On October 23rd, 2014, we updated our
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bore master cylinder • breather port • bypass port • bypassing compensating port diagonal split master cylinder • dual split master cylinder fast-fill master cylinder • filler port hydraulic system • master cylinder Pascal’s Law • pedal free play • pedal height • pedal reserve distance • piston assemblies
KEY TERMS: Continued
quick take-up master cylinder replenishing port self-apply • step-bore master cylinder vent port
In addition to mechanical advantage provided by leverage, all vehicles use hydraulic pressure to increase brake application force.
Continued Figure 69–1 Hydraulic brake lines transfer the brake effort to each brake assembly attached to all four wheels. All braking systems require that a driver’s force is transmitted to the drum or rotor attached to each wheel. The force exerted on the brake pedal varies due to strength and size of the driver. Engineers design brake systems to require less than 150 lb of force (68 kg) from the driver, yet provide the force necessary to stop a heavy vehicle from high speed.
NONCOMPRESSIBILITY OF LIQUIDS
Hydraulic systems use liquids to transmit motion. For all practical purposes, a liquid cannot be compressed.
Continued Figure 69–2 Because liquids cannot be compressed, they are able to transmit motion in a closed system. No matter how much pressure or force is placed on a quantity of liquid, its volume will remain the same. This fact enables liquids in a closed system to transmit motion.
Figure 69–3 Hydraulic systems must be free of air to operate properly.
Even though piston A is moved 1 in., piston B will not move if the load on it is greater than the pressure of the air in the system.
Continued Liquids cannot be compressed, but any air trapped in the system can be compressed. The hydraulic system is air-contaminated. If the load on piston B is 50 pounds per square inch (psi), the movement of piston A must compress the air in the system to that same pressure before piston B will move. A brake system must be air free or there will be serious problems.
The hydraulic principles that permit a brake system to function were discovered by a French physicist, 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.” Assume a force of 10 lb is exerted on a piston with a surface area of 1 square inch (sq. in.). Since this force measured in lb or Newtons (N) is applied to a piston with an area measured in square inches (sq. in.), the pressure is the force divided by the area or “10 pounds per square inch” (psi). It is this “pressure” that is transmitted, without loss, throughout the entire hydraulic system. See Figure 69–4.
Figure 69–4 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. Continued
If you know two out of the three factors, you can calculate the other using this formula:
Continued See the formula on Page 826 of your textbook. A practical example involves a master cylinder with a piston area of 1 sq. in., and one wheel cylinder with an area of 1 sq. in., and one wheel cylinder with a piston area of 2 sq. in. See Figure 69–5.
Figure 69–5 The amount of force on the piston is the result of pressure multiplied by the surface area. Continued
The real “magic” of a hydraulic brake system is the fact that different forces can be created at different wheel cylinders. More force is necessary for front brakes than for rear brakes because, as the brakes are applied, the weight of the vehicle moves forward. Larger (area) pistons are used in wheel cylinders (calipers, if disc brakes) on front wheels to increase force to apply front brakes. Not only can hydraulics act as a “force machine” (by varying piston size), but the hydraulic system also can be varied to change piston stroke distances. On a typical vehicle, a driver-input force of 150 lb (660 Newtons) is boosted both mechanically (through the brake pedal linkage) and by the power booster to a fluid pressure of about 1700 psi (11,700 kPa).
With a drum brake, the wheel cylinder expands and pushes the brake shoes against a brake drum.
Figure 69–6 Drum brake illustrating the typical clearance between the brake shoes (friction material) and the rotating brake drum represented as the outermost black circle. Continued NOTE: 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. The distance the shoes move is about 0.005–0.012 in. (5 to 12 thousandths of an inch) (0.015–0.30 mm).
With a disc brake, brake fluid pressure pushes on the piston in the caliper a small amount and causes a clamping of the disc brake pads against both sides of a rotor (disc).
Figure 69–7 The brake pad (friction material) is pressed on both sides of the rotating rotor by the hydraulic pressure of the caliper. Continued The typical distance the pads move is only about 0.001–0.003 in. (1-to 3-thousandths-of an inch) (0.025–0.076 mm).
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 reads:
Figure 69–8 Mechanical force and the master cylinder piston area determine the hydraulic pressure in the brake system. Continued The result in this case is 100 psi of brake system hydraulic pressure.
Doubling the area of the master cylinder piston cuts the hydraulic system pressure in half. Conversely, if the same 100-lb force is applied to a master cylinder piston with only half the area (0.5 or 1/2 sq. in.), the equation will show the system pressure is doubled:
Continued However, if the same 100-lb force is applied to a master cylinder piston with twice the area (2 sq. in.), the equation will read:
Application Force and Piston Size While size of the master cylinder piston affects hydraulic pressure of the entire brake system, weight shift and bias require the heavily loaded front brakes receive much higher application force than the lightly loaded rear brakes.
Pascal states that pressurized liquid in confined space acts with equal pressure on equal areas . 100 psi from the master cylinder will result in 100 psi of friction assembly application force. Continued
Figure 69–10 Differences in brake caliper and wheel cylinder piston area have a major effect on brake application force.
It is piston surface area , not diameter, that affects force.
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. Continued
Piston Size versus Piston Travel In disc brakes, mechanical force available to apply the brakes is four times greater because of size differences between master cylinder and caliper pistons.
Continued Some hydraulic energy is converted into increased mechanical force. The tradeoff is the larger caliper piston with greater force will not move as far as the smaller master cylinder piston. Hydraulic energy converted into mechanical motion is decreased . The relative movement of pistons within the brake system can be calculated with the following equation:
Figure 69–11 The increase in application force created by the large brake caliper piston is offset by a decrease in piston travel.
The results show that if the master cylinder piston stroke is 1 in., the caliper piston will move only 1/4 inch.
Figure 69–12 The decrease in application force created by a small wheel cylinder piston is offset by an increase in piston travel.
If caliper piston area were reduced to 2 sq. in., application force would increase to only 200 lb, the caliper piston would travel 1/2 inch for a 1 in. master cylinder stroke.
With a dual- piston cylinder, the total travel is divided between the two pistons. Continued
Hydraulic Principles and Brake Design When a brake system is designed, the hydraulic relationships discussed above play a major part in determining the sizes of pistons within the system. The piston sizes selected 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 friction assemblies.
The master cylinder is the heart of the entire braking system. No braking occurs until the driver depresses the brake pedal. The brake pedal linkage applies the force of the driver’s foot into a closed hydraulic system. Master Cylinder Reservoirs Most vehicles built since the early 1980s are equipped with see-through master cylinder reservoirs, which permit owners and service technicians to check the brake fluid level without having to remove the top of the reservoir. Some countries have laws that require this type of reservoir. See Figure 69–13.
Figure 69–13 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.
Reservoir capacity is great enough to allow for the brakes to become completely worn out and still have enough reserve for safe operation.
The typical capacity of the entire braking system is usually 2 to 3 pints (1 to 1.5 liters). Vehicles equipped with four-wheel disc brakes usually hold 4 pints (2 liters) or more. Continued
Master Cylinder Reservoir Diaphragm The entire system is filled with brake fluid to the “full” level of the master cylinder reservoir, which is vented to the atmosphere so the fluid can expand and contract without difficulty as would be the case if the reservoir were sealed.
Continued CAUTION: The master cylinder should never be filled higher than the recommended full mark to allow for fluid expansion that occurs normally when the brake fluid gets hot due to the heat generated by the brakes.
Continued Being open to the atmosphere allows the possibility of moisture-laden air coming in contact with the brake fluid! Moisture in the air is readily and rapidly absorbed into the brake fluid because brake fluid has an affinity (attraction) to moisture (water). Master cylinders use a rubber diaphragm or floating disc to help seal outside air from direct contact with brake fluid. 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.
Figure 69–14 All master cylinders should have a vent hole on the outside cover that allows air between the cover and the rubber 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 fluid.
Whenever servicing a brake system, be sure to check the vent hole is clear on the cover to allow air to get between the cover and the diaphragm. Continued
The boss explained to the beginning tech 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 here.
Don’t Fill the Master Cylinder Without Seeing Me! If a customer notices that brake fluid is low in the master cylinder reservoir, the vehicle should be serviced—either for new brakes or to repair a leak.
As the brakes wear, the brake piston moves outward to maintain the same distance between friction materials and the rotor. As disc brake pads wear, brake fluid level goes down to compensate.
If the master cylinder reservoir is low, there may be a leak that should be repaired.
Figure 69–15 Master cylinder with brake fluid level at the “min” (minimum) line.
