry to eliminate any handling problem in the event of a brake circuit failure




Figure 94-24 Front-wheel-drive vehicles use a diagonal split master cylinder. In this design one section of the master cylinder operates the right front and the left rear brake and the other section operates the left front and right rear. In the event of a failure in one section, at least one front brake will still function.





• Quick Take-Up Master Cylinders– Quick take-up master cylinders include a

larger diameter primary piston (low-pressure chamber) and a quick take-up valve





• Quick Take-Up Master Cylinders– This type of master cylinder is also called

dual-diameter bore, step-bore, or fast-fill master cylinders





• Quick Take-Up Master Cylinders– A spring-loaded check ball valve holds

pressure on the brake fluid in the large diameter rear chamber of the primary piston





• Quick Take-Up Master Cylinders– When the brakes are first applied, the

movement of the rear larger piston forces a larger volume of brake fluid forward past the primary piston seal and into the primary high-pressure chamber





• Quick Take-Up Master Cylinders– The extra volume of brake fluid “takes up”

the extra clearance of the front disc brake calipers without increasing the brake pedal travel distance




Figure 94-25 Quick take-up master cylinder can be identified by the oversize primary low-pressure chamber.




DIAGNOSING ANDDIAGNOSING ANDTROUBLESHOOTINGTROUBLESHOOTINGMASTER CYLINDERSMASTER CYLINDERS




Diagnosing and Troubleshooting Diagnosing and Troubleshooting Master CylindersMaster Cylinders

• Visual inspection should include checking the following items:1. Brake fluid for proper level and

condition.2. Vent holes in the reservoir cover should

be open and clean.





• Visual inspection should include checking the following items:1. The reservoir cover diaphragm should not

be torn or enlarged.2. Check for any external leaks at the brake

lines or at the pushrod area.





• NOTE: If the rubber cover diaphragm is enlarged, this is an indication that a mineral oil, such as automatic transmission fluid or engine oil, has been used in or near the brake system, because rubber that is brake fluid resistant expands when exposed to mineral oil.




Figure 94-27 Some seepage is normal when a trace of fluid appears on the vacuum booster shell. Excessive leakage, however, indicates a leaking secondary (end) seal.





• After a thorough visual inspection, check for proper operation of pedal height, pedal free play, and pedal reserve distance





• Proper brake pedal height is important for the proper operation of the stop (brake) light switch

• Pedal reserve height is easily checked by depressing the brake pedal with the right foot and attempting to slide your left foot under the brake pedal





• Free play is the distance the brake pedal travels before the primary piston in the master cylinder moves





• Too little or too much free play can cause braking problems that can be mistakenly contributed to a defective master cylinder, such as the following:– Spongy brake pedal





• Too little or too much free play can cause braking problems that can be mistakenly contributed to a defective master cylinder, such as the following:– Lower-than-normal brake pedal





• Too little or too much free play can cause braking problems that can be mistakenly contributed to a defective master cylinder, such as the following:– Sinking brake pedal




Figure 94-28 Pedal height is usually measured from the floor to the top of the brake pedal. Some vehicle manufacturers recommend removing the carpet and measuring from the asphalt matting on the floor for an accurate measurement. Always follow the manufacturer’s recommended procedures and measurements. (Courtesy of Toyota Motor Sales, U.S.A., Inc.)




Figure 94-29 Brake pedal free play is the distance between the brake pedal fully released and the position of the brake pedal when braking resistance is felt. (Courtesy of Toyota Motor Sales, U.S.A., Inc.)




Figure 94-30 Brake pedal reserve is usually specified as the measurement from the floor to the top of the brake pedal with the brakes applied. A quick-and-easy test of pedal reserve is to try to place your left toe underneath the brake pedal while the brake pedal is depressed with your right foot. If your toe will not fit, then pedal reserve may not be sufficient.




MASTER CYLINDER MASTER CYLINDER SERVICESERVICE




Master Cylinder ServiceMaster Cylinder Service

• If disassembling a master cylinder, perform the following steps:– STEP 1: Remove the master cylinder from

the vehicle, being careful not to drip or spill brake fluid onto painted surfaces of the vehicle. Dispose of all old brake fluid and clean the outside of the master cylinder.

– STEP 2: Remove the reservoir, if possible.




Figure 94-31 Using a prybar to carefully remove the reservoir from the master cylinder.





• If disassembling a master cylinder, perform the following steps:– STEP 3: Remove the retaining bolt that

holds the secondary piston assembly in the bore.





• If disassembling a master cylinder, perform the following steps:– STEP 4: Depress the primary piston with a

blunt tool such as a Phillips screwdriver, a rounded wooden dowel, or an engine pushrod. Use of a straight-blade screwdriver or other non-rounded tool can damage and distort the aluminum piston.





• CAUTION: If holding the master cylinder in a vise, use the flange area. Never clamp the body of the master cylinder.

