on most rear disc brake vehicles you have to screw the piston back into the caliper, as it is also used as the handbrake adjuster. the piston willl probably have at least 2 cuts on it probably 4 these are for a key to fit into, which makes screwing them back in a bit easier, go to a good tool stockist and you can pick a universal tool up for a few �'s, I have 2, I was expensive and is difficult to use, the other is the simplest thing you ever saw, its a cube, that fits onto a 3/8 drive ratchet, and each side has a different key, it fits almost every make of car.