706D Low Oil Pressure

Our 706D (D282) makes about 25 psi oil pressure when cold and slowly goes down to less than 5 psi (nearly zero) when hot. This motor got a complete overhaul last winter including sleeves, pistons, rings, main bearings, and rod bearings. We replaced standard bearings with standard. The head was also completely gone over. We did not remove the timing cover to replace camshaft bearings. The low oil pressure problem has existed since we got the tractor 15 years ago, and I thought the overhaul would improve things.
I verified the pressure using two different gauges - just in case. Then I replaced the oil pump with a remanufactured one and still no change. I"ve since removed the oil filter housing and oil cooler and found no problems or blockage anywhere. What else can I check??
Some may say that if it"s run this long with low oil pressure, keep running it that way, but we found significant wear on the main and rod bearings during overhaul and it doesn"t make me feel very good doing field work looking at a pressure guage reading 0 psi!
One final note: At some point in the distant past, this motor became a hybrid with a D236 block and a D282 crankshaft. Everything about it is a D282, except that the block is stamped D236.
Thanks for the help!

Which oil filter base do you have. If it is the cast iron one with external plug for relief valve I doubt the problem is in relief valve. If you have the pot metal one I would bet that your seat for the relief valve is eroded or you have the springs mixed up. Those flat seats and flat valves are not the best idea IH ever had. Also, might have wrong base mounting gasket as there are three different gaskets. Other wise, time to apply oil or air pressure to oil gallery and see where oil is going. I am going to assume you checked bearing clearances and that they were good when assembling .

If there was significant wear on the mains I would have ground the crank. I would take the pan down and plastigauge it or mic the crank. I'd be willing to bet that the crank was ground 10 under years ago and they used the wrong bearings. You used ones that were too small as a result.

Was there any deep low main bearing rumble in the engine?

Did you use plasti-gage? What was the clearance on the bearings?

I have the filter housing with the external plug. The spring looks fine. I can take it apart again, use a flashlight and look at the seat.

We didn't use plasti-gage since we were replacing standard bearings. I can check that - thank you.