The caliper pistons (in the small bikes) are either bakeolite or some form of epoxy. I use a bicycle pump to push them out (thru the bleeder), and Ace Hardware for the "O" ring.
If you have a square profile to the ring then you can remove it, fill the groove with grease, invert the ring (inside out), and reinstall it. I've never had an issue with either method, and I've rescued a few scoots that were completely frozen. 1000 grit in the bore as long as you do NOT remove metal, and soap, and water wash it afterwards.