Here is the deal. I ordered new pistons for for my M calipers and of course the listing was WRONG. Information listed on the web site showed the pistons for the pin type caliper which they are. Part that is listed is really for LATE cup pistons. Seems like the early calipers had something called a knock back. I am assuming that is the spring in the piston of the caliper. If the late calipers do not have the pin and use the cup piston why not remove the pin and replicate the late caliper? I gently worked the pins back and forth not damaging the piston bore and removed them. Test fitted the pistons and they work just like they should. When you drop the pistons in you can feel them forcing air out of the chamber.

Is my thinking flawed and did I waste a caliper?

Well I'm no Eric Shea,,but I did stay at a Holiday Inn Express. I'd say it's okay.

They're already starting to rust...

Do you have a brake booster in the circuit? Cup style pistons came along with brake boosters. That booster circuit keeps 2-3psi residual pressure on the circuit eliminating the need for knockback pins and mechanisms inside the pistons.

That said, a lot of people use A-Calipers on their cars.

Mark... I just noticed. Your right side back 1/2 is wasted anyway. You have massive cracks in your dust boot flange.

Click to view attachment

All is not lost... if you get it plated up, it makes an attractive paperweight and conversation piece.

That sucks! I had that happen to me. Before I knew about Eric's service, I bought cheap reman calipers. Within a year one had a piston seized in the bore. I was able to get it unstuck by JUMPING on the pedal with the pad out, but it did the same thing as above. Cracked the bore and once that happens, there's nothing to hold the seal in there securely. Not safe to drive with at all, so had to source a replacement. Sorry Mark.

Yes, this set of calipers have been pure hell. I soaked the one with PB Blaster for two weeks trying to get the piston out. With a BFH, heat from a torch and and 110 lbs of air pressure it moved. I though I saw cracks last night but I was too tired to go back out an check. When I bead blasted the one side this afternoon is when I saw the damage clearly.

The good news is these are going in the scrap pile and I am buying a set of Eric's rebuilt to go along with the PMB rears I have already.