Duo-Servo Drum Brakes
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
| • | 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 shoes 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. |
