If the fluid has never been changed then it's way over do. This may not work, but you have nothing to lose trying it.
Put the old pads back on and the rotor back on. Then change out the brake fluid by pumping the brake pedal and with a clear tube that fits over the bleed nipple and the other end into a clear bottle (I use an empty pop bottle) open the valve closing the valve when the pedal hits bottom. Replete until you see clear brake fluid come out. Of course you need to keep filling the reservoir to prevent running it dry and allowing air into the system.

This may flush out any contamination that is causing the piston from retracting. I would do all three calibers as well.