It the car has brake drums it may have a leak in the wheel cylinder. You will have to remove the brake drum to check it.
Purchase a caliper rebuild kit from an automotive store or dealership. Remove the brake bleeder bolt using a flare-nut wrench. If needed, utilize gentle tapping and penetrating oil to loosen the part without breaking it. Disconnect both the steel and rubber brake lines using a flare-nut wrench. Replace these lines if they are cracking or worn before putting the calipers back onto the car. Strip the calipers of pads, shims, springs and sliders or pins. Remove the outer dust seal. Place a piece of wood that is slightly thicker than both brake pads stacked together in the caliper behind the piston. Release low-pressure compressed air into the inlet port. This should cause the piston to pop out.
Lubricate the new piston that came in the rebuild kit with brake fluid. Insert the new piston into the caliper using moderate finger pressure.
Replace the outer dust seal. Replace the pads, shims, springs, and sliders or pins. Use the new parts that came in your repair kit, and discard your old parts. Reconnect the steel and rubber brake lines. Replace the brake bleeder bolt. Test the brakes to make sure they are no longer leaking.
Remove the hubcap and the tire. Raise the car using a jack so that the wheel is off of the ground. Remove the lug nuts and the wheel. Spray the steel brake line fitting with penetrating oil to loosen any existing rust.
Remove the rubber plug behind the backing plate. Loosen the self-adjuster (star wheel) to lower the brake shoes. If you turn the self-adjuster in the wrong direction, then the drum will get tighter and will not turn. Use a small flat head screwdriver to release the adjuster arm, if needed. You may need to use a hammer to hit the middle of the drum to loosen any rust that surrounds the center piece. Remove the drum. Place a drip pan beneath the brake shoes. If the brake shoes are covered in fluid, you will need to replace them as well. Spray the area with brake cleaner to eliminate dirt and fluid.
Prepare a vacuum hose to prevent brake fluid from leaking out of the steel brake line. Place a screw or bolt in one end. Locate the steel brake line on the plate where it screws into the wheel cylinder and use a line wrench to loosen the brake line fitting. Remove the fitting. Place the vacuum hose over the line to prevent leakage.
Locate the two retaining bolts on the backing plate that hold the wheel cylinder in place. Use a socket wrench to loosen the bolts. Remove the old wheel cylinder. Thread the steel brake line fitting into the new cylinder. Screw it in as far as you can by hand. Insert the bolts back into the backing plate and screw them in to secure the new cylinder.
Keep topping off the master cylinder with more fluid. It should never be more than half empty.
Repeat the process until all four wheels have been bled. Again, you must bleed each brake one at a time.
