No oil pressure

Hi, working on a turbo 2900 that has been stored 3+ years, added new oil and antifreeze, bled fuel system, freed up shutdown solenoid arm and it starts and runs with no issues except the minor detail of no oil pressure. I have a trusted mechanical gauge installed in place of the sender, if gauge is unhooked it is flowing a very weak stream of oil. I have had the pressure valve behind the pump out, oil pump looks new, screen on pump is clean, relief valve on oil pump is free and clean, drive gear on cam is fine, oil cooler is not leaking, and I do not know what else to check. I do not believe this to be a problem that had been ongoing with this engine because the rods and mains all look like new, however I have no history on this engine. What did I miss?

Sounds like it's a pre-1972 version with a camshaft driven oil pump and single oil filter, correct?? Oil flow goes like this......oil sucked thru the oil pump strainer screen and discharged thru the steel tube into the block. Cold oil relief valve on the pump only opens when pressures exceed 80 to 100 psi and returns oil to the pan.....should be able to check that with 150 psi air compressor I'd think.....it can't have anything stuck inside it holding it open. From the oil pump discharge tube, oil flows up thru block passages and out to the oil filter base. Oil flows thru the correct P/N ? filter and back into the block and travels rearward about six inches and then out again thru an elbow to a large metal tube that carries the oil around the back of the block to the eng oil cooler. Oil into the rear port of the cooler and out the front port thru another short tube into the RH side of the block. It enters the block there and fills the main oil galley which runs from the very rear of the block all the way to the front of the block. At the rear of the blocks main oil galley there is a threaded plug....and at the front of that same main oil galley there is another threaded plug, EXCEPT it has a small hole drilled in it, to let small amounts of oil lube the timing gears. IF THIS PLUG IS MISSING, YOU'LL HAVE NO OIL PRESSURE. Someone had to have the engine apart and left the plug out to have this scenario. So, assuming the oil pump is OK fine and moving oil, the only three possibilities are the cold oil relief on the pump.....the pressure regulator behind the injection pump (missing parts) or the front main galley plug missing. Where is your oil pressure gauge plumbed in?? the RH side on the main oil galley??

If I recall correctly,when I did an overhaul on my 200 several years ago,the rocker arm assembly has a plug on each end of it. I'm thinking they are cork? I forgot to put them back in during reassembly and engine had very little oil pressure because oil was flowing out of each end of rocker arm. Only ran it a few minutes,but with no pressure building up in rocker arm,it scorched rocker arm. Found a used one and replaced plugs with new ones and it worked fine.