It's not often you will get a system that will hold perfectly over night and it really shouldn't need to be perfect as long as it doesn't get low enough for air to get into the system then the initial priming should be all that is needed. If all the air is purged out when you are testing then very little leakage is needed to drop the pressure over night so even the smallest leak on the gauge/connection itself could be the biggest problem. Yes, the rubber fuel line should be pretty obvious but pinching that off 100% is not easy either. Sounds like you have a good connection to get proper readings though if the gauge drops immediately when the pump stops.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)