• If disassembling a master cylinder, perform the following steps:– STEP 5: Remove the snap ring and slowly

release the pressure on the depressing tool. Spring pressure should push the primary piston out of the cylinder bore.




Figure 94-32 Whenever disassembling a master cylinder, note the exact order of parts as they are removed. Master cylinder overhaul kits (when available) often include entire piston assemblies rather than the individual seals.





• If disassembling a master cylinder, perform the following steps:– STEP 6: Remove the master cylinder from

the vise and tap the open end of the bore against the top of a workbench to force the secondary piston out of the bore. If necessary, use compressed air in the outlet to force the piston out.





• Inspection and Reassembly of the Master Cylinder– Inspect the master cylinder bore for

pitting, corrosion, or wear– Most cast-iron and aluminum master

cylinders cannot be honed





• Inspection and Reassembly of the Master Cylinder– Thoroughly clean the master cylinder and

any other parts to be reused (except for rubber components) in clean denatured alcohol





• Inspection and Reassembly of the Master Cylinder– If the bore is okay, replacement piston

assemblies can be installed into the master cylinder after dipping them into clean brake fluid.• STEP 1: Install the secondary (smaller)

piston assembly into the bore, spring end first.






assemblies can be installed into the master cylinder after dipping them into clean brake fluid.• STEP 2: Install the primary piston assembly,

spring end first.






assemblies can be installed into the master cylinder after dipping them into clean brake fluid.• STEP 3: Depress the primary piston and

install the snap ring.






assemblies can be installed into the master cylinder after dipping them into clean brake fluid• STEP 4: Install the retaining bolt.






assemblies can be installed into the master cylinder after dipping them into clean brake fluid• STEP 5: Reinstall the plastic reservoir.






assemblies can be installed into the master cylinder after dipping them into clean brake fluid• STEP 6: Bench bleed the master cylinder.

This step is very important. Failure to get all of the air out of the master cylinder can cause a lower than normal brake pedal and a lack of proper braking.





• NOTE: While most master cylinder overhaul kits include the entire piston assemblies, some kits just contain the sealing cups and/or O-rings. Always follow the installation instructions that accompany the kit and always use the installation tool that is included to prevent damage to the replacement seals.




Figure 94-33 Piston assembly.




Figure 94-34 To reinstall the reservoir onto a master cylinder, place the reservoir on a clean flat surface and push the housing down onto the reservoir after coating the rubber seals with brake fluid.




Figure 94-35 Bleeding a master cylinder before installing it on the vehicle. The master cylinder is clamped into a bench vise while using a rounded end of a dowel rod to push on the pushrod end with bleeder tubes down into the brake fluid. Master cylinders should be clamped on the mounting flange as shown to prevent distorting the master cylinder bore.





• Installing the Master Cylinder– After the master cylinder has been bench

bled, it can be installed in the vehicle– Tighten the fasteners to factory

specifications– Bleed the system as needed





• NOTE: Brake fluid can drip from the outlet of the master cylinder and could drip onto the vehicle. Brake fluid is very corrosive and can remove paint. Use fender covers and avoid letting brake fluid touch any component of the vehicle.




Figure 94-36 Installing a master cylinder. Always tighten the retaining fastener and brake lines to factory specifications.




FREQUENTLY FREQUENTLY ASKED QUESTIONASKED QUESTION

• How Much Brake Fluid Is Moved When the Brake Pedal Is Depressed?– During a typical brake application, only

about 1 teaspoon (5 ml or cc) of brake fluid actually is moved from the master cylinder and into the hydraulic system to cause the pressure buildup to occur.

?BACK TO

PRESENTATION




REAL WORLD FIXREAL WORLD FIX

• Bigger Is Not Better– A vehicle owner wanted better braking

performance from his off-road race vehicle. Thinking that a larger master cylinder would help, a technician replaced the original 1-in.-bore-diameter master cylinder with a larger master cylinder with a 1 1/8-in.-borediameter master cylinder.

BACK TO PRESENTATION

• After bleeding the system, the technician was anxious to test-drive the “new” brake system. During the test-drive the technician noticed that the brake pedal “grabbed” much higher than with the original master cylinder. This delighted the technician. The owner of the vehicle was also delighted until he tried to stop from highway speed. The driver had to use both feet to stop!

• The technician realized, after the complaint, that the larger master cylinder was able to move more brake fluid, but with less pressure to the wheel cylinders. The new master cylinder gave the impression of better brakes because the fluid was moved into the wheel cylinders (and calipers) quickly, and the pads and shoes contacted the rotor and drums sooner because of the greater volume of brake fluid moved by the larger pistons in the master cylinder.

• To calculate the difference in pressure between the original (stock) master cylinder and the larger replacement, the technician used Pascal’s law with the following results:

• The difference in pressure is 119 PSI less with the larger master cylinder (573 - 454 = 119).

