between the switches and the dual stable multivibrator is momentarily grounded and the multivibrator changed from one state to another as will be apparent to one skilled in the art the dual stable multivibrator is arranged to permit the flow of current to the solenoid coil 30c in a first state and cut the flow of current thereto when in the second state whereby the spool of the three way electromagnetic valve is moved from the first operative position when said multivibrator takes the first state and to the second operative position and returned when said vibrator takes the second state thus in operation when the control system comprising the switches 37 and 38 and the dual stable multivibrator 36 are energised by closing the main switch 34 the multivibrator immediately enters the first state and energises the solenoid coil 30c this unfailingly occurs because the capacitor connected to the switches is always discharged during non operation of the pressurised fluid supply system the piston assuming its home position when the pressure differential there across is negligible in this position the switch 37 is constantly closed by projection 16a to ensure the earthing or grounding of the capacitor upon energisation of the solenoid coil the spool 30d is moved to the second operative position whereupon the vacuum prevailing in the induction manifold is transmitted to the vacuum chamber of vacuum motor 14 the piston 15 is moved via the pressure differential thereacross toward the slave cylinder 7 urging the piston 8 in a similar direction to displace the.