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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.
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