If all else fails, use a valve spring compressor, with the screw end on the piston and the other end over the handbrake spring retaining bolt on the back of the caliper, lock it and screw it down nice and tight. Now firmly grip the edge of the piston with a large pair of Molegrips, ensuring you fold back the rubber seal first, and turn clockwise slowly.
Tighten the screw a bit, adjust the molegrips and turn again. Repeat until the piston has gone down into the caliper.
This has worked fine over the past few years on 2 Fiat Cromas (both 56 plate), and both my own and the daughters 156's. Quick simple and easy.
