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.
Bosch Master, ASE Master, L1
33 years MB technician