A vehicle owner wanted better braking performance from his off-road race vehicle. Thinking that a larger master cylinder would help, a tech replaced the original 1-in.-bore-diameter master cylinder with a larger master cylinder with a 1 1/8-in.-bore-diameter master cylinder. After bleeding the system, the tech was anxious to test-drive the “new” brake system. During the test-drive the tech noticed that the brake pedal “grabbed” much higher than with the original master cylinder. This delighted the tech.
Is Bigger Better? - Part 1 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 tech 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 master cylinder and the larger replacement, the tech used Pascal’s Law with the following results:
Is Bigger Better? - Part 2 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 correct braking effort, pressure, pedal travel, and stopping ability. A tech should never change sizing of any hydraulic brake component on a vehicle!
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.
Too Much is Bad
MASTER CYLINDER OPERATION
The master cylinder is the heart of any hydraulic braking system. Brake pedal movement and force are transferred to the brake fluid and directed to wheel cylinders or calipers.
Continued Figure 69–16 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.
The master cylinder is also separated into two pressure-building chambers (or circuits) to provide braking force to one-half of the brake in the event of a leak or damage to one circuit.
Figure 69–17 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 and secondary chambers, and therefore uses a fluid level switch that activates the red dash warning lamp if the brake fluid level drops. Continued
Both pressure-building sections of the master cylinder contain two holes from the reservoir. The SAE term for the forward (tapered) hole is the vent port , and the rearward straight drilled hole is called the replenishing port . Various vehicle and brake component manufacturers call these ports by various names. The vent port is the high- pressure port. This tapered forward hole is also called the compensating port . The replenishing port is the low-pressure rearward, larger diameter hole. The inlet port is also called the bypass port , filler port , or breather port .
Figure 69–18 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). Continued
At-Rest Position The primary sealing cups are between the compensating port hole and the inlet port hole. In this position, the brake fluid is free to expand and move from the calipers, wheel cylinders, and brake lines up into the reservoir through the vent port (compensation port) if the temperature rises and the fluid expands. If the fluid was trapped, the pressure of the brake fluid would increase with temperature, causing the brakes to self - apply . See Figure 69–19.
Figure 69–19 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. The heated brake fluid can expand and flow back into the reservoir through the vent ports.
The pistons (primary and secondary) are retained by a clip at the pushrod end and held in position by return springs.
Figure 69–20 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 sealing cup.
Applied Position When the brake pedal is pressed, pedal linkage forces the push rod and primary piston down the bore of the master cylinder.
As the piston moves forward, the primary sealing cup covers and blocks off the vent port (compensating port). Hydraulic pressure builds in front of the primary seal as the pushrod moves forward. The back of the piston is kept filled through the replenishing port.
Figure 69–21 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. Continued
Released Position Releasing the brake pedal removes the pressure on the pushrod and master cylinder pistons. A spring on the brake pedal linkage returns the brake pedal to its normal at-rest (up) position. The spring in front of the master cylinder piston expands, pushing the pistons rearward. At the same time, pressure is released from the entire braking system and the released brake fluid pressure is 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.
See Figure 69–22.
Figure 69–22 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.
DUAL-SPLIT MASTER CYLINDERS
Dual split master cylinders use two separate pressure-building sections. One section operates front brakes and the other section the rear brakes on vehicles equipped with a front/rear-split system.
Continued The nose end of the master cylinder is the closed end toward the front of the vehicle. The open end is often called the pushrod end of the master cylinder. See Figure 69–24.
Figure 69–24 The primary outlet is the outlet closest to the pushrod end of the master cylinder and the second outlet is closest to the nose end of the master cylinder. Continued
Some manufacturers operate the front brakes (which do the most braking) from the “nose end” section (secondary piston end) of the master cylinder. The secondary piston has only one pressure-building seal. The primary piston (pushrod end) requires two (2) seals to build pressure. The nose end of the master cylinder is considered the more reliable of the two pressure-building sections.
Continued If the rear section of the system fails, the primary piston will not build pressure to operate the secondary piston. To permit the operation of the secondary (nose end) piston in the event of a hydraulic failure of the rear section, the primary piston extension will mechanically contact and push on the secondary piston. NOTE: On vehicles equipped with front and rear split master cylinders, the front brakes may or may not be operated from the front chamber. GM typically uses the front (nose end) chamber for the front brakes and the rear (pushrod end) for the rear brakes. Many other makes and models of vehicles use the rear chamber for the front brakes. If in doubt, consult the factory service manual for the exact vehicle being serviced.
Figure 69–25 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 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.
The secondary piston, will not be able to build pressure due to the leak in the system.
