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.
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 aluminium is now the preferred material.