What is a Wheel Cylinder? The wheel cylinder is the hydraulic component that actuates the brake shoes in a drum brake assembly. Hydraulic pressure from the master cylinder pushes the pistons outward moving the semi-circular brake shoe in contact with the brake drum. This contact creates the friction that slows your vehicle. Wheel cylinders are only used in drum brake equipped vehicles and therefore are becoming less commonplace.
The most common failure of a wheel cylinder is leakage. Over time the wheel cylinder piston seals will deteriorate and begin to allow fluid to pass. To check for wheel cylinder leakage the dust boot is opened to inspect. As seen in the figure the red arrow points to fluid that has bypassed the seal and filled in behind the dust boot. This wheel cylinder will require replacement. After the new cylinder is installed the braking system will be refilled and the air removed through the bleeding procedure. A wheel cylinder replacement is the perfect time to service the remainder of the brake fluid to save some money and take advantage of the overlapping labor. Brake fluid needs to be serviced every three years or less.