Couple thoughts, is the piston returning in the master cylinder? if not, its not going to get its next sip of fluid. Is the brake lever returning to its normal position on its own? That is the first way to tell if its returning, if its not remove the line at the master and give it a little bump of compressed air.
As far as the caliper, I am a Polaris man, they only use one caliper piston, this sounds like a dual piston, one on each side of the Rotor? and only one piston is moving? something to try is, if you get the master cylinder free'd up and working, sounds like whan you pushed the caliper pistons in, when the one hit the bottom it got stuck in some goop in the bottom of the bore, push both pistons back, and put a C-clamp on the piston that is moving ok, then pump the brakes and see if you can get the stuck piston moving again, I would even squeeze it back in and pump it back out several times to free it up. you may even want to go so far as remove the caliper and the bleeder screw and flush it out as best you can with some brake clean and compressed air, then put it all back together and fill with fresh fluid.
98 Polaris XC600 (M10 suspension, Boyesen reeds, SLP clutch kit, SLP can)
95 Jag 440 (daughters sled)
"Speed is expensive, How fast do you want to go?"
"You must be fast, cause when I passed you I was Haulin @$$!"