You could remove the pistons and let them soak in a 50/50 mix of ATF and acetone for a while.
You could heat them up a little bit to see if they get looser than they are, than use PB or 50/50 after the pistons are out.
Sometimes they can be stubborn, it'll take some patience....and no sticking small screwdrivers and stuff into it . Doesn't work.
I have heated the rings with a propane torch uniformly , laid the piston on it's side on a 2x4 and used a 2nd 2x4 on the other side and rolled them between the 2x4's just to get them a little looser , that way the mix can get in behind the rings.
And I still have 1 500 piston that have refused to break loose 2 years ago.
