Which of the following is NOT a function of the relay emergency valve?

The relay emergency valve plays a crucial role in the air brake system, particularly in trailer braking. Its primary functions include relaying air from the trailer reservoir to the trailer brake chambers to facilitate braking and directing air pressure effectively.

When considering the functions of the relay emergency valve, it is important to recognize that activating the tractor engine is not among its designated roles. The valve is specifically designed to manage air pressure communication between the trailer and the tractor, ensuring optimal performance of the braking system.

In contrast, the other options directly relate to how the relay emergency valve operates within the air brake system. It relays air from the trailer reservoir to the trailer brake chambers and stops air pressure from going back to the tractor to ensure that the trailer can brake independently when necessary. Additionally, directing trailer reservoir air pressure to the trailer brakes is a fundamental function that ensures safety and reliability in braking performance.

Thus, the choice indicating the activation of the tractor engine is not a function of the relay emergency valve accurately reflects a misunderstanding of the valve's purpose within the air brake system.