On my old Zafira and now on my 'new' Picasso, the rear brake calliper pistons MUST be screwed back in to the callips to accept fresh pads. Why is this? Some say... it's to prevent the O rings being damaged/folding over themselves if the pistons are just pushed back in (but why should rears be different from fronts?), and others that it's to do with the handbrake also being on the disc brakes; the handbrake works by turning the piston housing so that it 'screws' out when operated, and that it stays in these incremently-out positions as the pads wear - so it then all needs screwing back in. Any ideas? Thanks. Very curious.