A drum brake assembly consists of a cast-iron drum, which is bolted to and rotates with the vehicle wheel, and a fixed backing plate to which the shoes, wheel cylinders, automatic adjusters, and linkages are attached (Figure 46-21). Additionally, there might be some extra
hardware for parking brakes. The shoes are surfaced with frictional linings, which contact the inside of the drum when the brakes are applied. The shoes are forced outward by pistons located inside the wheel cylinder. They are actuated by hydraulic pressure. As the drum rubs against the shoes, the energy of the moving drum is transformed into heat. This heat energy is passed into the atmosphere. When the brake pedal is released, hydraulic pressure drops and the pistons are pulled back to theirunapplied position by return springs.