CDL Vehicle Combination Practice Test

When driving a set of doubles, which shut-off valve needs to be closed in the last trailer?

1. Front

2. Middle

3. Back

4. Emergency

The correct answer is: Back

In the context of driving a set of doubles, the correct choice is to close the shut-off valve in the last trailer, which is at the back. The purpose of shutting off the valve in the last trailer is primarily to ensure that the air supply to the last trailer's brakes is preserved while preventing any potential leaks or issues that could affect braking performance. When the shut-off valve in the back trailer is closed, it isolates that trailer's braking system from any potential air loss that might occur from a failure or leak elsewhere in the combination. This way, even if there is a malfunction in the air system of the first trailer or elsewhere, the last trailer will still have the necessary pressure to operate its brakes effectively. This enhances safety when maneuvering or when stopping, as it ensures that all trailers maintain their braking capabilities. If the shut-off valves in other positions (the front or middle) were closed, it could lead to diminished braking power in either the front or both trailers, which may endanger the vehicle's handling and safety. Therefore, keeping the last trailer's valve shut protects the integrity of the overall air brake system in the double combination.