Audi S4's Erratic Brake Pedal: Warning Signs & Fixes

I had a very similar issue with my 2014 Audi S5 last year. Having some experience with basic brake system maintenance, I initially thought it might be related to worn brake rotors, but it turned out to be more complex. After noticing the inconsistent brake pedal feedback, I took it to my regular workshop. The diagnostic revealed an undervoltage issue affecting the brake system electronics, specifically the hydraulic brake control unit. This was causing irregular pressure distribution in the brake circuits. The total repair came to 240 Euro, which covered: Electronic system diagnosis, Repair of voltage-related issues, Testing of the brake system, Final safety check Since your symptoms sound nearly identical, its worth checking the electrical system before assuming its purely a mechanical brake issue. However, to provide more specific guidance, could you share: Any recent electrical issues or battery problems? Does the issue occur more frequently when other electrical systems are in use? Have you noticed any flickering dashboard lights? Current mileage on your brake rotors? The squeaky brakes or unusual sounds could also provide important clues about whether this is an electrical or mechanical issue.

Thanks for the input regarding the electrical system. My mileage is now at 86479 KM and I checked those points you mentioned. No electrical issues or battery problems noticed. The brake pedal behavior happens consistently regardless of other systems being used. The brake pads were replaced during the last service and the dashboard shows no warning lights or flickering. Had a closer inspection of the brake system yesterday and noticed a small fluid spot near the master cylinder. Starting to think this might be related to a hydraulic brakes issue rather than electrical. The disc brakes themselves look fine visually, but that fluid leak could explain the inconsistent pedal feel.

Thanks for the additional details. Given the fluid spot you found near the master cylinder, this does point more towards a hydraulic issue than the electrical problems I encountered with my S5. The inconsistent pedal feedback makes perfect sense now. A leak in the master cylinder will definitely cause varying brake pedal feel, as the system cant maintain consistent pressure. Unlike worn brake rotors or brake calipers which typically cause more predictable symptoms, a hydraulic leak creates that inconsistent pedal response youre experiencing. In my case, after the electrical fix, I actually ended up having a similar master cylinder issue about 6 months later. The repair involved: Replacing the master cylinder, Bleeding the brake system, New brake fluid, System pressure test The total cost was around 580 Euro including parts and labor. Its not a repair you want to postpone, as the brake pedal behavior will likely worsen over time. While the brake shoes and other mechanical components might be in good condition, any compromise in the hydraulic system needs immediate attention. Suggest getting this checked soon, especially since youve confirmed theres a visible fluid leak. Would be helpful to know if the fluid spot is growing or if youve noticed any changes in brake fluid levels in the reservoir?

Thanks for the detailed response about the master cylinder issue. I checked the brake fluid reservoir this morning and noticed it was slightly below the minimum mark, which makes sense given the leak. The fluid spot hasnt grown significantly over the past few days, but Ive been placing cardboard underneath to monitor it better. The varying brake pedal feel seems most noticeable after the car sits overnight, which aligns with what you mentioned about the hydraulic system losing pressure. My brake calipers and brake pads visually appear fine, so Im convinced now this is definitely a master cylinder problem rather than a mechanical issue with the disc brakes themselves. Given the safety implications and your repair cost estimate of 580 Euro, Ill schedule a workshop visit for next week. Better to address this now before it gets worse or causes a more serious brake system failure. Will update once I get the official diagnosis and repair completed.