Thanks for the great tips. I had the same problem with a 2000 model Renault Secnic 4WD. I could do it manually as per John's suggestion with a pair of needle-nose pliers, but it was difficult to press down hard enough at the same time as rotating. In the end I bent a stiff 4mm wire into a 'Y' shape so the two prongs fitted into the grooves on the brake piston and the single end fitted into the power screwdriver. As the driver could power the turning action I was able to press the driver heavily into the brake poston - all fixed in one quick zap! (My tip for the next bloke!) Good luck.
Ben from Bathurst, NSW, Australia, May 2010