It is not likely that fluid pressure is holding it.

If that is the case your best bet would be to get a rebuilt caliper (or new) as even if you do break it loose it will continue to be a problem.

The way we get the loose (for rebuilding purposes) is to remove the caliper completely and place a special bleeder in place that allows us to pump the caliper full of high pressure grease from a grease gun. Then clean it all up and replace the seals if the metal is not corroded.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician