I did mine recently and had the same problem. I re attached the hose to push the piston nearly all the way out then disconnected the hose and pulled the piston out using soft grips. The piston are ceramic so be careful. Everything looked ok but on closer inspections there was like a thin varnish build up on the piston and dirt in the groove that the seal sits in. A quick clean of both (and the caliper bore) with solvent and 1000 grade yet and dry paper followed by reassembly using lots of fresh brake fluid as lube to make sure the real stayed seated and the piston was I nice sliding but tight fit and works perfectly.
If you can't fix it with a hammer, you've got yourself an electrical problem!