I have a feeling "goes soft" if just the normal amount of pedal drop when the booster kicks in.

Like mentioned above the pedal should be nigh impossible to push until the engine is started and then you will be able to press the pedal maybe 1-1.5" I never measured.

When the engine's running, there will be some amount of pedal travel from initial engagement until the pedal won't depress more.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)