approximately then the spring 18 merely needs to be rated so that its force prevents the piston 16 being displaced until the hydraulic fluid pressure is substantially equal to said threshold in the variant embodiment of fig3 and 4 items 9 19 24 25 26 27 31 32 and 33 of fig2 can be seen however in this case the chamber 25 is made in the form of a cylinder for a slide a slide 34 is displaceable within the chamber 25 and replaces the valve 28 30 the slide 34 is subjected to the action of the spring 31 the slide 34 has two internal chambers 35 and 36 which open out to the periphery of said slide and which are separated from each other by a non return valve 37 itself loaded by a weak spring disposed inside the chamber 35 a duct 39 inside the slide 34 provides communication between the chamber 35 and the hose 19 the slide 34 can move between two extreme positions inside the chamber 25 in the first of these extreme positions as shown in fig3 the spring 31 is at maximum extension and the chambers 35 and 36 are in communication with the ducts 26.