59 is energized the plunger 58 moves the member 50 toward and into sealing engagement with the opening 57 thereby preventing air under pressure in the interior chamber 18 between the pistons 15 17 from being evacuated to the outside of the housing 3 referring now specifically to fig1 of the drawings the proportional control valve 1 of the invention is shown just as it is reaching its fully open state the normally closed inlet solenoid 49 has been opened to admit pressurized air into the interior chamber 18 between the pistons 15 17 to force them apart at the same time the normally open outlet solenoid has been closed to prevent evacuation of the air from the interior chamber 18 the pressure of the air in the interior chamber 18 applies to the pistons 15 17 a force greater than the return force of the main springs 29 31 thereby causing the main springs 29 31 to be compressed as the pistons 15 17 reach the outer limit of their travel as the pistons 15 17 travel toward their maximum open position the rack attached to each piston rotates the pinion 5 which is mounted on the control valve shaft thereby turning the control valve shaft to its fully open position the main valve has a shaft coupled to the control valve shaft and is hence also fully opened the present invention also provides for the control valve 1 and hence the main valve to be fixed in a partially opened position as can be seen in fig2 there while the control valve 1 is in a partially open.