[djpateman]

Generally when a piston seizes in a caliper there can be two situations. First is total seizure and the piston never moves and the pad will not act on the pad, nor the pad on the rotor so braking action is reduced. Second is partial seizure where hydraulic pressure can force the piston out, but the piston is not able to retract. In this case the pad is forced against the rotor and must be worn down until the rotor is no longer dragging on the pad. This will result in hot wheels and lots of brake dust. I had one interesting caliper (came to me to repair) where the piston partially seized and the piston face was poking through the metal backing of the pad and wearing on the rotor; it could go no further since the piston was past the seal and the caliper leaked rather than pushing the piston out further.
Often there are 3 things at work to retract the piston. The rubber seal ring distorts as the piston is pushed out, and this acts to pull the piston back. The brake rotor will wobble a small amount and push back at the pad to create the venting clearance, and in so doing also acts to push the piston back. Early calipers in the 60's and 70's had a pin in the back of the bore, and an internal mechanism inside the piston that was spring loaded; the mechanism gripped the pin, and as the piston was pushed out the spring was compressed, when pressure was released the spring drew the piston back.