10. 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
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12. Figure 69–5 The amount of force on the piston is the result of pressure multiplied by the surface area. Continued
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27. 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.
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36. 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
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42. 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.
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44. 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
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46. 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.
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51. 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
52. 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
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56. Figure 69–28 Quick take-up master cylinder can be identified by the oversize primary low pressure chamber. Continued
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58. 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
59. 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
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62. 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.
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71. 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.
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75. 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.