Rebuild Wheel Cylinders
After decades of use, the rubber cups in wheel cylinders wear out and allow brake fluid to leak past them. The brake fluid drips down onto the drum and brake shoes, ruining the shoes and diminishing stopping power. As the system gradually loses brake fluid, a complete loss of braking is probable.
Wheel Cylinder Rebuild Kit
Wheel cylinders should be replaced or rebuilt in pairs. Before you start the wheel cylinder rebuild, order two rebuild kits. Don't rebuild just one side.
Remove Wheel Cylinder From Vehicle
If you haven't done so already, jack up the vehicle, place jack stands under the frame, then remove the tires and brake drums. The brake shoes may also have to be removed. Never work on a car supported by the jack alone.
Before trying to remove the wheel cylinder, spray the mounting bolts and brake line fittings with WD-40 or similar penetrant. Let it soak so it has time to work. The longer you let is soak the better it works. Sliding a drip pan or piece of cardboard underneath will catch any overflow and brake fluid.
Flare-nut wrenches, also called line wrenches, were designed to remove hydraulic fittings. A standard wrench will work, but you stand a chance of stripping the fitting. After loosening, but before taking the fitting off, remove the two mounting bolts that hold the wheel cylinder in place. Then unthread the fitting by hand and remove the wheel cylinder. This is a precautionary measure to prevent damaging the steel lines.
Read: How To Fabricate Brake Steel Lines
Once the fitting is off, brake fluid will start running out. Have a drip pan or plastic tray ready. To prevent the line from leaking, cover the end of the line with a suitable plug.
Wheel Cylinder Disassembly
Once the wheel cylinder is off the car, it's time for disassembly and inspection. The outer dust boots usually pry off easily but may rip during the process. New ones should come with the rebuild kit.
dust boot - piston - spring cup - *spring* - spring cup - piston - dust boot
The pistons inside old wheel cylinders are often stuck. Try shooting compressed air into the line fitting. Make sure you're holding both ends of the cylinder in a rag, or the pistons will fly across the garage!
If compressed air doesn't unstick them, stand the cylinder upright, spray with penetrating fluid, and let it soak overnight.
A digital camera is very useful during the removal and installation process.
Pushing out the old piston with a socket.
Wheel Cylinder Inspection
After cleaning with brake parts cleaner, look inside the bore. If you can, run a fingernail inside the bore. A smooth bore surface with a light amount of pitting can be honed out. If the pits feel deep, the bore surface can only be repaired by re-sleeving or replacing the wheel cylinder.
Wheel Cylinder Honing Tool
The next step is to hone and polish the bore to a fine finish. The purpose of honing is to help the cylinder cup make a good seal against the bore. There are several ways to accomplish this. The most popular method is to use a cylinder honing tool with a variable speed drill.
Hone Wheel Cylinder
Using a variable speed drill at low speed using brake fluid as a lubricant, run the honing tool the full length of the bore in a gradual up and down motion. Lubricate the honing stones with clean brake fluid. Try to make a "cross-hatch" pattern. Run the hone long enough to smooth out the bore, but be careful not to over-hone.
After honing, wipe out the cylinder and clean off the stones. Look inside the cylinders and see if all pits and corrosion are gone. Repeat the honing process until the bore is smooth and free of scratches. When done, wash out the bore with soap and water to remove all traces of the hone grit.
Another way to hone wheel cylinders is by hand, or more accurately, by finger. Wrap a piece of emery cloth around a finger and have at it. Switch from coarse grit to medium grit and then to fine.
A smooth bore means a tight seal with less chance of leakage.
Wheel Cylinder Reassembly
Check your rebuild kit and make sure the cups are the same size as the old ones. Pour a little clean brake fluid into a container, and coat the inside of the bore with brake fluid.
First, put the spring into the center of the bore, then place the spring cups on either end (flat side out). The spring will fit into the recess on the back of the spring cup.
Place the piston into the bore and rotate it back and forth several times to coat it. Use the piston to push the rubber cup further into the cylinder.
Install Rubber Dust Boots
Installing the rubber dust boot on the metal piston can be tricky. You can rip the new rubber with a small screwdriver trying seat the dust boot around the lip of the cylinder.
First the spring, then spring cups (each side), pistons (each side), dust boots (each side)
A better idea is to use an appropriate-sized socket and slip the dust boot over the width of the piston (see illustration). Once it's installed, twist it back and forth a couple of times to make sure it's seated all the way around.
You can also use a socket to help install the dust boot on the piston.
Now re-install the bleeder screw, you can use a little anti-seize if you'd like.
Install Wheel Cylinder Back On Vehicle
Mount the rebuilt wheel cylinder onto the backing plate and hand-tighten the mounting bolts leaving the cylinder loose. First, hand-tighten the brake line fitting into the cylinder, then tighten the mounting bolts, then tighten the fitting.
A fresh-rebuilt wheel cylinder with a smooth bore should last as long as the original one did. But remember, similar to engine oil being changed, brake fluid also needs to be changed (much less often). A look in any owner's manual shows manufacturers' recommended maintenance of replacing brake fluid.