BMW X6 Brake Problems: Failing Booster or Bad Sensor?

Hey there! I had a somewhat similar issue with my 2010 BMW X5 last year (I have some experience working on cars but nothing too advanced). The symptoms you're describing sound very familiar, my X3-owning actually warned me about this. Had the same trouble, weak brakes, warning light, and that annoying pump noise. Took it to my regular workshop and turns out the brake system wasn't properly bled, causing air to get trapped in the lines. Fixed it for about 90 Euro, and the difference was like night and day. The brakes became firm again, and that irritating pump noise disappeared completely. But before jumping to conclusions, could you share: Did you notice any brake fluid leaks? When was your last brake fluid change? Does the brake pedal feel spongy? This info would help better understand if you're dealing with the same issue I had or if it's something else entirely.

Thanks for sharing that experience! My brake fluid was actually changed during my last service at 129882 KM, about 3 months ago. No visible leaks that I can spot, but yeah, the pedal definitely feels spongy, almost like stepping on a wet sponge. It's concerning because it wasn't like this before. When I press the brake pedal, it goes almost to the floor before really grabbing. I'm thinking it might be more serious than just air in the lines, but I'm hoping I'm wrong! Interesting that your issue was solved with just a proper bleeding, that would be the best-case scenario cost-wise. Did you have any warning lights on the dash when your issue occurred? Just trying to compare symptoms here.

Hey again! Yeah, I totally remember the anxiety when my X5 was acting up like this. Actually, what you're describing about the pedal going almost to the floor before grabbing, that's exactly what I experienced! And yes, I had warning lights too, got that annoying brake system warning on the dash. Looking back at your symptoms and comparing them to what I went through, I'm starting to think it might be more than just trapped air in my case too. While bleeding the system helped initially, the problem came back after a few weeks. Turned out I had a failing brake master cylinder, that was the real culprit. The spongy pedal was a dead giveaway, I just didn't want to believe it at first! The master cylinder replacement was definitely pricier than just bleeding the system (ended up costing around 800 bucks with parts and labor), but it completely solved the issue. Haven't had a problem since. Given that you just had your brake fluid changed 3 months ago, and you're still having these issues, I'd bet you're looking at something similar. Might be worth getting the master cylinder checked out. These BMWs are great cars, but when the brake system starts acting up, it's better to tackle it head-on rather than hoping it'll sort itself out. Want me to share what symptoms to watch for that confirmed it was the master cylinder in my case?

Thanks so much for that detailed follow-up! It's actually a huge relief to hear someone else went through the exact same thing. That price for the master cylinder replacement isn't as bad as I was fearing, I was honestly bracing for much worse. The fact that your symptoms match mine so closely, especially that spongy pedal feel and the warning lights, makes me think you're onto something with the master cylinder diagnosis. And yes, please do share those additional symptoms! I want to be as informed as possible before heading to the mechanic. It's super helpful having this info from someone who's already been through it. The more I know, the better I can explain what's going on and avoid any unnecessary repairs. Really appreciate you taking the time to share your experience in such detail!