Sounds like it is going to be a hassle to fix.

You can't use gravity. You may have been able to do it with two people, but that wouldn't necessarily have flushed what was in the ABS controller. Once you got air in the ABS controller you were hooped.

Not a toothless hillbilly here, but I have worked on vehicles since the mid seventies, and have done brake jobs on vehicles from the twenties (a truck with mechanical linkages to the rear drum brakes) to current models. But I don't flush the fluid in my own BMW brakes. I don't need the hassle, and the dealer has trained personnel and the appropriate tooling to do it quickly and easily.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue