ABR wrote "Have you tried putting the seal on the piston a bit further down than where it would sit normally , then inserting the piston into the caliper body? You may find this will help as the inside circumference of the seal will be tensioned over the outside circumference of the piston." ??? If the seal isn't fully bedded into the calliper, you can't push the piston through it!! You certainly can't fit it using that method.! OP, make sure the grooves in the calliper are perfectly clean, with no dirt or oxide in the bottom of the grooves. The seals are a precise fit and a little bit of crud will stop them seating. You will always get an arc when fitting them, just push the arc in and the seal will be seated properly. Do not refit pistons unless they are perfectly clean, highly polished and smeared with red rubber/caliper grease. If you still struggle, pm yer number and I'll talk you through it.