It is NOT a true open center system. When the aux valve levers are in neutral there is NO flow through the aux valves. The flow is controlled by an unloading valve in the regulator valve block. That block contains the relief valve(heavy spring loaded one) a regulator piston with a ball and seat underneath it and then right in the middle is a screwdriver headed screw with a orfice and a screen on it. That orfice is the flow to the regulator valve piston when in neutral. The flow through the orfice forces the piston down and opens the ball valve dumping oil to sump. When you move the aux valve lever to raise or lower imp, the first thing that happens is it uncovers a land on the spool which is hooked parallel to a channel to the orfice screen. This allows much more oil to flow to sump through that channel than the orfice can provide to the regulator piston and therefore the ball seats and you go on demand.
If that orfice or screen plugs the regulator piston cannot dumup oil to sump and therefore you remain on pressure even in neutral.
Also, if one of the single to double adjustment roll pins are bent or broken that same channel in valve will be open all the time same as operating the lever so you go on demand (high pressure).
Might as well go a little farther here. The reason you can set it to double or single action is that in single action that land never uncovers when you move the lever to drop positon and therefore system stays on low pressure. That is the main reason IH used that system was for the single action control without having a float positon in the valve.
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