If you loosen all the pistons up and thoroughly clean externally before popping any out, then the others should pop out more easily even with a bit of air in the system.
I have yet to find a piston so stuck that hydraulic brake pressure cant get it moving, but check that piston to body clearance on the trailing edge, mine was well corroded and jamming against the piston. All nice and free now though.
Brake callipers - splitting
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