Wheel Cylinder
A hydraulic cylinder in a drum brake system that pushes the brake shoes outward against the brake drum.
The wheel cylinder is the hydraulic actuator in a drum brake system. It is mounted inside the brake drum assembly, bolted to the backing plate, and connected to the brake lines coming from the master cylinder.
Inside the wheel cylinder are two pistons, rubber seals, and a central spring. When you press the brake pedal, pressurized brake fluid enters the cylinder, pushing the pistons outward in opposite directions. The pistons press against the tops of the brake shoes, forcing them outward against the spinning brake drum to slow the vehicle.
The most common failure point for wheel cylinders is internal fluid leakage. Over time, the rubber seals degrade from age and moisture in the brake fluid, allowing fluid to leak past the pistons and contaminate the brake shoes. This reduces braking efficiency, causes a spongy pedal, and can lead to a complete loss of hydraulic pressure if the reservoir runs dry.
Signs of a failing wheel cylinder include visible brake fluid leaking on the backing plate or tire, a spongy brake pedal, or rear brakes that grab or lock up. Replacing a wheel cylinder is standard procedure when rebuilding rear drum brakes, followed by bleeding the brakes.