All good suggestions. If the pistons move at all, most likely the air will work.
But if not, the extreme approach is to squirt some brake cleaner or Gumout (any volatile solvent) into the cylinder, then block both ports with bolts & nuts, and heat the body of the cylinder. The aluminum cylinder expands with heat faster than the steel piston, so this creates clearance for the piston to move. And the solvent will boil, creating pressure to move it. Be sure to have the open end aimed where it won't do any damage, since you are in effect creating a mortar. Eye protection is a must. You may also get a small ball of flame as the solvent burns, but just keep cool and it will go out in a moment.
I've done this many times on cylinders that are too badly corroded to disassemble with shop air. Although there is still a piston up in the rafters somewhere, it's worked well for me. And I feel it actually does less damage to the cylinder than forcing the piston out cold (with grease, for example), due to the increased clearance.