The hydraulic system is an "open center" type system. That means that the oil leaving the hydraulic pump goes through the various valves and then returns to the reservoir. As long as no valve is putting a demand on the system the oil just returns to the reservoir and you will have no pressure in the system. When a control valve is moved the oil is diverted through that valve to what ever is attached to it, a motor, cylinder, power steering, etc. The pressure will then rise to whatever is needed to move the cylinder or motor and no more. If a cylinder should reach the end of it's stroke so that it can move no farther, the pressure in the system will rise until the relief valve opens. At that point you will hear the squealing caused by the oil escaping past the valve. That will be the maximum pressure the system capable of delivering.
If your power steering works when the system is on high pressure, caused by the hitch system, but not working any other time, the steering system is working but not getting the priority flow it needs. The priority valve works on flow, not pressure. The steering system is also an open center system so when no load is placed on the steering there will be no pressure rise there. However, when demand is placed on the steering system, the pressure will rise, but only in the oil that is flowing from that side of the priority valve. The remaining flow to the hitch will still pass out of the valve with no rise in pressure.
What it sounds like is happening is that when you activate the steering the flow through that part of the system stops and all oil flows to the hitch. When the hitch valve causes the pressure to rise it is also rising in the steering system to allow it to work.
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Today's Featured Article - Identifying Tractor Noises - by Curtis Von Fange. Listening To Your Tractor : Part 3 - In this series we are continuing to learn the fine art of listening to our tractor in hopes of keeping it running longer. One particularly important facet is to hear and identify the particular noises that our
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