Why not pop the pistons out?

It's a piece of cake to get them back in again, and it'll give you the chance to clean them all up inside as I bet they're not as smooth as they ought to be.

On the plus side of pulling the pistons out you might even find that they're only leaking (if that's why you want to chance) because they're all crusty with old fluid. I've brought some real old nasty calipers back from the dead with a ground up saw blade for a scraper, a bit of elbow grease and an old Scotchbrite! They old crusty seals cleaned up good as new (ish).

The caliper on my CR500 Super retard is a 15 year old MC21 block of wood, but with a total strip down, a clean up and back together it's like new.