Drum brakes use two brake shoes mounted on a stationary backing plate. These shoes are positioned inside a circular drum which rotates with the wheel assembly. On these vehicles, the shoes are held in place by one large retractor spring. This allows the brake shoes to slide toward the drum (when the brake are applied) while keeping the linings and drums in alignment. The shoes are actuated by a wheel cylinder which is mounted at the top of the backing plate. When the brakes are applied, hydraulic pressure forces the wheel cylinder's pistons outward. Since the small projections on the brake shoe bear directly against the wheel cylinder pistons, the tops of the shoes are then forced against the inner side of the drum.
The W-Body vehicles covered by this manual may have either rear disc or drum brakes, depending on the vehicle, model and options. On these vehicles, the drum brake is a leading/trailing design. In the leading/trailing brake, the force from the wheel cylinder is applied equally to both shoes. Torque from the brake shoes is transferred through the backing plate, to the axle flange. Adjustment is automatic when the brakes are applied. When pressure within the wheel cylinder is relaxed, return springs pull the shoes back away from the drum.
Most modern drum brakes are designed to self-adjust themselves during application when the vehicle is moving in reverse. This motion causes both shoes to rotate very slightly with the drum, rocking an adjusting lever, thereby causing rotation of the adjusting screw.