I think your are working on the rear of the car, so I will show you what needs to be done to compress the piston back into the caliper for the rear axle. This requires a special tool that will turn the piston in as it is being compessed, the parking brake has a ratchet system built into it and requires the piston to be screwed back in, see pictures below.
David DeLeon, VW Master Technician
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