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We are not talking about any system other than the fuel system on the 98.5 to 2002 dodge/cummins.
If the system is left intack/ unmodified workings, if the pressure drops, so does voluume. No if and or buts here.
But if Bosch says 8-10 PSI is needed to get that volume, that is thier specs on it.

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Correct; in the context of this discussion/issue, the pressure can be used to determine if the lift pump is adequate or not.

Dave

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On an unmodified fuel system, I'd agree with that. You can see if the lift pump meets the specs of the people (Dodge and Bosch) who designed the system that has such a high failure rate and can't agree on how to test it; pressure VS volume.

The original poster has a modified system, which appears to have low fuel pressure. At this point is the fuel pressure low enough to cause a problem? A solid probably. Best way to fix it? My suggestion would be to use the FASS pump with the relocator kit to put it on the frame rail. I would return the tank to stock condition. I would make sure that the connections that were made where the factory lift pump was originally are correct and not restricting flow. Lastly, I would try to take my pressure readings at the inlet to the injector pump if at all possible, since that takes all the restrictions in the supply system out of the picture. This may not be possible, due to the possibility of the injector pump creating a pulsation while it draws in the fuel.

My appologies to the O/P for getting off track and getting caught up discussing theory.