Final answer:
All vehicles with air brakes are required to have a low air warning signal to alert the driver of potentially dangerous low air pressure in the brake system.
Step-by-step explanation:
It is indeed true that all vehicles equipped with air brakes must have a low air warning signal. This safety feature is mandated to ensure that the driver is warned when the air brake pressure falls below a certain level, which is usually 60 psi (pounds per square inch). The low air warning signal can be either a visual alert, an audible alarm, or both. It's a critical component because if the air pressure gets too low, the brakes could fail, leading to potentially dangerous situations. The low air warning serves as an early indication that the air brake system needs attention before it becomes a safety hazard.