CDL Vehicle Combination Practice Test

What is the consequence of crossing air lines when attaching to an old trailer?

1. You will have increased brake efficiency

2. If the trailer has no spring brakes, you can drive away, but you will not have trailer brakes

3. The trailer will automatically brake

4. There will be no effect on the braking system

The correct answer is: If the trailer has no spring brakes, you can drive away, but you will not have trailer brakes

When crossing air lines while connecting to an old trailer, the correct consequence is that if the trailer does not have spring brakes, you can drive away, but you will not have trailer brakes. This situation arises because of the function of the air brake system in combination vehicles. In a typical setup, the emergency line is usually red and is meant to engage the trailer brakes in case of an air system failure. The service line is blue and is responsible for supplying air to the trailer brakes for normal operation. When these lines are crossed, the air supply to the trailer's braking system is disrupted. Therefore, if you were to drive away without the trailer brakes functioning properly, it could lead to a serious risk since you would only rely on the tractor's brakes. The presence or absence of spring brakes can change the dynamics. If the trailer is equipped with spring brakes, they would automatically engage in case of an air pressure drop, ensuring some level of braking even if the lines are crossed. However, if there are no spring brakes, the trailer would remain without effective braking capability, leaving the driver without control over the trailer braking system. This understanding highlights the importance of correctly connecting air lines and the potential hazards involved when they are mistaken.