I'm starting to have this too... Brake works fine and then at the end of a long-ish stop, the pedal pushes back on my foot and braking force seems to lighten up. I can stop, but the forces my leg has to exert on the brake pedal are very, very large.

So is that the pump or the booster?

Oh yeah, just remembered that when I started the car before this issue arose, there was a suction sound that emanated from the engine compartment. It was short lived and had a wheezing sound to it. Hmmm....

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Kirk
Las Vegas
2016 X5 40e Mineral White/Black Dakota Leather, ZLL, ZCW, ZDA, ZDB, ZPP, multi contour seats, rear side window shades, HK stereo
2011 E90M3, 6-speed manual