• The stopping power of the brakes was reduced because the larger diameter master cylinder piston produced lower pressure (the same force was spread over a larger area and this means that the pressure [PSI] is less).

• All master cylinders are sized correctly from the factory for the correct braking effort, pressure, pedal travel, and stopping ability. A technician should never change the sizing of any hydraulic brake component on any vehicle!




TECH TIPTECH TIP

• Don’t Fill the Master Cylinder Without Seeing Me!– The boss explained to the beginning

technician that there are two reasons why the customer should be told not to fill the master cylinder reservoir when the brake fluid is down to the “minimum” mark, as shown in FIGURE 94–13.


Figure 94-13 Master cylinder with brake fluid level at the “max” (maximum) line.

.1 If the master cylinder reservoir is low, there may be a leak that should be repaired.

2. As the brakes wear, the disc brake piston moves outward to maintain the same distance between friction materials and the rotor. Therefore, as the disc brake pads wear, the brake fluid level goes down to compensate.

• Therefore if the brake fluid is low, the vehicle should be serviced—either for new brakes or to repair a leak.




TECH TIPTECH TIP

• Too Much Is Bad– Some vehicle owners or inexperienced

service people may fill the master cylinder to the top. Master cylinders should only be filled to the “maximum” level line or about 1/4 in. (6 mm) from the top to allow room for expansion when the brake fluid gets hot during normal operation.


• If the master cylinder is filled to the top, the expanding brake fluid has no place to expand and the pressure increases. This increased pressure can cause the brakes to “self-apply,” shortening brake friction material life and increasing fuel consumption. Overheated brakes can result and the brake fluid may boil, causing a total loss of braking.




TECH TIPTECH TIP

• Always Check for Venting (Compensation)– Whenever diagnosing any braking

problem, start the diagnosis at the master cylinder—the heart of any braking system. Remove the reservoir cover and observe the brake fluid for spurting while an assistant depresses the brake pedal.


Normal operation (movement of fluid observed in the reservoir)• There should be a squirt or movement of

brake fluid out of the vent port of both the primary and secondary chambers. This indicates that the vent port is open and that the sealing cup is capable of moving fluid upward through the port before the cup seals off the port as it moves forward to pressurize the fluid.

No movement of fluid observed in the reservoir in the primary piston• This indicates that brake fluid is not being

moved as the brake pedal is depressed. This can be caused by the following:

a. Incorrect brake pedal height—brake pedal or pushrod adjustment could be allowing the primary piston to be too far forward, causing the seal cup to be forward of the vent port. Adjust the brake pedal height to a higher level and check for a too-long pushrod length.

b.Defective or swollen rubber sealing cup on the primary piston could cause the cup itself to block the vent port.

• NOTE: If the vent port is blocked for any reason, the brakes of the vehicle may self-apply when the brake fluid heats up during normal braking. Since the vent port is blocked, the expanded hotter brake fluid has no place to expand and instead increases the pressure in the brake lines.

• NOTE: The increase in pressure causes the brakes to apply. Loosening the bleeder valves and releasing the built-up pressure is a check that the brakes are self-applying. Then check the master cylinder to see if it is “venting.”




TECH TIPTECH TIP

• The Brake Pedal Depressor Trick– The master cylinder can be used to block

the flow of brake fluid. Whenever any hydraulic brake component is removed, brake fluid tends to leak out because the master cylinder is usually higher than most other hydraulic components such as wheel cylinders and calipers.


• To prevent brake fluid loss that can easily empty the master cylinder reservoir, simply depress the brake pedal slightly or prop a stick or other pedal depressor to keep the brake pedal down. When the brake pedal is depressed, the piston sealing cups move forward, blocking off the reservoir from the rest of the braking system. The master cylinder stays full and the brake fluid stops dripping out of brake lines that have been disconnected.

Figure 94-26 A brake pedal depressor like this can be used during a wheel alignment to block the flow of brake fluid from the master cylinder during service work on the hydraulic system.

• NOTE: Try this—put a straw into a glass of water. Use a finger to seal the top of the straw and then remove the straw from the glass of water. The water remains in the straw because air cannot get into the top of the straw. This is why the brake pedal depressor trick works to prevent the loss of brake fluid from the system even if the brake line is totally disconnected.




TECH TIPTECH TIP

• Check for Bypassing– If a master cylinder is leaking internally,

brake fluid can be pumped from the rear chamber into the front chamber of the master cylinder. This internal leakage is called bypassing. When the fluid bypasses, the front chamber can overflow while emptying the rear chamber.


• Therefore, whenever checking the level of brake fluid, do not think that a low rear reservoir is always due to an external leak. Also, a master cylinder that is bypassing (leaking internally) will usually cause a lower-than-normal brake pedal.




WARNINGWARNING

Use extreme care when using compressed air. The piston can be shot out of the master cylinder with great force, which could cause personal injury.


Halderman ch094 lecture

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