A wheel cylinder is a component in a drum brake system. It is located in each wheel and is usually positioned at the top of the wheel, above the shoes. Its function is to exert force onto the shoes so as to bring them into contact with the drum and stop the vehicle with friction. The wheel cylinders are usually connected to the shoes with small bird-beak shaped rods.
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. Some designs use two single piston wheel cylinders, one at the top of the drum and one at the bottom, each connected to one brake shoe.
Wheel cylinders must be rebuilt or replaced if they show signs of leaking. Wheel cylinders used to be made of cast iron. However, they were more prone to rusting and aluminum is now the preferred material.