I want to add, wear eye protection, I was glad I did when using this method yesterday. Grease, resudual brake fluid, or water can end up coming right at you forcefully, remaining air pockets can be, in a sense, explosive. I drape a rag over the whole caliper to dampen, absorb flying brake fluid water grease while applying air pressure when using it instead of grease gun

Determine which piston is stuck, push the one that popped out easily or moves easily back in,

use a c clamp or devise a method to immobilize the piston that moves easily, then apply force with the grease gun or air compressor, and the force will be delivered to the stuck piston, yesterday I was amazed, the force may have damaged my c clamp it was a pretty serious amout of force, I opened the bleeder screw and grease was squirting out, I repositioned the c clamp and gave it another round of force with the grease gun. The process permanently tweaked the C clamp

After the stuck piston moved out some distance, I used some controlled blows with a pin punch plus hammer, on the leading edge of the piston, it has a rim the retains the rubber boot and allows pin punch force at a 45 degee angle do part of the force moves the piston out, part pushes it sidewise, back and forth one side then the other to get it the rest of the way out. I wanted to minimize grease use, its a messy clean up, once it was far enough out to get at it.

Place something soft, wood, rubber, i used a bit of rag yesterday .... so the pistons arent damaged if they strike one another with force due to residual compressed air pockets , also that can happen if air pressure is used, the grease force is much more controlled all by itself , unless pockets of air in the caliper are compressed , and is much messier if water isn't filling most of the caliper, air pockets inside the caliper continue to exert possible a lot of force, can break loose suddenly in an uncontrolled way

I go thru all this to rebuild the calipers for the cost of the few little parts in the rebuild kits. I see ********eAZ has new Ate calipers for $300 or so.