Chapter 12 Disc Brake System Components and Operation 179 Copyright by Goodheart-Willcox Co., Inc. Caliper Operation As discussed in Chapter 5, all calipers operate in roughly the same manner. When the brake pedal is pressed, the master cylinder develops hydraulic pressure in the brake system. Th is pressure travels to the caliper and causes the piston to move outward. Piston movement forces the pads into con- tact with the rotor, causing friction that slows the rotor and the vehicle. Th e piston seal prevents leaks and retracts the piston slightly when hydraulic pressure is removed. Th is allows the linings to just clear the rotor. Higher Pedal Effort Unlike drum brakes, disc brakes have no servo action. Servo action means that wheel rotation helps the brakes to apply. With no servo action, disc brakes require more pedal force to achieve the same braking eff ect as drum brakes. For this reason, power assist units are used on most modern vehicles. Integral braking (rotor) and hub Inner bearing Wheel stud Outer bearing Spindle Shoe and lining Wheel Boot Seal Piston Mounting bolt Adapter Mounting bolt Splash shield Steering knuckle Seal Caliper Spindle Caliper Splash shield Rotor Wheel Chrysler Figure 12-5. A disc brake assembly that incorporates the brake rotor and hub into an integral (one-piece) unit. Compare this to the sectional rotor/hub assembly shown in Figure 12-4. Ford Figure 12-6. An exploded view of a front brake assembly that uses a splash shield and vehicle wheel to protect the brake assembly from dirt, water, stones, etc.