A control unit for brake-slip-controlled operation of the brake device in a state in which braking torque is transferred from the rear to the front wheels by the coupling arrangement, at least one rotational speed sensor in a drive train of the vehicle for inputting into the control unit rotational speed signals representing the rotational behavior of the coupled front wheels and rear wheels, at least one acceleration sensor for inputting into the control unit acceleration signals representing the vehicle longitudinal acceleration, and/or a vehicle GPS device for inputting into the control unit position signals representing positions of the vehicle, the control unit determining at least one first variable, which is characteristic of a vehicle reference speed and/or a vehicle reference acceleration, based on the acceleration and/or position signals, and so as to determine a second variable, which is characteristic of the rotational behavior of the coupled front and rear wheels, based on the rotational speed signals, and to calculate the brake slip of the coupled front and rear wheels based on the first and second variables.