Drum Brakes are still used by car manufacturers on rear brakes and it is likely that your vehicle has a set unless it is a high performance or non-base model with 4 wheel disk brakes.
Drum brakes are pretty basic and relatively easy to understand and work on. The rear hub that is attached to your axle will hold your wheel studs to attach your tire.
If the vehicle is front wheel drive the axles will spin freely unless the brakes are applied.
Over the hub a bowl shaped drum is attached with the open end of the bowl shape pointing in towards the car. The sides of the drum are flat and inside there are two moon shaped brake pads that are forced out against the drum sides to make the vehicle stop.
To force the brake pads or shoes out to the walls of the drum fluid is forced into your wheel cylinder that has two pistons. When pressure is applied at your brake peddle the fluid forces the brake shoes into the walls of the drum.
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Wheel Cylinder Design
The wheel cylinder is made of 6 basic parts.
The Housing which is tube shaped measuring approximately 1 inch in diameter and three inches long.
The Pistons there are two pistons one to push the front and back brake shoes.
The o-rings or piston seals that wrap around the piston and make a tight seal.
A spring that sits between the two pistons to keep them apart and in place.
A bleeder valve on the back which allows purging of air from your brake lines.
Finally there are dust boots over the ends of each side of the wheel cylinder to keep brake dust and moisture out.
Repairing A Wheel Cylinder
Making repairs to your wheel cylinders is pretty simple.
First you need to remove the wheel and drum to access your brakes.
Then you need to remove your brake shoes to reduce the possibility of contamination from brake fluid. If your wheel cylinders are already leaking and coated your pads you may need to either clean them with brake cleaner or replace them.
Once your springs and pads are off you can use a small screwdriver to peal back the dust boot carefully making sure not to tare the boot. You can probably do this just with your fingers and that would be better.
Look behind the boot for brake fluid moisture. If there is a lot of brake fluid you probably will need a seal kit.
Using a flat vice grip lightly clamp your RUBBER brake line where it enters into your wheel cylinder from behind. If you are unlucky to not have a rubber line in this area you will need to work quickly so the fluid in your master cylinder does not run empty.
Remove one of the pistons and the spring between them then push the opposite side piston out with your finger or something soft DO NOT USE A SCREWDRIVER or something that can scratch the inside of the wheel cylinder chamber.
If you have a seal kit then you can remove the old o-ring seals and apply new ones to the pistons.
Look inside the wheel cylinder and make sure there is no corrosion on the sides of the piston chamber. If there is a large amount of scaring or deep pitting then replacing the seals may not work and you will probably need to replace your wheel cylinder with a new one.
Assembling the Wheel Cylinder
Apply a small amount of brake fluid to the new o-rings when you install them and to the piston before you reinstall it in the wheel cylinder for lubrication purposes.
Push one piston into the cylinder as far as you can and then line up the spring on the center of the piston that is in the chamber. Now press the opposite side piston into the chamber by first lining up the spring and pushing it into the chamber.
When both pistons are in place center them and then roll the dust boot onto the ends of the wheel cylinder.
DO NOT USE BRAKE CLEANER, CARBURETOR CLEANER OR SOAPS TO CLEAN ANY OF YOUR WHEEL CYLINDER PARTS!
It will cause damage to the seals and will contaminate your brake fluid. I strongly suggest that you do not use water or any cleaner other then brake fluid and a 100% new clean paper towel.
If you need to clean your parts use a dry lint free paper towel.
Bleeding The Brakes
After you open your wheel cylinder to inspect or repair it …. or if you find you need to replace it you will need to bleed the brake line at the wheel you were working on.
Using a vacuum pump or by pumping the brakes with the aid of a helper follow the standard steps to bleed your brakes.
I suggest that you push at least 40 ML of fluid through the brakes to make sure there is no air in the cylinder or the lines.
Final Note
Remember that you have to work relatively quickly or the fluid in your lines will leak out as you make the repair.
Keep the lid on your master cylinder as you work and it should help to retain the fluid in the system to a small degree by creating a slight vacuum.
Final Note
As noted above it is extremely important that you do not contaminate the brake fluid, the seals or surfaces inside the wheel cylinder while you make the repair.
Once the dust boots are on the wheel cylinder you can clean the backing plate and brake shoes with a standard brake cleaner. If you have ABS Sensors then you need to make sure you use a ABS Friendly cleaner or the fumes can harm the sensor.
This is a relatively easy repair if you can work steady and relatively fast.
If all of the fluid drains from your master cylinder then you will need to follow the procedure to bleed the master cylinder and all your brake lines at each wheel. I would believe you could only perform the bleeding on one side but it is important enough to take the time and rebleed the full system if it runs dry. It is not so difficult to do and will take only a few pints of fluid.