Abstract: A pilot-operated, spring-centered, directional control valve includes a valve body having a bore, chambers at opposite ends of the bore, a valve spool slidably disposed within the bore, a hollow piston slidably disposed within each of the chambers and biased by springs inwardly toward the spool, and a secondary piston slidably disposed within one of the primary pistons so that pilot pressure directed into the outward ends of the chambers acts on the end of the spool or the end of the secondary piston to selectively operate the valve. The various components of the valve have differing effective cross-sectional areas upon which pilot pressure acts. The control valve is advantageously employed in a hydraulic system controlling front wheel drive in a vehicle.

Abstract: There is disclosed a hydraulic control system including body automatic and manual control for controlling the position of a grader blade for a motor grader. The hydraulic system is supplied fluid from a single pump and is provided with means to isolate the fluid supply from the automatic portion of the system when the manual portion of the system is activated.