I agree with X53Jay4.8is. If the vehicle is able to decelerate normally (while the RPMs are up) but then not hold the brakes (at idle) it sounds like you have a vacuum leak, there's a possibility for an issue with the booster, but I've had experience with a lot of vacuum issues with other BMWs and never any with the master cylinder or the booster.


My 86' C20 pick up had horrible brakes and was showing the same symptoms that your X is showing. We replaced the brake master cylinder, rebuilt the calipers, and bled the brakes many times. The engine needed replaced and when we did the swap (all of the vacuum lines were replaced) The brakes came back and the truck stopped like it was new.

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Running:
2001 E53 3.0i Black 5MT Style 87, Eibach Swaybars, Headers, Turner Monoball Bushings, Oil pump nut fix, E60 short shifter + ZHP shift knob, Sport Swap 204K (G-power supercharger coming soon)
2011 E70 35d Blue Style 214 3" exhaust, Dudmd stage 2, 2" HR lowering springs
2012 E70 35d Black Style 215

Sold:
2002 e39 540i 6MT Black sport, M5 LSD, M60 Intake manifold
2003 e39 525i Auto Grey Sport