The thing that points me to the vacuum pump is that it stops fine at higher speeds, but at lower speeds, there's not enough vacuum built up from the engine speed and the vacuum pump is not functioning properly (or like Doru said the seals are bad) and it's not able to give the booster the oomph it needs to help you stop.

This is EXACTLY what mine was doing.

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• 2008 X5 3.0si Alpine White