With -many modern braking systems for automobiles and other vehicles, disc brakes are commonly used on front wheels to increase stopping power. On rear wheels, however, drum brakes are often used, which necessitate the use of wheel cylinders. Within the drum brake, the wheel cylinders push against the brake shoes, which in turn press against the inside of the brake drum, causing friction to slow the vehicle.
Wheel cylinders work as part of a hydraulic braking system; hydraulic fluid, usually an oil of some sort, is pressed through a narrow tube, which in turn pushes fluid against the pistons in the wheel cylinders. The cylinders then push out against the brake shoes at varying pressures, depending on how much pressure is being placed on the fluid from the brake pedal. This type of system allows for easy modulation of braking power, as well as decreased effort needed to actuate the brakes.
Each rear wheel has its own wheel cylinder. They are mounted inside the brake drum on a fixed mounting point near the top of the drum that will not flex when the wheel cylinders actuate. The wheel cylinders are shaped like a short tube or cylinder, with pistons protruding from either side. These pistons are seated within the cylinder and surrounded by rubber gaskets to prevent leaking of hydraulic fluid. When actuated, the pistons are forced outward by the fluid being forced through the cylinder. When the fluid recedes back into the cylinder and back through the hydraulic lines, the pistons also retreat, releasing the brake shoes from the inside of the drum.
A wheel cylinder works in much the same way as a master cylinder on a disc brake, but is generally not as strong and is more prone to failure after excessive wear. A wheel cylinder is relatively easy to replace, however, and less expensive than working on a disc brake with a master cylinder. Wheel cylinders are also an access point for the hydraulic fluid in the braking system. Each cylinder has a built-in bleed screw, which, when loosened or removed, allows access to the braking fluid. If a cylinder needs repair or replacement, the brake lines must be bled, or flushed with new brake fluid. Because the brakes will not function properly if air is present in the brake lines, the bleed screw on the cylinder is loosened as brake fluid is run through the system. This way, any air in the system can escape through the bleed fitting or the fluid reservoir near the engine.