If the brake hoses are pretty old (10 years or so) it may be you have an internally failed hose. Deteriorated rubber in the hose can act as a check valve holding the piston out. You can push the piston back manually, but when you apply the brake, the pad stays against the rotor, dragging and creating lots of heat. Check that this is not happening by applying the brake, then trying to spin the wheel by hand with the wheel off the ground.

Also, sometimes a little bit of rust in a critical spot can prevent the pads from seating properly.

Just a couple of things to check...good luck with it.