the engagement end 26 of the pin 21 has reached a lower position as indicated at 35 in the path of movement thereof from there the engagement end 26 of the coupling pin 21 moves under a spring force towards the upper detent location 32 and when the piston 12 is depressed in the cylinder portion 11 along the groove portion 31a towards the further lower location 36 from which the spring force moves the pin 21 into the detent location 36 the pin comes into retaining contact in the detent location 33 so that the spring 14 cannot displace the piston 12 upwardly again unless the pin 21 is moved into the position as indicated at 35 from which then it again passes along the groove portion 31b into the upper detent location 32 as a result of the spring force the movement of the coupling pin 21 through the guide groove 31 in the specified direction indicated by the arrow 34 can be produced by a suitable action on the pivotal arm 22 which for example presupposes the existence of a suitable device for pivoting the pivotal arm 22 towards the right or the left respectively depending on whether the coupling pin 21 is disposed in the proximity of the upper or the lower detent location 32 or 33 respectively another possibility is for the engagement end 26 of the coupling pin 21 to be of a configuration which promotes movement of the coupling pin 21 through the guide groove 31 in one direction free running mode while blocking such movement in the opposite direction in the preferred embodiment described and illustrated herein the movement of the engagement end 26 of the coupling pin 21 in the direction indicated by the arrow 34 is compelled.