Since it was "rebuilt" 2 years ago, the oil pump in my 1979 Yanmar YSM-12 (sailboat) diesel has lost its prime with increasing frequency. To get it pumping oil again each time, I have had to remove the triangular oil pump cover (revealing totally-dry pump cavities) and fill the pump cavities with oil. After quickly replacing the pump cover and 3 screws, and re-starting, oil pressure comes imediately up (to 70 psi cold, then it subsides to its running level of about 18 psi) and it'll run all day faultlessly. It has never failed to maintain pressure once it has it.
Guided by several experienced Yanmar mechanics, I have over time replaced both oil pump rotors and all related seals and gaskets, let the newly-started engine run with zero oil pressure (and the siren screaming) for up to one minute to see if it would pull up a prime on its own, and (finally last fall) taken the side of the engine to a diesel engine machine shop and had the pump shaft opening drilled out and bushed for a closer fit. This Spring it started with immediate pressure and I thought my troubles were over. But it has since progressed from one failure to get any oil pressure per 10-15 starts at the beginning of the sailing season to a failure after sitting over night only once every few days, to current failures with every over-night sit. As yet, it has failed only once during a 4-6-hour day sailing. I've currently got the side of the engine off again (for the third time) and am performing simulations with the side sitting (at the correct angle) in a pan of oil at "running depth". With pump chambers even half full, it will regain a prime and pump oil up (past the filter and the pressure regulator) until it runs out the feed holes: (1) to the rest of the engine; and (2) to the oil pressure meter. How to stop drain-down?? Am I the only person ever to have this problem!!??