New to the forum but GXP owner for 3 years. Tried everything to rectract rear caliper pistons for pad replacement including opening bleeder and disconnecting e-brake cable, have zero piston compression. Anyone come across this mystery before? Seems to be an internal mechanism preventing piston retraction.

You are twisting them in with the correct tool, right? Rear disc brakes do not compress in like front ones. You need the proper tool that attaches to a 3/8" ratchet and it turns them back in. They are available at most parts stores.

Tool -

Yep, don't ever try a C-clamp on the rear piston. Get the piston reset tool and twist while pushing to reset.

Thanks folks!! Figured that one out after bashing my head off the ground a few times, tried a home-made custom spanner, worked like a charm.

Which reminds me, a friend borrowed my tool like above and I need it back to do my rears.