What indicates that a vehicle has failed the brake test?

A vehicle that produces a metal-to-metal sound during braking is a clear indicator that it has failed the brake test. This sound typically signifies that the brake pads have worn down completely and are making contact with the brake rotors, which not only diminishes braking efficiency but can also lead to severe damage to the braking system itself.

In contrast, the other options do not directly indicate a failure in braking capability. A humming sound may arise from various mechanical issues but doesn't specifically point to brake system failure. Shuddering during braking can indicate uneven wear or other issues, but it is not as definitive as the metal-to-metal contact, and malfunctioning brake lights signal an electrical or bulb issue rather than the performance quality of the brakes themselves. Thus, the presence of a metal-to-metal sound stands out as a definitive sign that immediate attention is required to maintain vehicle safety on the road.