The only thing I can think that you might try is to put it back on and work the brakes a few times and then take it apart and try again. It sound like the piston is stuck and it may break loose.

If you don't want to go to the trouble of sticking it back on just put the pads back in the caliper and put a 1/4in. piece of flat stock in and hold it while working the pedal or leaver.
Good luck to ya!

 

O.K. problem solved. Had to disassemble the caliper and then the pistons retracted. Don't know why they were stuck but I finally got them free. Thanks to all for your suggestions but nothing worked until I took the caliper apart. Go figure!!