Which item is a requirement for vehicles with air brakes regarding low pressure?

For vehicles equipped with air brakes, having a visual or audible warning system is essential for alerting the driver when air pressure is too low. This is critical because low air pressure can impair the braking system's effectiveness, posing a safety risk. A red light, a mechanical arm (often referred to as a wig-wag), or a buzzer functions as a reliable alert mechanism to notify the driver immediately and promptly address any potential issues with the braking system.

In the context of safety regulations, these warning devices help prevent the operation of a vehicle with insufficient air pressure, ensuring that the driver can take necessary precautions before it leads to braking failure. This proactive measure is crucial for maintaining the operational integrity of vehicles with air brake systems.

While other items, such as pressure gauges or manual backup systems, may serve important roles in vehicle operation, they do not specifically meet the requirement aimed at signaling low pressure directly to the driver in an immediate and actionable way.