Check that the brake cables are attached correctly to the rear discs and under tension via spring. For standard Hand brake adjustment you need to remove the inner stowage box (Screws and push fit) from the centre console. You will see the tension nut attached to both cables. It helps if you have a rachet spanner (I think its 12 or 13mm cant remember off-hand). A standard spanner will suffice but its takes quite a few turns to tension the cable - bit awkward!
As a guide the wheels should bite at one click, but you will need about 3-4 clicks for MOT. Check the cable again after the brakes have bedded in
When you re-wound the piston did you release the cap on the fluid reservior. If not, you may have put the pistons seals under pressure so check that they are not leaking.