Oil pump priming question

There are also plugs at the front of both oil galleries. They are covered by the cam plate. The cam plate helps block that area, but if those plugs are missing, that has been known to leak enough to lower pressure. Timing cover has to come off to inspect.

As for the air suction, if the oil pump is not seating square on the rear main bearing cap, then that will cause both suction and output pressure problems.

And the pressure relief bypass valve in the oil pump could be jammed open.