11 while its end close to the tappet projects into a spring chamber 13 the piston being urged into pressure chamber 11 by a spring 14 supported in spring chamber 13 return port 3 leads from valve chamber 5 to spring chamber 13 where a line 15 leads to a reservoir not shown in fig1 there is disposed at the orifice of the return port 3 opening into spring chamber 13 a throttle 16 formed as a sharp edged diaphragm and restricting the cross section of passage of the return port 3 through which the tappet 9 extends in fig2 there is likewise disposed a throttle 16 39 at the orifice of return port 3 opening into spring chamber 13 this throttle being formed by the return port 3 having over part 17 of its length a diameter only slightly larger than the diameter of tappet 9 the gap present in this partial area 17 between tappet 9 and the cylindrical wall of the return port 3 forms the throttle the mode of operation of the valves illustrated in fig1 and 2 is as follows if the pressure in pressure chamber 11 is below a predetermined change point pressure the valves will be in their illustrated positions which is the first valve position in this case piston 10 is displaced into pressure chamber 11 with tappet 9 extending through return port 3 but not as far as to the.