Abstract: A vehicle hydraulic system is disclosed comprising a pressure source supplying pressurized hydraulic fluid to a discharge header, a vehicle braking system having a hydraulically operated boost piston assembly for supplementing a force exerted by a vehicle operator to operate brakes of the vehicle braking system; a boost pressure control valve for selectively directing pressurized hydraulic fluid from the discharge header to boost piston assembly and venting hydraulic fluid from the boost piston assembly; and a sensor for receiving an input from a vehicle operator and supplying an electrical signal to the boost pressure control valve to control the operation thereof.