The pistons should press back in quite easily, but they do readily seize up.
I'd go round them all in turn levering each right in, then very gently pumping it out agin with a series of very short pedal pumps. Do NOT floor the pedal, or you will cause internal leakage of the master cylinder and create another job.

Work the pistons backwards and forwards until they move freely. You can apply a bit of fluid around the rim as a lubricant.
If you find one that is seized and won't lever, get the pad out and put in an old worn pad - ie something thinner. Then stand on the pedal until the piston moves. Then lever it in, and keep repeating until you get it free.

BTW I'd have unscrewed the nipple and then tried poking with a bit of wire, without the 20 psi!