When you do the front pads, you just GENTLY push the piston back into the housing. Make sure you loosen the brake fluid cap on the reservoir, and put some rag around the top to catch any fluid that could spill out. The rear calipers wind back in, so you will need a caliper wind-back tool, I bought mine from a local motor factors for about £20ish. Don't release the bleed nipples as this will allow air into the breaking system, and then they'll need to be bled through....