Husky did you remove the screw top , from top up tank.
Just spray the defending piston with a little wd40 and screw down on the C clamp till it goes back in . Then keep your eye on it from time to time and inspect it for leaks.Sounds like piston is sized up .Is the rubber gaiter ok on the end of caliper piston ,this stops damp getting in the bore of cylinder in the caliper.