hydraulic motor 35 the pressure medium conveyed by the hydrostatic piston engine 12 is used in the first embodiment illustrated in fig1 there is provision for the hydraulic motor 35 to be acted on with the pressure produced in the supply line 14 to this end a releasable non return valve 42 is provided in the hydraulic motor connection line 36 the non return valve 42 is arranged in the hydraulic motor connection line 36 in such a manner that it opens in the direction towards the supply line 14 consequently when pressure is produced in the supply line 14 or the excess pressure line 18 the releasable non return valve 42 is loaded in the closing direction a release line 43 is provided for releasing if the release line 43 directs sufficient pressure the releasable non return valve 42 is brought into the open position thereof regardless of the pressure relationships in the excess pressure line 18 and the hydraulic motor connection line 36 the release line 43 is connected to the discharge line 20 downstream of the pressure limitation valve 19 preferably the release line 43 connects the releasable non return valve 42 to a location of the discharge line 20 upstream of the parallel arrangement of the pretensioned non return valve 26 and the throttle location 27 owing to the pretensioned non return valve 26 and the throttle location 27 a higher pressure can be provided in the portion of the discharge line 20 that is formed upstream of this parallel arrangement this higher pressure relative to the tank volume 22 is supplied to the releasable non return valve 42 by means of the release line 43 the pressure in the line portion of the discharge line 20 upstream of the parallel arrangement is.