Has anybody experienced an oil pump that is slow to prime and/or develop initial oil pressure? This issue seems to occur only at initial start-up after the truck has been sitting idle. The pump loses its prime. Once the pump is primed and operating, I get plenty of pressure and constant pressure.

For example, yesterday I started the engine and it ran for several seconds without oil pressure. I promptly shut the engine down because I don't what to damage it. I removed the distributor and used an electric drill to turn the pump and quickly got pressure and plenty of oil from the rockers. I then replaced the distributor and re-started the engine and instantly saw oil pressure. The pressure stayed high for 20 minutes while I ran the engine. I stopped and re-started the engine and again saw pressure instantly. However, the following morning, same issue. No pressure at initial start-up and I'm afraid to let the engine idle for any length of time to see if it will prime itself.

So my question is, how quickly should I be seeing pressure at a cold start-up? Is this normal, or should I replace the pump? What keeps the pump primed? Is there a foot valve or seal that is leaking? Thanks!