Wheel cylinders are found inside the brake drums.
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.
Replacing the cylinder seals, or sometimes cups, is wasting money, as they will also wear in time.
CBC can resleeve the cylinder bore with a stainless steel sleeve and replace all the seals and boots. The stainless steel is non porous which prevents further corrosion.