Control stage for driving the motor of a pump unit in a pump-back antilock brake system

In the case of an antilock brake system working on the pump back principle with static brake circuit (19, 22) and a tandem master cylinder (16) as brake unit, in order to alleviate noticeable kick- back on the brake pedal (17) when the antilock control comes into play, a control stage (58) is provided which from the occurrence of a pressure reduction signal for at least one brake pressure regulating valve (24, 26, 27) of the ABS generates an operating voltage for the drive motor (37) of the pump back unit (31, 32), responding as the duration of this signal increases, which after a predetermined rise time can attain a maximum value correlated with the maximum speed of rotation of the engine (37) and is then held at this value at least until decaying, otherwise increasing to that value which is attained within the duration of the pressure reduction signal. From the instant the pressure reduction signal begins to fall, this control stage (58) relays a constant or stepped reduction of the operating voltage of the motor (37) until it comes to a stop, in such a way that the time integral of the operating voltage is greater than the time integral over the rise time of the operating voltage of the drive motor (37) of the pump back unit (31, 32).