Drum Brake System: Description and Operation

This drum brake assembly is a leading/trailing shoe design. Both brake shoes are held against the wheel cylinder pistons by the upper return spring and the fixed anchor plate by the lower return spring. When the brakes are applied, the wheel cylinder pistons move both shoes out to contact the drum. With forward wheel rotation, the forward brake shoe will wrap into the drum and become self-energized. With reverse wheel rotation, the rear brake shoe is self-energized. Force from the brake shoes is transferred to the anchor plate through the backing plate to the axle flange. Adjustment is automatic and occurs on any service brake application. Also, with leading/trailing brakes, it is normal for the front (leading) shoe to wear at a faster rate than the rear (trailing) shoe. Do not switch the position of shoes that have been in service as this may render the self adjustment feature inoperative and result in increased pedal travel.

Important

- Replace all components included in repair kits used to service this drum brake.
- Lubricate parts as specified.
- Do not use lubricated shop air on brake parts as damage to rubber components may result.
- If any hydraulic components is removed or disconnected, it may be necessary to bleed all or part of the brake system.
- Replace shoe and linings in axle sets only.
- The torque values specified are for dry, unlubricated fasteners.
- Perform service operations on a clean bench free from all mineral oil materials.

CAUTION: When servicing brake parts, do not create dust by grinding or sanding brake linings or by cleaning wheel brake parts with a dry brush or with compressed air. A water dampened cloth should be used. Many brake parts contain asbestos fibers which can become airborne if dust is created during servicing. Breathing dust containing asbestos fibers may cause serious bodily harm.