I have a set of Avid Juicy 3 calipers that I have sucessfully replaced the pistons and seals on. Getting the pistons out of one of the calipers was not so easy though, and I ended up drilling the middle of the piston, putting in a self tapping screw and pulling out the piston with a mole wrench, like the cork from a bottle. This was OK because I was renewing the pressure foot as well as the seals. The downside was the little dimple left in the caliper body where the drill overshot or the screw pressed.
I have a couple of other caliper bodies in need of some work with a similar problem and wonder whether there is a better method or a trick to ease the piston/pressure foot out (potentially so it can be reused!).
I have tried compressed air with no success and hydraulically seperating the caliper halves with rubber over the port then pumping the bleed port with dot fluid and the syringe, also no go. I feel like I'm missing something but it may be that the caliper is completely siezed and the corkscrew is all that's left.