dannymax is correct. If the pistons are not frozen in the cylinder then an air compressor @100 psi should pop 'em-out. If that doesn't do it, try more air pressure. Be aware that if you use air pressure, and you are careless about it, you may end-up 'painting the walls' with DOT 3 or 4! As the pistons pop-out you may end up releasing the remaining brake fluid in the caliper. I usually try the air compressor first, and I'll use something like a screwdriver blade between the caliper pads, to stop their excessive movement where one side pops-out before the other, a common occurrence.
I also use a shop rag or an old bath towel around the caliper to prevent the
splurt! of rapidly-propelled brake fluid going everywhere.
If you're a novice around DOT 3 or 4, they are great at removing paint, so have plenty of paper towels or clean shop towels to wipe-up any that spills onto painted surfaces.
I have a post about using the grease-gun on the opened bleeder valve, to remove stuck pistons. A hand-pump grease gun can develop nearly 2,000 psi so twenty-times an air compressor's pressure. Yes you have to clean the caliper's inner passages of grease but I've never had a caliper not release a piston or pistons.
A vintage SOHC Honda 750-4 surrendering to my grease gun.
Probably 40 year-old crystallized brake fluid.
A rig to test the rebuild before replacing it on the bike.