CAUTION
Brake shoes may contain asbestos, which has been determined to be a cancer causing
agent. never clean the brake surfaces with Compressed air! Avoid inhaling any
dust from any brake surface! When cleaning brake surfaces, use a commercially
available brake cleaning fluid.
Drum brakes employ two brake shoes mounted on a stationary backing plate. These shoes are positioned inside a circular cast iron drum which rotates with the wheel. The shoes are held in place by springs; this allows them to slide toward the drum (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 inside 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 the pressure within the wheel cylinder is relaxed, return springs pull the shoes away from the drum.
Most modern drum brakes are designed to self-adjust 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 and thereby causing rotation of the adjusting screw via a star wheel. This on-board adjustment system reduces the need for maintenance adjustments but most drivers don't back up enough to keep the brakes properly set.