two methods... compressed air sometimes works, just remove the brake line and you can blow it out... sometimes. it's actually easier to remove the caliper completely and keep the lines connected and pump the master cylinder until the piston is pushed completely out. the other option if you have access to a welder is to drill a hole on the backside and hammer the piston out with a piece of hardened steel... then reweld the hole, only to be used as a last resort