Low oil pressure?

brian6pac said

The oil gets sucked up the pick up into the pump, and out of the pump through the engine and comes out the bearings, rocker arms and around the lifters and that's pretty much it. If the plug is in it goes through the filter and out to the galley's, and goes out the bearing's, if the plug is out the filter just fills with oil and the oil goes to the galley's and goes out the bearing's. Are you getting the picture! If you don't understand hydraulics you wont understand.

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I understand hydraulics pretty well.
If you take a look at the schematic you posted, you are assuming that when the oil leaves the filter that it will go upward against gravity to the main oil galley. Why should it go up. Why not go down. The plug is not there, it is free to go down. That is the dual purpose of the plug, to steer the direction of flow. Without the plug there is an optional flow path that is not necessarily toward the main galley.
This optional flow path has very large diameter passages so the oil can just free flow around in a loop with little if any pressure.
Oil pumps will not make pressure until there is a restriction.