Not sure how helpful this is:
When I stripped my brake callipers down recently. I found the easiest way (for me) was to drain the fluid from the system and remove the callipers entirely. If you can, place the calliper in a vice (or vicelike grip) and using some old pads place a thinnish (10mm) piece of wood between the pads.
To push out the pistons on the bench, I use an air line (foot pump or compressor) attached to the bleed nipple, the piece of wood ensures that all pistons will come out to the same height, eventually.
Depending on the age/mileage, you might want to consider splitting the callipers and replacing the piston seals with a new set. It's not expensive and is easy to do. (With care you can work out the extended pistons from each calliper half). As long as the piston barrels are not marked, then you can reuse them.
Lots of brake cleaner and an old toothbrush and when you fit the pads back, I always use copper grease on the backs of the pads.
I did this recently to a set of 10 year old callipers with 40k miles on them and the pistons only had some surface rust spotting which came off immediately with some grey scotchbrite.