What is a Rear Wheel Cylinder?

Rear wheel cylinders are a critical component of a vehicle's braking system. Although small in size, they perform very critical tasks that enable a vehicle to brake and stop effectively. Understanding the basic functions and designs of a rear wheel cylinder, and the negative impact that a faulty or worn rear wheel cylinder can have on a vehicle's stopping ability is crucial for any auto enthusiast.

A rear wheel cylinder on a vehicle is a very critical component of the braking system. Used with drum brake systems, which are most commonly installed on the rear of a vehicle, rear wheel cylinders are small, cylindrical parts made up of a steel outer casing that houses the rubber plunger and rubber seals that make up the internal workings of a rear wheel cylinder. The brake line, which is what carries brake fluid from the brake master cylinder reservoir directly to the brakes, attaches directly to the rear wheel cylinder. A rear wheel cylinder has a small metal port that receives incoming brake fluid and directs it to its inner parts.

The main functions of a rear wheel cylinder are to accept the incoming brake fluid from the brake line and redirect it into usable force against the drum brake shoes. When the brakes are applied on a vehicle, the brake fluid is pressurized and sent directly to both the front and rear brakes. The brake fluid is what supplies the physical push to activate the braking action of the brake shoes against the rotating vehicle tire. A rear wheel cylinder has a small plunger mechanism located within its housing that parcels out the incoming brake fluid in response to the force being applied to the brake pedal. The plunger mechanism adjusts the amount of brake fluid acting against the brake shoes. When the brake pedal is pressed firmly, the plunger allows more brake fluid to exert against the brake shoes; when the brake pedal is lightly pressed, the plunger mechanism allows only a small portion of brake fluid to contact the brake shoes.

All rear wheel cylinders function in the same way. The only real difference between various wheel cylinders is the size of the cylinder used in a particular braking system. Different braking systems use different sizes and shapes of wheel cylinders. Heavy duty braking systems tend to use bigger, more powerful rear wheel cylinders. Lighter duty braking system use, smaller, less powerful rear wheel cylinders. A larger rear wheel cylinder will be able to accommodate more brake fluid and exert more force on a particular braking system. A smaller rear wheel cylinder will be less powerful and capable.

Worn or defective rear wheel cylinders can pose a serious braking hazard and undermine a vehicle's ability to be stopped effectively. Leaks are a common problem with old, worn out rear wheel cylinders. The rubber lining and inner plunger mechanisms can erode and lose their ability to effectively seal brake fluid as it passed from the brake line into the wheel cylinder assembly. When this happens, brake fluid spills out and is lost. This reduces braking power, and it allows outside air to enter the braking system, which further reduces braking power and leads to a soft, mushy brake pedal. Timely repair of a worn out rear wheel cylinder is necessary for safe, effective braking action.

Rear wheel cylinders are small in size, seldom measuring more than 4 to 5 inches in length and 2 to 3 inches in width. Attached directly to the top of the brake back plate, a rear wheel cylinder fits snugly between the top portions of the two drum brake shoes. Due to their small size and convenient location, installation and removal of a rear wheel cylinder is easy.