depressing force fn has been briefly applied the tailgate remains in the position reached and is at rest as shown in fig7 even when the depressing force fn is removed from the tailgate 581 with regard to the magnitude of the raising force fh and the depressing force fn only the upper limit values have been indicated hereinabove in consideration of the fact that also a weak person is able to apply these forces nevertheless it should be mentioned that these forces fn and fh ought not to be reduced willy nilly they ought to be sufficiently great that accidental pushing of the tailgate or wind forces cannot give rise to unintended movements on a basis of the situation shown in fig7 if it is desired to lower the tailgate in the direction of closure as indicated by solid lines in fig6 then it is necessary to apply a lowering force fs to the tailgate as shown in fig7 then the pressure p1 in the working chamber 506 increases and this increased pressure acts on the sealing piston 512 and the sleeve member 570 as a result of this increased pressure the sleeve member 570 together with the sealing piston 512 is displaced upwardly in fig5 until the sealing ring 571 has passed beyond the stop 575.