Wrenched

If a piston is trying to seize, in my experience, there's no liklihood that trying to free it by moving it back and forward will succeed. The caliper will need to be rebuilt, polishing out the rusty bores, and then change the fluid regularly to combat the moisture-absorption problem, or consider silicone fluid.
Did you eliminate the possibility of the pads 'walking' back or being pushed back, creating a long pedal? Is your car discs all round or drums at the rear?