The piston is early version unit w/o 4 notches on front to accept stamped SS clip for the inner pad. (Should have a wire clip doing this.) I think this version is part of brake recall.
Is started to "blown out" the back because can't self adjust.
Front should have vent plug to keep out most crap makes worse rust problems when is missing.
More proof? put piston in the bore w/ park screw and everything on it except the big spring and "o-ring" seals.
Very likely you can't "retract" the piston fully by hand even w/o the big spring so new pads and rotor can fit. IOW piston height ≥ 5mm - 3/16in
Get rebuilt caliper from AZ etc w/ a good warranty. AZ uses Phone # to track warranty nation wide.
See my Cave, Rear Piston notes
, How to I tell when my piston is retracted enough?
and rest of section. Do You Recall?
links at bottom but don't bother trying to go to GM for 30+ year old car for a recall if even applies to you.
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave