A tractor includes a vehicle coupler which couples with a trailer brake coupler to connect it to a pressure source for the trailer brake system. The pressure source is driven by a controllable drive unit, and an electronic control unit controls the activation of the drive unit. The vehicle coupler includes a first sensor which is connected to the control unit and which senses when the trailer coupler is coupled to the vehicle coupler. The control unit controls the activation of the drive unit in response to the sensor in order to prevent the pressure source from being driven unnecessarily.