Recently freshened up a 340. Primed it on the stand to check everything was ok, for leaks etc. showed over 60psi with the drill priming check
Put it in the car and when we first started it all was well with 75psi. On the next day started the car and no oil pressure. Quickly shut it down
Checked the oil pump drive shaft. It was fine. Used the drill/priming rod and a mechanical gauge no pressure. Drained the oil to see if the pickup tube was still in place with a inspection camera. It was
Pulled the engine back out pulled the pan and pump found that the pressure relief piston was stuck in the open position was a small burr on it and about every third time we pushed it over it would hang up. Crazy. Good thing it did it in the garage and was caught quickly not going down the road at 65mph