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 aluminium alloy. Some are sleeved with stainless steel to be longer-wearing and more resistant to corrosion. Cast iron and aluminium cylinders are suspectible to pitting and potential fluid loss if the brake fluid is contaminated, particularly with water vapour. This also lowers the boiling point of the brake fluid.
The wheel cylinder cups seal the cylinder against fluid loss. They operate under difficult conditions of extreme pressures and temperatures.
They may be fitted with a spreader and 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.
A flexible cover, or boot, allows for piston movement, and also keeps out dust and moisture.
Majority of manufacturers that utilised drum brakes used a larger diameter wheel cylinder on the front braking stations while using a smaller diameter wheel cylinder on the rear. This allows for the braking proportion for the vehicle, normally 60/40; 60% on front brakes and 40% on the rear brakes.
Leaking wheel cylinders are the cause of many brake problems such as unreliable stopping, brake shoe damage and even partial brake system failure.
Whenever you open a hydraulic brake system and introduce air into the lines, master cylinder or wheel cylinders you will need to bleed the air out of the system.
Depending on the manufacturer, rear wheel cylinders can be piped independantly of each other or in series. Independently piped systems are the most common. These have a bleeder screw on each cylinder to allow the expulsion of any trapped air bubbles in the system. Air bubbles trapped in the wheel cylinders will affect the functionality of the brake stations.
Source: CDX Global & Wikipedia - en.wikipedia.org
