The wheel cylinder is located inside the brake drum, and bolted to the backing plate. It converts hydraulic pressure from the master cylinder into mechanical force that pushes the brake linings against the brake drum.
Wheel cylinders are either:
They are usually made of cast iron or aluminum alloy, and they operate under difficult conditions of extreme pressures and temperatures.. Some are sleeved with stainless steel to be longer-wearing and more resistant to corrosion. Contamination, particularly from water, lowers the boiling point of the brake fluid and may cause pitting and fluid loss. The wheel cylinder cups seal the cylinder against fluid loss.Wheel cylinders may be fitted with a spreader and a light expansion spring to keep the lips in contact with the cylinder during retraction and while at rest. This helps keep air out of the system. Most wheel cylinders are fitted with bleed nipples to allow air to be bled from the system after assembly, and a flexible cover, or boot, allows for piston movement and also keeps out dust and moisture.