Proportioning control valve with spring biased control piston

Abstract From the Canadian patent

A proportioning control valve for a fluid pressure braking system for a tractor-trailer combination vehicle permits unproportioned communication of control pressure when the tractor is pulling a trailer but proportions communication of control pressure when the tractor is operated without a trailer to establish a proportionally lower pressure level at the outlet of the valve. When the pressure level at the control port exceeds a predetermined level established by a spring the, control valve proportions at a rate which increases the pressure level at the outlet at a rate greater than the rate of increase of control pressure until the unproportioned pressure level is attained.