Applying the brakes causes the wheel cylinder
piston to force the leading edge of the primary brake shoe and lining assembly
against the rotating brake drum. The primary brake shoe tries to rotate
with the drum and transfers force to the secondary shoe and lining assembly
through the adjusting screw assembly. The secondary shoe's lining leading
edge grips the drum and tries to rotate just like the primary shoe. Since
the shoes cannot rotate, they wedge themselves into the drum. The rotating
torque from the shoes increases the braking force applied by the wheel cylinder.
Because of this wedging action, the design is a duo-servo (as opposed to
a single-servo design where the wheel cylinder pressure alone is the source
of braking force).
The brake shoes transfer the torque through the backing plate to the
axle flange. Brake adjustments are automatic and occur during reverse brake
applications.
Servicing Information
When servicing drum brake components, do the following:
| • | Replace all components included in the repair kits. |
| • | Lubricate the parts as specified. |
| • | Do not use lubricated shop air on the brake parts. Shop air can
damage the rubber components. |
| • | If necessary, bleed all or part of the brake system after any
hydraulic component has been removed or disconnected. |
| • | Replace brake pads in axle sets only. |
| • | The torques specified are for dry, unlubricated fasteners. |
| • | Perform service operations on a clean bench. Verify that the bench
is free from mineral oil and other contaminants. |
| • | Use extreme care when doing any work around antilock components
in order to prevent damage or misalignment. |
| • | Pump the brake pedal several times in order to make sure it is
firm prior to moving the vehicle. Do not move the vehicle until a firm pedal
has been obtained. Check the brake fluid level in the master cylinder after
pumping the brakes. |
