CDL Vehicle Combination Practice Test

If you cross air lines on a new trailer that has spring brakes, what will happen?

1. You will be able to release the trailer brakes

2. You won't be able to release the trailer brakes

3. The trailer brakes will activate unexpectedly

4. You will receive a warning alarm in the cab

The correct answer is: You won't be able to release the trailer brakes

When air lines are crossed on a new trailer that has spring brakes, the correct outcome is that you won't be able to release the trailer brakes. This is because spring brakes are designed to engage automatically when there is a loss of air pressure in the air brake system. If the air lines are crossed, it disrupts the normal functioning of the brake system and prevents the release of the trailer brakes. In a properly functioning system, the air lines control the release of the spring brakes through air pressure. When those lines are crossed, the air pressure does not reach the brakes as intended, resulting in the brakes remaining engaged. This condition is crucial for safety because it prevents the trailer from rolling away unintentionally and ensures that the brakes are applied effectively during situations such as a vehicle stopping on a slope. Understanding this function is vital for safe trailer operation and managing braking systems in combination vehicles.