The rear caliper pistons must be screwed back down the handbrake rods at the same time as you push them in. This is utterly different to the front calipers, where simple brute force will do. Squashing the rear pistons in with pliers, a clamp or a sledgehammer will do no good at all, you have to twist at the same time.
IF your pistons have well-defined key holes you might be able to get away with using long-nosed pliers to push and turn with. From a pit, or with the car on a lift. If you don't mind skinned knuckles, lots of blood, sweat, tears and pain.
I've done it once, on an old Golf GTI. I recommend you get a windback tool.