VW Passat Hybrid's Spongy Brake Pedal Mystery Fixed

I had a similar issue with my 2018 VW Arteon Hybrid last year and have some experience working with hybrid brake systems. The symptoms you describe match what I encountered, particularly that spongy brake pedal feel that got progressively worse. The root cause in my case was actually an undervoltage issue affecting the hydraulic brake system controller. While I initially suspected worn brake rotors or mechanical issues with the disc brakes, the problem was electrical. The workshop diagnosed it using their diagnostic system and found the voltage regulation system needed recalibration and partial replacement. The repair took about 4 hours and cost 240€ including parts and labor. Since this was fixed, the brake pedal feel returned to normal and has remained consistent. However, to better assist with your specific case, could you provide: Current mileage on your Passat, Whether any warning lights appeared on the dashboard, If the issue occurs more when the car is cold or warm, Whether the regenerative braking still works normally These details would help determine if your issue is similar to what I experienced or if it might be related to other brake system components. While waiting for the additional information, I recommend getting the brake fluid levels checked, as low fluid can also cause similar symptoms in hybrid brake systems.

Thanks for sharing your experience. Our cases seem quite different. At my recent 80000km service just 2 weeks ago, they actually checked the brake fluid levels and all mechanical components of the brake system. All seemed fine then. Since then, I noticed the spongy brake pedal only appears after longer highway drives. The brake calipers feel slightly warm when this happens, and the pedal gradually returns to normal after cooling down. No warning lights have appeared on the dashboard yet. The regenerative braking works perfectly fine, suggesting the hybrid components are not the issue. This makes me think the problem might be related to the metal brakes or brake shoes rather than the electronic brake management system you mentioned. Going to schedule another inspection focused specifically on the mechanical brake components next week.

Thanks for the additional details about your brake system behavior. Given that the issue occurs after highway driving and is accompanied by warm brake calipers, this points more toward a mechanical rather than electrical issue, quite different from my previous voltage-related problem. The symptoms you describe strongly suggest brake fluid expansion due to heat buildup. While your brake fluid levels are correct, the fluid might be absorbing moisture over time, leading to a lower boiling point. This commonly creates a spongy brake pedal feel when the hydraulic brakes heat up during extended use. On my Arteon, I faced this exact situation about six months after my electrical issue was fixed. The solution was a complete brake fluid flush and replacement, which cost 120€. The workshop also found slight wear on the disc brakes, but not enough to require replacement. For your situation, have the technician specifically check: Brake fluid condition and water content, Brake caliper sliding pins for proper movement, Air in the hydraulic brake lines, Disc brake surface for uneven wear patterns These checks should identify if heat-related brake fluid expansion is causing your spongy pedal feel.

The pattern you describe with warm brake calipers and spongy pedal feel after highway drives does match what multiple users here have experienced with VW hybrids. The brake fluid expansion theory makes a lot of sense, especially since the issue resolves after cooling. My brake system was inspected today and the technician confirmed moisture contamination in the brake fluid. The hydraulic brakes were indeed affected by heat buildup during longer drives. They did a complete fluid flush and found one partially seized brake caliper that was causing uneven heating. Total cost came to 280€ including the new brake fluid, caliper service, and labor. The squeaky brakes I had started to notice are now gone too. The pedal feel is back to normal, even after extended highway driving. Will report back after a few weeks of driving to confirm the fix was successful. Really appreciate everyone sharing their expertise here, helped me better understand the issue and what to look for during the inspection.