On drum brakes, the wheel cylinder is the hydraulic part that pushes the shoes out - and when it fails, the signs are fairly clear. Here are the symptoms of a bad wheel cylinder.
Fluid behind the drum
The most common failure is a leak - the internal cups wear and brake fluid seeps out, showing as dampness behind the drum or on the inside of the tire; see what is a wheel cylinder and fluid leak signs. A leaking wheel cylinder soaks the shoes and ruins them.
A low or sinking pedal
A leaking wheel cylinder loses fluid and pressure, so the brake pedal can feel low or slowly sink, and the reservoir level drops. Weak or grabby rear braking is another clue.
Contaminated shoes
Fluid from a leaking cylinder soaks into the brake shoe lining, making that brake grab, pull, or fade; see how drum brakes work. Contaminated shoes must be replaced, not cleaned.
What to do
Replace a leaking wheel cylinder promptly, replace the soaked shoes, and bleed the system; see what is a brake shoe. Do both sides if the mileage is high. Keep the front discs healthy with quality pads and hardware.