behind the rubber seal of the piston it should be copper grease in there i didnt describe it well not the actuakl pistio n i push the piston out qa good inch or so and gently lift the rubber seal up then using a syringe pump some copper grease inside refit the seal compress the piston back then out again then back clean up any residue it doesnt damage the seals half youre brakes are lubricated with copper grease
copper grease is acidic though? so over time its gonna damage the seals, if not cause of that then it will dry them out and damage that way. also copper grease isnt a lubricant..its an anti seize compound.
i have yet to see any part of a caliper with copper grease in it, only place ive seen it is on the back of pads and even that isnt recommended these days. for 2 reasons, it doesnt burn off and it damages seals. if any grease should be behind the seals it should be red brake grease?
from car bibles...
Copper grease and rubber
Whilst copper grease such as Copaslip works well in the short term to solve brake squeal, long-term, it has an adverse affect on the rubber dust seals of the caliper pistons. This can lead to the seal deteriorating or failing completely. If that happens, it leaves the piston and it's surface exposed to the very elements from which it should be protected. Just so you know.
i know if i bought a car and found copper grease inside the caliper seals i would be ditching them for new ones.