.... I had the exact same problem with mine ages ago, the piston just refused to come out, even used a piston puller,compressed air and finally put it all back and tried using the braking system to push it out, no joy, so what I did was drill a hole in the center of the piston, then tapped a thread for a long bolt , then just tightened the bolt up, this pushed the piston away from the caliper 
ruins the piston, but of you have spares its no problem!
Matthew
Matthew
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