If a driver applies 20 pounds of brake application using the hand valve, which brakes are engaged?

When a driver uses the hand valve to apply the brakes, it primarily engages the trailer brakes. This action is designed to provide additional braking force to the trailer independently of the tractor, which is particularly useful in situations like backing up a trailer or controlling the trailer's speed when driving downhill. The hand valve allows for better maneuverability and control over the trailer, especially in tricky driving conditions.

In contrast, the foot brake application effects both the tractor and the trailer brakes, ensuring that the entire vehicle slows down uniformly. The hand valve is designed specifically to enhance the functionality and safety of trailer braking when needed without automatically involving the tractor brakes. Thus, when 20 pounds of brake application is made via the hand valve, it exclusively engages the brakes on the trailer.