A wheel cylinder is a component in a drum brake system. It is located in each wheel and is usually at the top, above the shoes. Its responsibility is to exert force onto the shoes so they can contact the drum and stop the vehicle with friction. What connects these wheel cylinders to the shoes are usually small rods shaped like a birds beak. It is very similar to a master cylinder and functions in pretty much the same way, consisting of just a simple little plunger on the inside. On older vehicles these will begin to leak and hinder the performance of the brakes, but are normally inexpensive and easy to replace.
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.