Procedure to increase the manipulation security for a bi-directional contactless data transmission. The controller receives AC power from a power source and supplies an AC power signal via two conductors to the units. The control system includes retracting speed limit means for limiting a retracting speed of the screw to a predetermined retracting speed limit value after a pressure control in the hold press process begins. This arrangement ensures output of a required braking force in response to a driver’s accelerator-off action, while taking into account the charge limit of the electric accumulator.