When does the brake system failure warning buzzer sound?

Answer: C — When the rear air pressure gauge drops below 70 PSI

The brake system failure warning buzzer sounds when the rear air pressure gauge drops below 70 PSI. This threshold is critical for ensuring that there is adequate air pressure in the braking system, which is essential for the safe operation of the vehicle. When the air pressure drops to this level, it indicates a potential failure or malfunction within the air brake system, which could jeopardize braking performance. Maintaining proper air pressure is vital because the brakes rely on this pressure to function effectively. If the pressure is insufficient, the brakes may not engage properly, leading to an increased risk of accidents. Therefore, the warning buzzer serves as an important alert for the driver to take corrective action, such as inspecting the air pressure and determining the appropriate measures to ensure the braking system is functioning correctly. Other options refer to conditions that do not directly relate to the specific operation or safety of the brake system, making them less relevant in the context of braking system failure alerts.