What does the emergency air line do in a combination vehicle?

The emergency air line in a combination vehicle serves two critical functions: it supplies air to the trailer air tanks and controls the emergency brakes. This line is designed to maintain a constant air supply to the trailer, ensuring that the brakes remain operational. In the event of a disconnection or failure of the service line, this emergency air line will automatically activate the trailer's emergency braking system, bringing the trailer to a stop. This system is a vital safety feature to prevent accidents caused by brake failure or separation of the tractor and trailer.

The functionalities of the other options do not pertain to the role of the emergency air line. For instance, regulating temperature in the trailer is managed by separate systems, while activating trailer lights and controlling hydraulic functions are not associated with the air line’s purpose. Each aspect of a combination vehicle's systems has distinctly defined roles, emphasizing the importance of understanding the specific functions of each component.