A wheel cylinder is a component of a hydraulic 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.
It is very similar to a slave cylinder and functions in much the same way, internally consisting of only a simple plunger. On older vehicles these may begin to leak and hinder the performance of the brakes, but are normally inexpensive and relatively easy to replace.