Brake Wheel Cylinders
The wheel cylinder's pistons can stick in their bores resulting in uneven pad wear. They contain a return spring, cups, and two pistons, one for each shoe. Be careful when checking behind dust boots, they're designed to keep leaks in and the environment out.
When the dust boot is damaged or worn, water enters the wheel cylinder resulting in rust and corrosion. This contamination can cause the piston to stick in its bore. It can be hard to notice without inspection because the other shoe is contacting the drum and slowing the vehicle.
Leaking seals are the most common problem found with wheel cylinders. When this happens, the wheel cylinder is typically replaced. Some iron wheel cylinders can be honed, and the seals replaced, but most are made of aluminum with a hardened bore that cannot be honed.