Wheel cylinders are found inside the brake drums.These are designed to transmit pressure from the pistons to the brake shoes via the brake fluid. When you put your foot on the brake pedal the brake shoes move outwards to contact the brake drum and so to slow down the vehicle. Most drum brakes are found on rear wheels, where the braking requirement is much less severe than on the front wheels.
Most wheel cylinders are made up of simple pistons inside a barrel. These barrels can become pitted or corroded over time from the hygroscopic (the brake fluid absorbs water) effect of the brake fluid. Corrosion makes very small, or sometimes larger, pit marks, or scores, on the bore of the cylinder and the marks cause leaks and the seals to wear away as they move, eventually reducing all brake pressure in the cylinder.