chapter 32 Disc Brake System
© SA 20 M 12 PL Jo E ne C s H & AP B TE art R let O tL N ea LY r n
I n disc brake calipers, the piston is sealed by a stationary square section sealing ring, also called a square cut O-ring FIGURE 32-11 . This O-ring has a square crosssection and is fi tted in a machined groove in the caliper. The O-ring is compressed between the piston and
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O-Rings
caliper housing, creating a positive seal to keep the high-pressure brake fluid from leaking out. It also prevents air from being drawn into the system when a low-pressure situation is created in the hydraulic system when the brake pedal is released quickly. hen the brakes are applied, the piston moves W outward, slightly deforming the O-ring seal FIGURE 32-12 . When the brakes are released, the elasticity or fl exibility of the seal causes it to return to its original shape. This action of the sealing ring retracts the piston to provide a small running clearance between the rotor and pads. As the brake pads wear, the piston needs to move outward a bit farther than the sealing ring can stretch or fl ex. The sealing ring is designed to allow the piston to slide through it in this situation, taking up the extra clearance and making the disc brakes self-adjusting.
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hen the brakes are applied, hydraulic pressure W f orces the piston toward the rotor. This takes up any clearance between the brake pad and rotor and starts to push the pad into the rotor. Since the caliper is free to move on the pins or slides, it gets pushed away from the rotor, pulling the outer brake pad into contact with the outside of the rotor. Once all clearance is taken up on the outer brake pad, the clamping force will increase equally on both brake pads, applying the brakes FIGURE 32-10 .
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Text that prepares students for the shop.
TECHNICIAN TIP
S ince the force generated by the O-ring to retract the piston is fairly small, any corrosion or buildup on the piston or bore will cause the piston to stick and not retract. This holds the brakes in the applied position, causing brake drag, overheated brakes, and poor fuel economy. Technicians can identify this situation by using an infrared temperature gun to measure the temperature of each brake rotor after test-driving the vehicle. The temperatures should be approximately the same on each side. If they are not, suspect a stuck or binding caliper.
FIGURE 32-10 32-10 Sliding/floating caliper application.
A
B
O-ring groove in caliper
FIGURE 32-11 32-11 A. Square cut O-ring and O-ring cut to show square section. B. Square cut O-ring groove in caliper.