What Mel said. No point unless the o-ring is leaking. When that happens most folks use very low air pressure to blow the piston out of the caliper. Use too much and it can tilt in the bore, catch the edge of the o-ring flange, and bend it or strip it off the piston.
No - the pistons are actually stamped one one side with "This side out", to prevent that.Is it possible that your pistons are in backwards?
Thanks again. Another question... having removed the calipers for cleaning, do I need to completely re-bleed the brake system again (pumping fluid from the bottom up)? Or can I just reconnect the (now empty) calipers, open the drain plugs at the bottom, and pump the pedals to force fluid down into them again (perhaps topping up the reservoir if required)?