The vacuum pump is where I'd look. For some reason BMW thought the 8 cylinder engine couldn't make enough vacuum so..... let's just say when mine failed after a 4 hour highway trip that in town traffic made my seat change into a traffic cone [emoji28]

If you hear a gurgling like water behind the dash it 100% the pump.

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"When the Team Chief said.... You're trapped in a hole with nothing but a goat and a slinky, what do you do? Stubby said, I'm not sure but it won't end well for the goat...." ~(Overheard) Last day, Phase 3, Q Course