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I think the end play in the shaft keeps the valve off center and working only as long as you keep turning the wheel. Say you turn the wheel a bit. The worm gear on the steering shaft pulls the valve actuator (the 'pin'.)to one side (I think of it as a screw threading into the sector gear) and that cracks the valve. That allows fluid to flow to the piston that provides the assist. (And opens a path for fluid to return to the reservoir.) Which pushes the ram a bit, turning the sector gear. Which moves the worm toward the centered position, pushing the 'pin' toward the centered position. When it gets to a certain point the valve centers. When the valve centers, fluid stops flowing to the piston. So you are not moving it more and more when you turn the wheel you are just keeping the valve off center to one side or the other as long as you turn the wheel. That is why its so important to not have any play where the pin fits the slot in the shaft. You want the 'pin' to move and fluid to start flowing the instant you turn the wheel. I just thought: Is there any chance the hoses from the pump and to the reservoir are crossed? That would be really easy to do at the back of the engine if the tractor was split recently because the hose connections are identical there at the back of the engine. I don't think it would work well backwards because there is a damper valve in the block where the pressure and return lines attach to the control valve. Sitting here at the computer, I can't tell you which connection at the valve should be pressure and which is return. Hope the manual tells.
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