Sometimes you can blow them out with compressed air blown into the brake line port with the bleeders shut. A thin piece of wood sandwiched between the inboard and outboard pistons (piston boots removed) will prevent damage from the pistons slamming into each other. If one side's piston starts advancing more than the other side, you can hold it in place with a C-clamp until the stuck one advances.

This method didn't work for me as they had been sitting too many years and my compressor is only capable of 100 psi on a good day. I had to McIver a tool that grips the inside of the piston and then extracts it. It only works on the front calipers though.

If it's really hard to remove the pistons, then they are probably toast anyway so extracting them through any available means is OK as long as it doesn't damage the caliper bores.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)