Just a couple of things....Is the caliper hooked up to the brake line,and if so, do you have the lid off of the fluid resevoir?If not,and the resevoir is full,there is no room for the fluid to displace when mashing in the piston...You could also loosen the bleed screw when pressing in the piston....Also,make sure the bolt that adjusts the parking brake is backed out...If not,it will keep the piston from moving back as well....If it is actually siezed,you may try soaking the whole caliper in pb blaster for a few days and see if it helps break it loose....Even if it breaks it loose,you will probably need to take it apart and clean the piston and the area where the piston rides...A new rebuild kit will be in order too....
Hoodlum