pipe 303 so that the opening means of the discharge valve are not actuated and the discharge valve remains closed when the coupling device 350 moves when the pivot arm 356 is not pulled the pre control valve 317 is opened when being flush or in alignment with actuator 351 the pressure medium which flows into pressure chamber 363 acts on the rear side of a membrane 382 which is pushed against the insert plate 301 thus establishing a pressure tight seal between the insert plate 301 and coupling device 350 for detaching the coupling the pressure chamber 363 is vented through a vent valve 383 which is actuated by a buffer rod 384 which is coupled with pivot arm 356 the individual parts of the discharge valve and the coupling device may consist partly of plastic or metal the square shaped pipe 303 is preferably an extruded profile at which the ribs 302 for receiving the insert plate 301 are provided the ribs 302 from at their upper side a running face 385 for rollers 354 of coupling device 350 a further rib 386 may be provided on line 303 for receiving curtain 387 fig4 the operation of the coupling device with the aid of hose 361 is a particularly simple solution by an inclined pulling of hose 361 the coupling is detached i e a coupling is prevented and the coupling device 350 is displaced on line 303 without opening the discharge valve when the coupling device 350 is moved without pulling hose 361 the coupling of orifice 364.