Wheel (Slave) Cylinder

The wheel cylinder, also called the brake cylinder or slave cylinder, is a cylinder in which movable piston(s) convert hydraulic fluid pressure into mechanical force. The piston(s) within the cylinder move the brake shoes or pads against the braking surface of the drum or rotor. There is one cylinder (or more in some systems) for each wheel. The cylinder is usually made up of a single-bore cylinder casting, an internal compression spring, two pistons, two rubber cups or seals, two rubber boots to prevent entry of dirt and water, and a bleeder screw (valve). In drum type brakes, the wheel cylinder is fitted with push rods that extend from the outer side of each piston through a rubber boot, where they bear against the toe end of each brake shoe. In disc brakes, the wheel cylinder is part of the caliper. As the brake pedal is depressed, it moves pistons within the master cylinder, forcing hydraulic brake fluid through the brake lines and into cylinders at each wheel. The fluid under pressure causes the wheel cylinders' pistons to move, which forces the shoes or pads against the brake drums or rotors. Two-way pressure is applied when the wheel cylinder is activated. Brake fluid enters the center of the cylinder, forcing the pistons apart. Pushrods at the piston ends then apply equal pressure to the brake shoes. A return spring pulls the pistons together when pressure is released.