Spongy Brake Pedal Points to Hydraulic System Issues

Having moderate experience with brake repairs, I faced a similar problem with my 2012 S6. The brake pedal started feeling spongy, which turned out to be more than just low brake fluid. The master cylinder was showing signs of wear, causing inconsistent pressure in the hydraulic brake system. Had it checked at a workshop where they did a complete inspection of the brake lines and found small leaks contributing to the issue. The fix involved: Full brake system inspection, Master cylinder replacement, Brake fluid flush, Pressure testing of all brake lines Total cost came to 480 Euro and solved the spongy pedal issue completely. The brakes now feel firm and responsive. To better assist with your situation, could you share: Any warning lights on the dashboard? When did you last have a brake fluid change? Do you notice any fluid spots under the car? Does the pedal feel worse when the engine is cold or warm? These details would help determine if your S7 is facing similar brake system problems or if its something else entirely.

The last service was done at 143000km about 2 months ago. Before seeing your comment I was concerned about the brake lines, but now I am convinced to check the master cylinder too. The brake warning light started flickering yesterday when braking hard. My brake pedal feels particularly spongy when the engine is cold and improves slightly after warming up. No fluid spots visible under the car yet, but the hydraulic brakes definitely need inspection soon. The last fluid replacement was done during the previous service. Given your experience with a similar vehicle and brake system issues, your detailed solution makes a lot of sense. Will schedule a complete inspection focusing on both the master cylinder and brake lines.

Thanks for providing those additional details. Since I went through almost identical brake system issues with my S6, the flickering brake warning light combined with the cold-start pedal behavior strongly suggests master cylinder problems, very similar to what I experienced. The symptoms match perfectly: spongy pedal feel especially when cold, gradual improvement after warmup, and warning light activation during hard braking. These are classic signs of a failing master cylinder affecting the hydraulic brake system. While my repair included multiple components, the master cylinder was the core issue. The cold-start behavior you describe is particularly telling, the seals in the master cylinder likely arent maintaining proper pressure until they warm up and expand slightly. Since your brake fluid was recently changed, the inconsistent pedal feel is almost certainly mechanical rather than fluid-related. Would still recommend a full brake system inspection to check for any developing leaks, but focusing on the master cylinder first makes sense based on the symptoms. For safety, consider limiting hard braking until the system is checked, as my experience showed these symptoms tend to worsen quickly once they start.

Thanks everyone for the detailed responses. The similarities between our brake system issues are really helpful. After reading through the advice and comparing symptoms, I called my regular workshop this morning. The flickering brake warning light combined with the cold-start spongy brake pedal feel has me convinced to get a thorough inspection done. They agreed the master cylinder is likely the main culprit affecting the hydraulic brakes, rather than just a brake fluid issue. Have scheduled a complete brake system check for next week. They will pressure test the brake lines and inspect the master cylinder as the priority. The workshop quoted around 500 Euro for a full master cylinder replacement if needed, which seems fair based on the repair costs shared here. Will definitely play it safe and avoid any aggressive braking until then. Really appreciate all the insights, helped me understand the probable cause and get this sorted before it becomes more serious.