Rear wheel cylinders are the hydraulic braking component for drum brakes. Internal pistons on both sides of the cylinder extend outward from inside the cylinder when demand is placed on the braking system. The pistons contact the horns of the shoes and push them outward to contact the inside diameter of the drum and help stop the vehicle. If a wheel cylinder is slightly compromised by leaking slightly, you can replace just the cylinder without removing the shoes and hardware. If the cylinder is blown out and has contaminated the rear shoes, you'll have to replace them. Cleaning them will not remove the brake fluid contamination.
Loosen the lug nuts on the wheel or wheels on which you're replacing the wheel cylinder with 1/4 turn with the lug wrench.
Place a wheel block in front of one of the front wheels. Lift and support the rear axle or frame using the jack and jack stands. Finish removing the lug nuts and then remove the wheel.
Spray the brake line connection to the wheel cylinder with a generous amount of penetrating lubricant. Place a drain pan beneath the wheel to catch the dripping lubricant. The older the connection, the more times you may want to repeat this procedure. Allow the lubricant time to soak in. Complete another step and then re-spray if necessary.
Remove the drum. If necessary, remove the rubber plug from behind the backing plate and readjust the brakes using a thin-bladed screwdriver and a brake spoon. This will turn the shoes inward and make the removal of the drum easier.
Use an appropriate-sized line wrench to break the brake fitting free from the wheel cylinder. This step must apply caution. The penetrating lubricant is intended to help the free-floating brake fitting move on the steel brake line. If the fitting is stuck and you do not apply enough patience, technique and penetrating spray, you can easily break the steel brake line and have to replace it. Once the brake fitting is free, use an open-end wrench to quicken the pace. Be sure the drain pan is still beneath the wheel to catch the dripping brake fluid.
Disconnect the two upper brake shoe return springs using the end of the brake shoe pliers to twist them off the seated retainer. If the shoes are contaminated, disassemble them entirely using a brake shoe spring tool. If you're not replacing the shoes, remove the two upper brake shoe return springs.
Remove the two retaining bolts or the retaining clip from the back of the backing plate holding the wheel cylinder in place. Remove the wheel cylinder. Install the new wheel cylinder and reverse the procedure to reassemble the rear brakes. Once everything is back in place, crack open the bleeder screw of the new wheel cylinder and let it gravity-bleed. As soon as brake fluid trickles steadily from the screw, close it.
Check the master cylinder for fluid level and top off with brake fluid if necessary. Replace the cap or cover to the master cylinder securely.
Employ an assistant to pump the foot brake pedal four to five times and then hold the pressure down onto the brake pedal. Crack open the bleeder screw to the master cylinder until the fluid purges from the screw. Tighten it and repeat this step as often as necessary until the brake pedal returns to its normal firmness and height. Check the master cylinder often if the brake-bleeding requires several attempts. Do not allow the master cylinder to run dry.
