"M" Caliper pistons

I am in the process of rebuilding my M calipers and I am having a hard time removing the pistons from the front calipers. I initially had them on the car, pads removed, pressing on the brake pedal. That removed one of the pistons, but of course there is no longer pressure behind the other three. Removed from the car, placed about 30psi of air from a compressor behind them with no luck. Any suggestions? The pistons and calipers look to be in good shape, is it possible that they warp or disform with heat and age? Are they seized?

I'll tell you how I did mine, which may not necessarily be the right way.

I mounted a caliper on my vice and placed a narrow block of wood between the pistons. By applying compressed air to the caliper, one piston would free itself (with a bang, be cafefull!!!). I then inserted the piston back into the caliper just enough to hold pressure and clamped it. Air pressure then could force the other one out.

My compressor is a dual stage unit that produces a working pressure of about 180 PSI. I'm not sure you need that much, but probably more than 30 PSI.

Do this carefully since when a piston comes out, it goes quick and with a bang. Don't get your fingers anywhere near the piston for obvious reasons and use the wood block to keep the pistons from hitting anything metal. I hope this helps.

Tim is right on. That's how I have done it, too. I take all four of my calipers to the local auto shop, and they "blow" the pistons out with compressed air. Follow Tim's advice, it has worked well for me in the past, and I've done it about 8 times.