BMW Z4 Brake System Warning: Bad Sensor or Pump Issue?

As a 2005 BMW Z3 owner, I ran into a similar issue last month. The symptoms match exactly what you're describing, spongy brake pedal, warning lights, and an overactive hydraulic pump. Initially thought it was the sensor too, but turned out the brake system was improperly bled during my last service. Air bubbles in the system were causing the pump to compensate constantly. Took it to my trusted mechanic who had to completely flush and properly bleed the brake system. They also found some contaminated brake fluid that needed replacing. It was quite serious since air in brake lines can lead to brake failure. Had to sort it immediately. The fix wasn't particularly complex, but required proper equipment and expertise to get all the air out of the ABS unit. Definitely get it checked soon, these symptoms shouldn't be ignored on BMWs since they have a complex brake system. Would recommend finding a shop that specializes in German cars.

Thanks for sharing your experience! That's really helpful to know it was an air bubble issue in your case. I'm dealing with similar symptoms right now and was worried it might be something more expensive. Just curious, do you remember roughly how much you ended up paying for the brake system flush and bleeding? Also, has everything been working smoothly since the repair or have you noticed any other quirks? I've got a Z4 rather than a Z3, but these BMWs seem to share quite a few common issues.

Hey there! Happy to give you an update on my Z3 situation. After getting it fixed, everything's been running like clockwork, no more weird noises or brake pedal issues. The total bill came to 90 € for the complete brake system flush and bleeding. Turns out I was right about the brake booster pressure sensor being faulty too, so they sorted that while they were at it. I know you've got a Z4, but these BMW brake systems are pretty similar across models. The mechanic really knew their stuff, they explained how air bubbles can wreak havoc in these systems. Since the repair, my brake pedal feels nice and firm again, and that annoying hydraulic pump noise has completely disappeared. If your symptoms are anything like what I experienced, you might be looking at a similar fix. It's definitely worth getting it checked out, the peace of mind alone is worth it. Can't put a price on reliable brakes, especially in these sporty BMWs!

Thanks for all this info! Just got back from the mechanic today actually. You won't believe it, my issue turned out to be different than expected. The brake fluid reservoir had a hairline crack, causing air to get into the system. That's why my pump was going crazy trying to maintain pressure. The fix wasn't too bad, about 180 bucks including parts and labor. Took them about 2 hours to replace the reservoir, flush the system, and bleed the brakes properly. They showed me the cracked part, it was pretty obvious once they pointed it out. The brake pedal feels solid now, those annoying warning lights are gone, and the car's running beautifully. What a relief it wasn't something more serious! Really glad I didn't put off getting it checked. Lesson learned, sometimes what seems like a major problem can have a relatively simple solution. Thanks again for helping me understand what I was dealing with!