Chap69

[object Object],[object Object],[object Object],OBJECTIVES: After studying Chapter 69, the reader should be able to: Continued

[object Object],[object Object],[object Object],OBJECTIVES: After studying Chapter 69, the reader should be able to:

[object Object],KEY TERMS: Continued

HYDRAULIC PRINCIPLES ,[object Object],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 ,[object Object],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. ,[object Object],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.

PASCAL’S LAW ,[object Object],Continued

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

[object Object],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

[object Object],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).

[object Object],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).

[object Object],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.

[object Object],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:

[object Object],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. ,[object Object],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

[object Object],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. ,[object Object],Continued

Figure 69–12 The decrease in application force created by a small wheel cylinder piston is offset by an increase in piston travel. ,[object Object],With a dual- piston cylinder, the total travel is divided between the two pistons. Continued

MASTER CYLINDERS ,[object Object],Continued

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. ,[object Object],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

[object Object],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. ,[object Object],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

[object Object],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. ,[object Object],[object Object],Figure 69–15 Master cylinder with brake fluid level at the “min” (minimum) line.

[object Object],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.

[object Object],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!

[object Object],Too Much is Bad

MASTER CYLINDER OPERATION ,[object Object],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.

[object Object],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

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

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. ,[object Object],Continued

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. ,[object Object],Continued

[object Object],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

[object Object],[object Object],Continued

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 ,[object Object],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

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. ,[object Object],Continued

[object Object],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: ,[object Object],[object Object]

DIAGONAL-SPLIT MASTER CYLINDERS ,[object Object],Continued

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

QUICK TAKE-UP MASTER CYLINDERS ,[object Object],Continued

Figure 69–28 Quick take-up master cylinder can be identified by the oversize primary low pressure chamber. Continued

[object Object],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

[object Object],DIAGNOSING AND TROUBLESHOOTING MASTER CYLINDERS Continued ,[object Object],[object Object],[object Object],[object Object]

[object Object],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. ,[object Object],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. ,[object Object],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. ,[object Object],Continued

[object Object],Continued NOTE: A lower than normal brake pedal may also be an indication of air in the hydraulic system.

[object Object],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 ,[object Object],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) ,[object Object],Step #3 Remove the retaining bolt that holds the secondary piston assembly in the bore. Continued

[object Object],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.

[object Object],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) ,[object Object],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) ,[object Object],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.

[object Object],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 ,[object Object],Figure 69–42 Installing a master cylinder. Always tighten the retaining fastener and brake lines to factory specifications. Bleed the system as needed.

SUMMARY ,[object Object],[object Object],[object Object],[object Object],Continued

SUMMARY ,[object Object],[object Object],[object Object],[object Object],( cont. )

Chap69

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Chap69