Drum brakes use two brake shoes mounted on a fixed support plate. These shoes are inside a round cast iron drum that rotates with the wheel. The pads are held in place by springs; this allows them to slide towards the drum during braking, keeping the linings aligned with the drum. The pads are driven by a wheel cylinder that is mounted on top of the support shield. When the brakes are applied, hydraulic pressure actuates the two drive connections of the wheel cylinder. Since these connections abut directly on the tops of the brake shoes, the shoes move towards the inner surface of the drum. After pressing the upper parts of the shoes to the drum, the lower parts of the shoes also come into contact with the drum by slightly rotating the entire assembly (known as servo action). When the pressure inside the wheel cylinder decreases, the return springs push the shoes away from the drum.
Most modern drum brakes are designed to be self-adjusting during braking. This system reduces the need for preventive adjustments and keeps the brake and pedal in good working order.