Whenever diagnosing any braking problem, start 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.
Always Check for Venting (Compensation) - Part 1 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 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. 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. 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.”
Always Check for Venting (Compensation) - Part 2 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:
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.
A defective or swollen rubber sealing cup on the primary piston could cause the cup itself to block the vent port.
DIAGONAL-SPLIT MASTER CYLINDERS
With front-wheel drive vehicles, the weight of the entire power train is on the front wheels and 80% to 90% of the braking force is achieved by the front brakes. This means that only 10% to 20% of the braking force is being handled by the rear brakes. If the front brakes fail, the rear brakes alone would not provide adequate braking force. The solution is the use of a diagonal split master cylinder . See Figures 69–26 and 69–27.
Figure 69–26 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. Continued
Figure 69–27 Typical General Motors diagonal split master cylinder. Notice the two aluminum proportioner valves. These valves limit and control brake fluid pressure to the rear brakes to help eliminate rear wheel lockup during a rapid stop. Continued
In a diagonal split braking system, 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. If one circuit fails, the remaining circuit can still stop the vehicle in a reasonable fashion because each circuit has one front brake. To prevent this one front brake from causing the vehicle to pull toward one side during braking, the front suspension is designed with negative scrub radius geometry. This effectively eliminates any handling problem in the event of a brake circuit failure.
QUICK TAKE-UP MASTER CYLINDERS
Many newer vehicles use low drag disc brake calipers to increase fuel economy. Due to the larger distance between the rotor and the friction pads, excessive brake pedal travel would be required before the pads touched the rotor. The solution is a a quick take - up master cylinder , which includes a larger diameter primary piston (low-pressure chamber) and a quick take-up valve. This type is also called dual - diameter bore , step - bore , or fast - fill master cylinders .
A spring-loaded check ball valve holds pressure on the brake fluid in the large diameter rear chamber of the primary piston. When the brakes are first applied, the movement of the rear larger piston forces this larger volume of brake fluid forward past the primary piston seal and into the primary high-pressure chamber. This extra volume of brake fluid “takes up” the extra clearance of the front disc brake calipers without increasing the brake pedal travel distance. See Figure 69–28.
Figure 69–28 Quick take-up master cylinder can be identified by the oversize primary low pressure chamber. Continued
At 70 to 100 psi, the check ball valve in the quick take-up valve allows fluid to return to the brake fluid reservoir.
Figure 69–29 The quick take-up valve controls fluid flow to and from the primary low pressure chamber. Continued The quick take-up “works” until 100 psi is reached, and a metering valve is not required to hold back fluid pressure to the front brakes. See Figures 69–30 and 69–31.
Figure 69–30 As the brakes are applied, reduced low-pressure chamber volume results in a pressure increase that causes fluid to bypass the primary cup seal. Continued
Figure 69–31 The one-way sealing abilities of both a spring-loaded check ball and a cup seal are used in the quick take-up valve. Continued
A thorough visual inspection is important when inspecting any master cylinder. The visual inspection should include checking the following items:
DIAGNOSING AND TROUBLESHOOTING MASTER CYLINDERS Continued
Check the brake fluid for proper level and condition. (Brake fluid should not be rusty, thick, or contaminated.)
Check that the vent holes in the reservoir cover are open and clean.
Check that the reservoir cover diaphragm is not torn or enlarged.
Check for any external leaks at the lines or at the pushrod area.
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
The Brake Pedal Depressor Trick To prevent brake fluid loss that can easily empty the master cylinder reservoir, depress the brake pedal slightly or prop a stick or other pedal depressor to keep the pedal down. When the pedal is depressed, the piston sealing cups move forward, blocking the reservoir from the rest of the system. The master cylinder stays full and the brake fluid stops dripping out of brake lines that have been disconnected. Figure 69–32 A brake pedal depressor like this, normally used during a wheel alignment, can be used to block the flow of brake fluid from the master cylinder during service work on the hydraulic system.
NOTE: If the 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 69–33 Some seepage is normal when a trace of fluid appears on the vacuum booster shell. Excessive leakage, however, indicates a leaking secondary (end) seal.
Figure 69–34 Pedal height is usually measured from the floor to the top of the brake pedal. Some vehicle manufacturers recommend removing the carpet and measure from the asphalt matting on the floor for an accurate measurement. Always follow the manufacturer’s recommended procedures and measurements.
After a thorough visual inspection, check for proper operation of pedal height , pedal free play , and pedal reserve distance . See Figure 69–35.
