accelerometer 13 the output thereof being processed by said central unit during the control and brake procedure when the power acting on the coupling has been decreased to a predetermined value the control procedure is interrupted and the present value is stored in the central unit said value is later used as an initial value at the next brake procedure when pushing is indicated that is when the brake power of the trailer vehicle is too low said central unit transmits a signal to said valve 14 to increase the operating pressure to the brakes of the trailer vehicle as said valve 14 is supplied with supplied pressure the operating pressure to the brakes of the trailer vehicle can if required exceed the operating pressure to the brakes of the tractor vehicle this is a considerable advantage because normally there are pressure losses in operating pressure lines of the tractor vehicle said losses decreasing the available pressure interval to achieve the prescribed deceleration higher pulling forces are allowed in the coupling even though the wear thereof is increased however such a control condition depends on a malfunction of the brakes of the tractor vehicle and such a condition is stored in the central unit as described above in respect of the present control condition of the brake pressure there is a reset of the central unit each time the driver brakes the vehicle with the parking brake thus the next braking procedure is started with a non adjusted operating pressure that is the same control pressure to the brakes of the trailer vehicle as to the brakes of the tractor vehicle the by passing of said valve 14 resulting in that the.