196 Auto Brakes
Figure 12-7. A brake caliper which is attached to the spindle (steering knuckle) with mounting bolts. (Chrysler)
Brake pad
Outer caliper
housing
Brake shoe
Piston dust boot
Wheel
mounting stud
hub bolt
Hub
Bearing
Steering knuckle
Teflon seal
Slinger
Splash shield
mounting
bolt
Caliper
mounting
bolt
Inner
caliper
housing
Piston
spring
Bleeder
screw
Seal
Fluid
passage
Rotor
Piston
seal
Piston
front wheels, Figure 12-7. Calipers have a dust boot over
every piston to keep dust and water away from the inner
seals, to retract the piston, and to reduce the possibility of
sticking.
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. This pressure travels to the caliper
and causes the piston to move outward. Piston move-
ment forces the pads into contact with the rotor, causing
friction which slows the rotor and the vehicle. The piston
seal prevents leaks and retracts the piston slightly when
hydraulic pressure is removed. This 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 effect as drum
brakes. For this reason, power assist units are used on most
modern vehicles.
Pad-to-Rotor Clearance
Modern disc brakes have almost no clearance
between the lining and rotor when the brakes are released.
Because residual pressure check valves are not used with
disc brakes, no static pressure is present to apply the pads.
Therefore, even with very low clearances, there is almost
no brake drag or wear.