Hydraulic control system

A hydraulic control system connectible to a pair of output lines, said system comprising:

• a source of fluid having a high-pressure side and a low-pressure side;
• means including a low-pressure limiting valve for maintaining a predetermined low-pressure differential between its input and output sides;
• means including a main valve connected to said lines, said sides, and said low-pressure valve and having a valve body displaceable from an end position for connecting each of said lines to a respective one of said sides and disconnecting said low-pressure valve from one of said sides and another position for disconnecting said lines from said sides and connecting said low-pressure valve across said sides;
• means including a medium-pressure limiting valve having an input connected to said high-pressure side and an output for maintaining pressure at its output below a predetermined medium-pressure level greater than said low-pressure differential and smaller than the pressure differential between said sides, said low-pressure valve and said medium-pressure valve being connected in parallel across said sides in said other position of said valve body;
• a differential pilot piston connected to said valve body and jointly displaceable therewith;
• and
• pilot-valve means connected between said output of said medium-pressure valve and said pilot piston for hydraulically actuating same and thereby displacing said body between its said positions, said pilot-valve means including a chamber at the smaller face of said piston and connected to said output of said medium-pressure valve, a pair of pilot valves, and a chamber at the larger face of said piston connectable through one of said pilot valves to said low-pressure side and through the other of said pilot valves to the other chamber of said piston.