Drum brakes employ tow brake shoes mounted on a stationary backing plate. These shoes are positioned inside a circular cast iron drum which rotates with the wheel assembly. The shoes are held in place by springs; this allows them to slide toward the drums (when they 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 two actuating links outward. Since these links bear directly against the top of the brake shoes, the tops of the shoes are then forced outward against the inner side of the drum. This action forces the bottoms of the two shoes to contact the brake drum by rotating the entire assembly slightly (known as servo action). 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 by means of a star wheel.
The duo-servo brake with pin and slot adjusters is a new design used only on the Chevette through 1979.
The rear drum brake system introduced in 1980 is a duo-servo and direct torque design. In the duo-servo design the force which the wheel cylinder applies to the shoes is supplemented by the tendency of the shoes to wrap into the drum during braking. With the direct torque design, torque from the brake shoes is transferred directly through the anchor pin to the control arm.