Most vehicles up until about 1970 had drum brakes on all four wheels after that time and up to today you are likely to see drum brakes on the rear of many vehicles and disk brakes on the front.
Drum brakes work when hydrolic brake fluid is forced through your brake lines from your master cylinder to the wheel cylinders that force brake shoe pads out and against the surface of the brake drum.
Although they can last many years there are times when a seal can brake or a piston can get frozen in the cylinder and this means you need to either replace or repair the parts inside.
Because most cars and even larger trucks have relatively inexpensive wheel cylinders it is often less expensive to replace them then to fix them if something more serious then a damaged boot or seal has occurred. Sometimes a new wheel cylinder is even less then a boot and seal kit so check your options when you need to buy parts.
Inspecting Wheel Cylinders For Leaks
Before you take your shoes off you can use your finger or very lightly a screw driver and peal back the edge of your brake cylinder dust boots.
You need to do this on both sides to look for brake fluid that is getting past the seals.
The Dust Boots do not retain the brake fluid and can not retain high pressures so if you have a damaged seal you are likely to see a lot of fluid leaking but if you have a seal that is about to go bad you may see some moisture.
If you do see moisture then it is time to replace the seals.
Inspect the dust boots for any tares that could allow dirt and moisture into the wheel cylinder.
Inspecting Seals and Cylinder Walls
If you do find that you have a leak then you have one of two problems or both. Either a piston seal has broke or there is damage to the cylinder walls that is allowing fluid to leak past good seals.
Cylinder walls like those of any piston chamber can wear out of round in which case you could use a honing tool to dress the walls and then replace the seals or it can become corroded from moisture getting past the dust boots and seals or when present in your brake fluid.
Before you begin disassembly of your brake cylinders you need to remove your brake shoes so they do not get contaminated with fluid. Do not think you can take just one side off and take a peek.
If your shoes are already soaked with fluid think strongly about replacing them but at least wash them thoroughly with brake cleaner to remove the fluid.
To inspect the cylinder walls you need to dissemble the wheel cylinder.
If you have a flexible brake line from your steel line that feeds the wheel cylinder you can clamp it off to stop fluid from leaking while you work if not then you will have to work fast so your master cylinder does not drain dry. If your master cylinder drains dry you may endup having to bleed the master cylinder along with purging all the fluid in your system to all four wheels. That is not fun so work fast or clamp the line off.
Remove the lip of the dust boot seal from the wheel cylinder housing. The Dust Boot is connected to the piston by friction so this will allow you to pull out the piston.
Remove one piston and remove the separator spring that sits between the two pistons.
Remove the opposite side piston in the same method you used to remove the first.
Inspect the inside wall of wheel cylinder. Look for dark spots which usually indicates corrosion.
Inspect the pistons for any corrosion.
Inspect the seals around the pistons for wear. They should stand proud of the piston noticeably and provide a good seal.
Repairing The Wheel Cylinder
In the introduction of this HowTo we suggested that the cost of many wheel cylinders whether remanufactured or new is often less then the cost of new seals and dust boots.
Review the price differences before you decide to make repairs.
You should replace the wheel cylinder if:
The wheel cylinder is concentric or out of round
There is a large amount of corrosion.
There are any internal lips or gouges in the cylinder wall.
You can repair the wheel cylinder if a dust boot is damaged or a seal is leaking.
Assembling The Brake Wheel Cylinder
After you have completed inspecting the wheel cylinder you should reassemble it to stop the leaking.
Clean the wheel cylinder with a clean paper towel with no chemicals on it.
DO NOT USE CHEMICALS INSIDE THE WHEEL CYLINDER it will contaminate your brake fluid and could cause seal damage.
Push one piston into the cylinder as far as possible then insert the separator spring from the opposite side.
When you insert the second piston make sure that the separator spring is properly seated on the piston.
Now center the pistons in the wheel cylinder.
You will now need to bleed that brake to remove the air from the wheel cylinder and line.