In a previous automotive life, I did more investigation and experimentation with the M54 CCV system than even I can believe. There's a fair bit of transferable knowledge to the N51/2 (less so with the turbo motors, as positive intake pressure requires more complex arrangements). There's a lot of confusion about these systems. In short, the crankcase is supposed to be sealed; if it isn't, you end up with a vacuum leak (that was your problem, gjm120). The CCV/PCV valve not only regulates crankcase pressure but also flow. Intake vacuum is much less of a factor due to the membrane in the CCV/PCV regulating crankcase vacuum. Spec on the M54 was 3-4 inches of water, which is barely perceptible.
For anyone interested in the result, just search for "02Pilot mod" or look at this thread over at BF for the ridiculously long full investigation:
https://www.bimmerforums.com/forum/s...CV-replacement