0 this process corresponds to curve i extending to the left of the reference point q 0 in fig2 according to the present invention the flow control valve assembly 10 is arranged so that the valve will be open approximately mid way when the bobbin 17 is at the reference position accordingly if the reference position is taken as the center of mean bobbin movement when the vehicle is runhing control can be effected over the necessary range with only a small energizing current the effect of the present invention will be obvious from a review of the flow control characteristics of the invention as shown in fig2 in comparison with the characteristics encountered in the prior art as depicted in fig4 it will be seen that an energizing current indicated at the vertical line h in fig4 is required to obtain the same opening degree or flow rate indicated at q 0 in fig2 where no energizing current is necessary in fig3 a and 3b each one example of the valve holes 21 and 22 is illustrated respectively a suitable number of the valve holes 21 and 22 respectively may be provided in the wall of the ferromagnetic core 16 for instance the overall opening area of the valve holes 22 can be increased to a suitable degree this permits the flow rate characteristics indicated by curve ii in fig2 to be obtained in other words the slope of the flow rate characteristic curve can be set or changed at will as a.