Those lines that I put in bold are what's bothering me. You talked about using your tractor, so I guess that the 3ph is working like it should. If the 3ph is working right, then the system won't go to relief pressure unless you severely overload the 3 point hitch. System relief pressure is 2000 psi. If you hooked the 3ph to something the tractor had no chance of lifting, it would simply lift the front of the tractor. This still wouldn't open the relief valve.
I know I sound like I'm rambling, but I said that to say this, there should be very little pressure with the lift down and the tractor at an idle.
I was wondering before and I may have missed your answer, (if I did, I apologize) did you put in the right, new, square shouldered o-rings between the pump and the mainfold?
Yes I installed new o-rings from NH.
The hissing or spewing sound you are hearing when you turn off the engine, can you see oil pushing out, or could it be sucking air as the oil drains down away from the pump?
I think you might be correct about it sucking air at this point; I will check before work tommorrow.
When the tractor is running, is the oil just seeping or is it spraying? Is it a drip or a stream?
There a very steady stream (no spraying), enough to make a trail across the cement.
Like Soundguy said, you can remove the threaded plug from the side of the pump head and put a gauge in there. The pressure there should not exceed 2000 psi.
I will look into a gauge tommorrow?
Thanks for the input
George
Mike