What does the trailer supply valve do?

The trailer supply valve is designed to control the flow of air from the tractor to the trailer’s braking system. When activated, this valve allows reservoir air pressure from the tractor to flow into the trailer's air system, enabling the brakes on the trailer to function effectively. This is crucial for safe operation, as proper pressure in the trailer’s air system ensures that the brakes can engage when needed, particularly during routine driving and in emergency situations.

In contrast, the other options do not accurately describe the primary function of the trailer supply valve. For instance, releasing all air pressure would not allow for any braking action, and controlling engine speed is not related to trailer brake functionality. The statement about emergency braking only suggests a limited function, whereas the trailer supply valve serves a broader purpose in maintaining overall trailer brake system readiness.