I had the same problem when trying to change the rear pads on my Zafira. You have to turn the pistons at the same time as pushing them back in to release the grip that the seals have on the pistons that stops you pushing them straight back in. Vauxhall produce a special tool for their mechanics to use but it's rather expensive so negates the whole reason to do it yourself. I used a G clamp to apply the pressure at the same time as twisting the piston with mole grips and it went in no bother. Be warned it was quite tricky and took a bit of experimentation to get it right. I can't remember if I loosened the bleed valve though. Good Luck.
andrew
October 2005