I would take the rocker arm shaft off and then blow compressed air down through the oil port. Then listen to the air flow. I have seen spun cam bearings cause this. I also have seen a chunk of dirt/carbon plug the port. Where using parts cleaner and compressed air will some times clear the passage way.
Has the rocker arm assembly been taken apart??? The rear rocker arm support is the only one drilled to allow the oil to flow up around it. If someone put the wrong support in the rear then you would not be getting any oil up to the rocker arm shaft.
There is not a check valve or anything else like that in this engine.
The oil comes up from the oil pump and then up through the oil cooler. Then down the line from the top of the oil cooler to the oil pressure rail/galley that runs the length of the motor on the right side. Then the block has passage ways to each cam bearing. Then the oil on all of the cam bearings but the back and front go to the main bearing only. The front passage goes to the main bearing but also on across to where the ventilator pump is on the older engines. The rear passage way goes to the main bearing and the internal passage way up to the rocker arm assembly. I just happened to have the JD 4000 series manual laying on my desk. LOL
Another thing I have done is drain all the engine oil. Put two quarts of new engine oil in then fill the motor up the rest of the way with diesel fuel. Start the motor and run it at low idle until it comes up to operating temperature. Drain the mix out and see how much dirt you get out of the block. If you have a plugged port this may clean it out.
If none of this helps than you have an internal failure. More than likely a spun cam bearing.
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Today's Featured Article - Uncle Cecil's Super A Lives Again - by Mike Purcell. A week or so out of most of my childhood summers was often spent with my Uncle Cecil and Aunt Sissie in the small East Texas town of Maydelle on their 80 acre farm. Some of my fondest memories of these visits are those of learning to drive a tractor at the helm of Uncle Cecil�s 1948 Farmall Super A. Uncle Cecil was the second owner of this wonderful little tractor, but it was almost as though he had adopted an infant. The original owner was a man from Minnesota who bought her from a local dea
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