No oil pressure

Air pressure from the oil sending unit hole to the pump inlet was clear with only a few drops of oil coming out. Air pressure in the reverse direction with the oil sending unit reinstalled sounded like an air leak in the pan but hard to tell the location accurately.

Pulled the oil pump drive and it's all intact and like new. Looks like a pan removal now. I hate when that happens. Will the pan come off by turning the engine so counterweights clear or does the exhaust have to come off and and engine raised. The net is full of all of the above.

My pump is an exterior mounted unit, On my first page of posting I gave some wear measurements on my pump but have not found any information as to how much wear causes pump failure. It's unfortunate that we weren't able to take all the failed pumps that were replaced and the low oil pressure issue solved with a new pump. Those pump measurements would be handy now.

I got the pan off today. No metal in the bottom. Nothing looks discolored from heat, thankfully. No debris clogging the pickup. Here is a pic of the pump. I gave some wear measurements earlier. With the moderate use this vehicle has seen, if a bearing has spun would it be most likely a rod or main?

Have you ever heard of an oil pump on one of these engines or any 4 stroke engine giving intermittent trouble and then operating normally for a time?

I didn't hear any knocking because I shut the engine off when the oil light came on. Wouldn't you expect a pump to push oil to the oil galleries and then to the pressure sensor even if the pump was in poor condition?

Since my last post I had a close look at the o-rings under the pump cover. The body of the large one looked more square than round in cross section. Maybe it was this way new. The smaller one was round in cross section and appeared to have a small dent in it. My local bearing store had o rings that would fit so I replaced them in the pump and gave the pump a good oiling. After reinstalling the pump I spun it with a drill with a container of oil under the pickup. I got oil dripping from various engine parts but no oil at the sending unit. Today I moved the crankshaft a few degrees so I could see further up at the front of the camshaft to check for any problems there. Then I removed the sending unit and my helper ran the pump. The oil appeared and gave me a mess to wipe up. I replaced the sending unit, checked the oil pressure light with the pump turning and the light went out. I have to clean up the pan, get gaskets, etc before I start the beast and see if all is well. Thank you for your responses. I'll post the later results.

'Thanks to all for being a sounding board for this sudden problem. It appears that all I did was change the oil pump o-rings which solved it. I was doubtful that oil pressure would return when I ran my pump with my drill and no oil came over the lifters but it did come out of the oil sending unit opening. With some concern I started the engine. The lifters got a bath like Niagara Falls. I hope the problem is solved so other spring work can get done.