D-Don, A couple of suggestions;

One, Don't try to remove a caliper piston with compressed air. If it doesn't come at first people invariably stand in front of the piston, bend over and look straight in to it. A 2 in. dia. piston with 100psi air behind it has about 300 psi pushing it directly at your face. You probably don't have enough spare eyes and teeth to do this very often.

Two; You can, as suggested above, use a grease gun, esp. the ones with the long handle. They will generate about 10K psi. If that doesn't break it loose opt for a new calliper. Some where along the line your time and labor is worth something.