Hey Geoff, here's those pics I promised you (I hope it's not too late!). I took these while rebuilding my ///M calipers, but the process should be the same.
Remove caliper, drain it (through the brake line hole) and place "upside down" on table. Place piece of wood (good wood: 1/2" thick plywood, not some crap particle board!) as shown. KEEP YOUR HANDS OUT OF THE WAY!!! Using a blow gun (set it low, 30-45 psi should work) pop that piston out of there. Wear safety glasses and please, keep your hands out of there. That piston shoots out like a bullet!
Then remove the inner O-ring:
And then you're going off to powdercoating. When it comes time to reassemble do the following:
Inspect the piston and clean it (I buffed mine) so it's nice and shiny!
Buy the rebuild kit. I could only find mine at the dealer$hip, but you might have better luck at an auto parts store.
Install the inner o-ring into the caliper and then coat it with new brake fluid. You need to coat (sparingly though) the shiny surface of the piston in clean brake fluid so it'll slide on and not catch on the ring or boot. Now here comes the fun part! You also need to install the edge of the boot into the large groove in the caliper. Here's a trick I use: stretch out the boot like so (see pic) and secure it with zip ties (this step will save you much frustration!):
Now with a small screwdriver (do not puncture boot!) seat the boot into the groove. It's important that the boot be fully
seated. The previous owner had the calipers painted and they didn't put the boot on properly, so water got inside and corroded the caliper, and that's why I had to rebuild them...both sides! Give the piston a little tug to make sure the boot is seated.
Then cut the zip-ties and push the piston straight
down. As you push, the boot will catch the groove in the piston and flip the right way. Then you're done.
And don't forget to bleed the brakes when you put them back on the car!