4 piston calipers question

I made a fitting with a brake hose and grease fitting to pump grease in to remove pistons. For the 4 piston caliper you will have to put clamps on the ones that are moving so the pressure can go to the other one.
I also recommend filling with brake fluid or water to minimize the amount of grease required.

Compressed air only works if the pistons are not stuck. When they are stuck you need hydraulic pressure.