[snap0964]

The std pistons are phenolic with a bonded metal ring that contacts against the pads - so they can look like metal.

Yeah, that's your best bet, initial test with caliper undone restrain one piston with a wood block or clamp, wind in the
other with a clamp, extend it with a wood block in place - obviously, be careful you don't pop it.

Does sound though that piston may be slightly angled with the seal over time, and binding.
Anyways, test first.