Low oil pressure?

Duane said

I was referring to the operation of the accumulator when the ball valve is opened with the engine not running or when pre oiling.
Canton explains in the instructions that some engines because of the way the oil pump is designed will allow the accumulator oil flow to go out through the pump and not to the engine. They recommend an inline check ball be placed in the engine outlet line side to force the direction of the accumulator oil into the engine. Much like the plug.
The small block Mopar oil pump apparently does not have this problem at least from a pre oil perspective. But I suppose if the engine sat unused for an extended period that some drain back would happen.
Regarding the pressure on both sides, the plug sits on a step in the bore, I agree that high pressure would not move the plug in the direction of the step, but the pressure on the supply side could maybe, potentially blow the plug out the other way because there is no step on the other side. I believe imho that this does not happen because after the engine builds oil pressure, the pressure on both sides of the plug are equal so it does not come out.
Hope that explains it better.

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Thanks for the thorough post; it is much appreciated. (No red X's LOL)

Just to add to this:
- Understood on the accumulator operation. The SBM pump looks like it would stop reverse flow to just slow leakage when sitting still based on how it operates. But indeed Mopar has used anti-drainback filters for decades just to limit the slow drainback when the engine is off. (They also used tall standtubes on the /6 filter base for the same reason: it limits the filter drainback problems. ) It's that feature that would get lost with the plug missing and might be the cause of the issues mentioned in your book.
- Look at the passages top and bottom on either side of the plug and realize that the upper one is after the filter. Whatever filter pressure loss occurs in the filter will make the upper passage's pressure lower than the lower passage's pressure. So there always will be a pressure difference, with the higher pressure being underneath the plug, before the filter. The difference may be very slight with a new filter and thin oil and idling speeds, but the pressure difference will always be such as to push the plug upwards. I would imagine that was the reason to put that step in a certain way.
- Leaving out the driver's side gallery plug is a whole 'nother animal and will indeed cause serious pressure loss; this issue keeps popping up from time to time here. Its place in the oiling chain is quite different, so can't be used to judge what would happen if the 'filter bypass' plug is left out.