Absolute rule!!

If the handbreak operates on the rear callipers the NEVER try to force the piston in with a "G" clamp.

They always need the pistons wound back in tho the calliper body.

Then as they're just callipers a clamp will put the pistons back into the calliper.

But I always crack the bleed nipple first.

This gives the nipple a "service" and makes sure it can be opened at a future date.

Expels any air and doesn't force fluid back into the lines.

It wouldn't be the first time a master or ABS unit is damaged by forcing fluid backwards.

And I'll repeat,

Any time brake bleed nipples are freed off is a good thing.

I normally give them a heat with a blow torch.

Properly serviced and lubed, sliding pistons should be able to be pushed home wih just gentle leverage with a relatively thin screwdriver, this applies whether they are single pistons with sliders or twin/quad opposed.

Never yet needed a G clamp, except when using 4 to hold all the shoes in to try and bleed the notorious (for bleeding) Landrover Series drum brakes.