Lost reverse, need help finding it. 727

That's how Chrysler should have made all their L/R servo pistons. On that one, I would use a trick I learned from what I found in a lot of overhaul kits. Some of them provide a green plastic circle about five inches in diameter to help roll the edge of the lip seals in. Just cut a smaller circle of plastic out of a milk carton or anti-freeze jug; about 3" in diameter and roll it around the servo bore; using gentle pressure on the piston, until it clears the groove. I used to use a feeler gauge, but it's pretty easy to slice the lip. Some pistons; like the direct clutch one; has that flange that makes it so you can't get to the seal edge. On that one the freeze method works great. I put a big rubber band over them.