What is the purpose of the hand valve in combination vehicle operation?

Answer: B — To test the trailer brakes

The hand valve, also known as the trailer hand brake or trolley valve, is specifically designed to allow the driver to apply the trailer brakes independently of the tractor brakes. This feature is particularly useful for testing the trailer brakes before driving, ensuring they are functioning properly. By pulling the hand valve, the driver engages the trailer brakes without affecting the tractor's brakes, making it an effective way to check the responsiveness and condition of the trailer braking system. Understanding the function of the hand valve is crucial for safe operation. When loading or unloading the trailer, or when parked on a slope, the hand valve provides a means to secure the trailer effectively without relying solely on the service brakes.