If I understand you correctly you have disconnected both hose at Y connector which connects directly to the source of the vacuum. You are not checking for fuel regulator vacuum at the fuel filter.

With both hoses connected to the same source for vacuum the only reason I can think of where one arm of the Y has vacuum and the other does not is that the fuel regulator side of that Y is plugged.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas