I'm rebuilding four calipers for a a long time dormant 914 6
Removing pistons can be a challenge. Especially rears.
Here is a simple way to remove pistons and pre-bleed
the master.
Notice how the resevoir plugs directly into the master.
You must plug the un-used circuit.
Please don't curse me for the silver pedal cluster, it is not my doing.

I use air to do the same thing you did. But I have 2 pieces of plywood that slide between the pistons so they don't come out too far. Then I walk them out by hand.