11 is conveyed via valve 19 to the hydraulic motor 20 during the activation of the electromagnet controlled valve 18 the pump 12 by means of the pressure controlled valve 44 is so controlled that the pressure of the fluid supplied to the motor 20 and the pressure in the pressure accumulator 44 coincide the pressure in the pressure accumulator during this brief transition period is maintained by means of the flow restrictor 42 at about the pressure which the motor energizing fluid had just prior to the start of the transition during the transitional connecting into circuit of the constant output pump 11 the pressure in the accumulator 41 and accordingly also the pressure upstream of valve 19 controlled by pressure regulator 44 decrease only slightly so that a jolt free transition is achieved when the pump 11 is not connected in the motor supply circuit the pressure accumulator 41 and the pressure controlled valve 44 additionally serves to prevent undesired sudden increases and decreases in the pressure of the motor energizing fluid and sudden undesired accelerations and decelerations of the vehicle in this situation the valve 44 changes the volumetric flow rate of the pump 12 as soon as the pressure difference between the pressure accumulator 41 and the pump outlet conduit 38 exceeds a predetermined value the change will be an increase or decrease as appropriate the output pressure of pump 11 after pump 11 has.