A Wheel Cylinder is part of the drum brake system. They are located in each wheel and are usually at the top, above the brake shoes. The purpose of the wheel cylinder is to put pressure on the shoes so they come in contact with the drum and use friction to stop the vehicle.
The wheel cylinder consists of a cylinder that has two pistons, one on each side. Each piston has a rubber seal and a shaft that connects the piston with a brake shoe. When brake pressure is applied, the pistons are forced out pushing the shoes into contact with the drum. Wheel cylinders must be rebuilt or replaced if they show signs of leaking.