Continued Proper brake pedal height is important for operation of the stop (brake) light switch.
Figure 69–35 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.
Free play is the distance the brake pedal travels before the primary piston in the master cylinder moves.
Most vehicles require brake pedal free play between 1/8 and 1 1/2 in. (3 to 38 mm). Too little or too much free play can cause braking problems that can be mistakenly contributed to a defective master cylinder. Continued
Figure 69–36 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.
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.
Spongy Brake Pedal A spongy pedal with a larger than normal travel indicates air in the lines. Check for leaks and bleed the air from the system as discussed later in this chapter. Lower Than Normal Brake Pedal A brake pedal that travels downward more than normal and then gets firm is an indication that one circuit of the dual-circuit hydraulic system is probably not working. Check for leaks in the system and repair as necessary. Another possible reason is an out-of-adjustment drum brake allowing too much pedal travel before the shoes touch the brake drum.
Continued NOTE: A lower than normal brake pedal may also be an indication of air in the hydraulic system.
Sinking Brake Pedal If the brake pedal sinks all the way to the floor, suspect a defective master cylinder that is leaking internally. This internal leakage is often called bypassing because the brake fluid is leaking past the sealing cup.
NOTE: A sinking brake pedal, on a vehicle equipped with an antilock braking system (ABS), could be caused by a defective dump valve.
DISASSEMLY OF THE MASTER CYLINDER
Many master cylinders can be disassembled, cleaned, and restored to service.
Continued NOTE: Check the vehicle manufacturer’s recommendation before attempting to overhaul or service a master cylinder. Many manufacturers recommend replacing the master cylinder as an assembly. Step #1 Remove the master cylinder from the vehicle, being careful to avoid dripping or spilling brake fluid onto painted surfaces of the vehicle. Dispose of all old brake fluid and clean the outside of the master cylinder.
Figure 69–37 Using a pry bar to remove the reservoir from the master cylinder. (Courtesy of Allied Signal Automotive Aftermarket)
Step #2 Remove the reservoir, if possible, as shown here.
Step #3 Remove the retaining bolt that holds the secondary piston assembly in the bore. Continued
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.
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 nonrounded 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. Continued
Figure 69–38 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. 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. CAUTION: Use extreme care when using compressed air. The piston can be shot out of the master cylinder with a great force.
Thoroughly clean the master cylinder and any other parts to be reused (except rubber components) in clean denatured alcohol. If the bore is OK, replacement piston assemblies can be installed into the master cylinder after dipping them into clean brake fluid.
INSPECTION AND REASSEMBLY OF THE MASTER CYLINDER Continued 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 69–39 Piston assembly. (Courtesy of Allied Signal Automotive Aftermarket)
Step #1 Install the secondary (smaller) piston assembly into the bore, spring end first.
Continued 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 instructions that accompany the kit and always use the installation tool included to prevent damage to the replacement seals.
Figure 69–40 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. (Courtesy of Allied Signal Automotive Aftermarket)
Step #2 Install the primary piston assembly, spring end first.
Continued Step #3 Depress the primary piston and install the snap ring. Step #4 Install the stop bolt. Step #5 Reinstall the plastic reservoir, if equipped, as shown at left. Step #6 Bench bleed the master cylinder. This step is very important. See Figure 69–41.
Figure 69–41 Bleeding a master cylinder before installing it on the vehicle. The master cylinder is clamped into a bench vise while using the rounded end of a breaker bar 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.
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 fluid bypasses, the front chamber can overflow while emptying the rear chamber. 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.
Check for Bypassing 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.
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.
Figure 69–42 Installing a master cylinder. Always tighten the retaining fastener and brake lines to factory specifications. Bleed the system as needed.
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.
Pascal’s Law states that: “When a force is applied to a liquid confined in a container or enclosure, the pressure is transmitted equally and undiminished in every direction.”
Master cylinder reservoirs are large enough for the brakes to be worn completely down and still have a small reserve.
The front port of the master cylinder is called the compensating port and the rear port is called the inlet port.
Brake system diagnosis should always start with checking for venting (compensation).
Dual split master cylinders that separate the front brakes from the rear brakes are used on rear-wheel-drive vehicles.
Diagonal split master cylinders that separate right front and left rear from the left front and right rear brakes are used on front-wheel-drive vehicles.
Some master cylinders can be rebuilt, but the cylinder bore should not be honed unless recommended by the manufacturer.