Spongy Brake Pedal Points to Dangerous Brake System Issues

Had a similar issue with my 2016 Audi S3 last year. With some experience working on brake systems, I recognized these symptoms but still needed professional diagnosis. The spongy brake pedal combined with vibration often suggests either worn brake pads or potential issues with the brake system electrical components. In my case, the workshop diagnosed an undervoltage problem affecting the brake control module, which was causing inconsistent brake pedal feedback. The repair included: Complete system diagnostic, Voltage regulator replacement, Brake system recalibration, Inspection of brake rotors and pads Total cost came to 240 Euro, including parts and labor. The repair resolved both the spongy pedal feel and vibration issues completely. Before suggesting specific solutions, could you provide: Current mileage on your S1, When was your last brake service, Whether the check engine or ABS light is illuminated, If the issue occurs more during cold starts or after driving These details would help determine if your issue matches what I experienced with the electrical system or if it might be related to mechanical brake components instead.

Thanks for sharing your experience. I had my last service at 85000 km (about 2 months ago) and currently at 88668 km. The symptoms you described with the electrical system sound different from my case since I have no warning lights on the dash. The spongy brake pedal and vibration in my hydraulic brakes seem to be getting worse, especially after highway driving. The squeaking from the disc brakes is now constant rather than intermittent. The brake calipers also look a bit corroded on visual inspection, though Im not sure if thats related. Just checked the brake fluid level and its slightly below the minimum mark, which might explain some of these symptoms. Will get it checked professionally to be safe, but wondering if anyone else found low brake fluid to cause similar issues?

Thanks for those additional details about your S1. After reading your description, I'm now even more convinced your issue differs from my electrical problems on the S3. The symptoms you describe strongly point to a hydraulic system issue, especially with the low brake fluid level. The combination of: Low brake fluid, Spongy pedal feel, Worsening symptoms after highway use, Constant squeaking from the brake calipers, Visible corrosion Usually indicates either a brake fluid leak or severely worn brake pads. In my S3, I actually had both issues about 6 months before the electrical problem. The metal brakes were worn enough to cause a small leak at the caliper seals. A quick test: check for drops or wetness around the wheels after parking overnight. If you spot brake fluid, thats likely your main issue. Either way, dont delay the workshop visit, low brake fluid can quickly lead to complete brake failure. My repair costs for the hydraulic brake system issue were around 380 Euro, which included: New brake fluid, Replacement of brake calipers, Fresh brake pads, Labor costs Worth every cent for the peace of mind of having proper braking power again.

Thanks everyone for the detailed responses. After checking under my car this morning as suggested, I noticed small drops near the front right wheel. Given all your feedback about my squeaky brakes and the symptoms with the hydraulic brakes, Im now pretty certain its a leak in the brake system causing these issues. Made an appointment at my regular workshop for tomorrow morning. The estimated cost of 400 Euro for new brake calipers, fluid, and labor seems in line with what others shared here. Will make sure they check the brake rotors too while theyre at it, just to be thorough. Really appreciate the help in narrowing down the problem. Having proper braking power is crucial, so better to fix it now before it gets worse. Will update the thread once I get the final diagnosis and repair completed.