SQ8 Brake Booster Failure Leads to ABS Warning

Having dealt with a similar brake system issue on a 2021 Q8, I can share some relevant experience. As someone with moderate experience in car maintenance, these brake performance problems need immediate attention for vehicle safety. My case also showed ABS warnings and decreased braking power. The hydraulic brakes were affected by improper bleeding during routine maintenance. The workshop diagnosed it using specialized diagnostic tools and found air bubbles in the brake lines. The fix involved: Complete brake system bleeding, Brake fluid replacement, Recalibration of the brake pressure sensors, Full system diagnostic scan Total repair cost was 90 Euro and took about 2 hours to complete. The harder brake pedal feel returned to normal immediately after the repair. To provide more specific advice for your case, could you share: Any unusual noises when pressing the brake pedal? Did this start suddenly or gradually? Have you had any recent brake work done? Does the pedal feel different when the engine is off vs running? While your warranty should cover this, documenting these symptoms will help the service department diagnose the issue faster.

Had my last service at 54000 KM and the issue started shortly after. There is a subtle hissing noise when pressing the brake pedal, and the pedal feels significantly different when the engine is off compared to when running. The brake performance degradation was gradual over about 2 weeks. The brake booster seems to provide less assistance than before, making the pedal extremely stiff. Based on the service history and symptoms, this sounds like it could be more serious than just air in the lines. When checking with diagnostic tools at home, I noticed the brake pressure readings fluctuate abnormally. Hoping the warranty covers any replacement parts needed.

Thanks for those details about the symptoms. Your description matches exactly what I experienced, and it does point to something more serious than air in the brake lines. In my case, after the initial diagnosis was inconclusive, the workshop found that the brake booster pressure sensor had completely failed. The diagnostic tools showed similar unstable pressure readings, and the characteristic hissing noise was a dead giveaway. The repair required: Full brake booster assembly replacement, New pressure sensor installation, System recalibration, Brake fluid flush The replacement parts were covered under warranty. The total repair took about 4 hours as they needed to verify all hydraulic brakes were functioning correctly after the fix. The good news is that after the repair, the brake pedal feel returned to normal, and the ABS warning disappeared. The hissing noise was completely gone, and the pressure readings stabilized. Since your symptoms are identical and the timing correlates with recent service, I strongly suggest having the dealer check the brake booster pressure sensor first. The fluctuating pressure readings from your diagnostic tools strongly indicate this as the root cause. Would recommend documenting all symptoms and diagnostic readings for your warranty claim.

Thanks everyone for the helpful input. After considering the symptoms and your experiences, I feel more confident about what to expect at the service appointment. The correlation between my brake pressure sensor readings and the symptoms you described is quite revealing. My next steps will be: Document all diagnostic readings from my ABS system, Record a video of the hissing noise, Note the exact dates when brake pedal issues started, Schedule warranty service ASAP for vehicle safety Really appreciate the detailed breakdown of the repair process and parts needed. Knowing it could take 4 hours for a complete brake booster and pressure sensor replacement helps me plan better. Will update once the warranty service is completed in case others experience similar brake system problems in the future.