How do you depress the piston back on rear brake caliper of a Renault megane?

Remove the caliper from the hub and with either pipe grips or a 12mm spanner on one of the four lobes turn clockwise to screw in, it will only do maybe 1 revolution then tap with a light hammer and do again. This takes ages but stick at it. Strangely both sides are Right Hand threads and you need gap of 40mm from piston to clamp to clear new pads, this was on Megane CC 2006. Andrew H