CDL Vehicle Combination Practice Test

What happens when the trailer breaks away and pulls apart both air lines?

1. The trailer brakes come on

2. The tractor moves forward

3. The electrical supply is lost

4. The trailer releases from the tractor

The correct answer is: The trailer brakes come on

When a trailer breaks away from the truck and pulls apart both air lines, it triggers the trailer's braking system to engage automatically. This is a safety feature designed to prevent the trailer from rolling uncontrolled if it becomes disconnected from the tractor. The trailer has an emergency brake system that activates when there is a loss of air supply, which happens when both air lines are damaged or disconnected. This results in the trailer brakes being applied, bringing the trailer to a stop. Understanding how the braking system functions in a vehicle combination is crucial for ensuring safety on the road. The other choices describe scenarios that do not occur because they do not involve the proper reaction of the trailer's braking system in the event of a breakaway. The electrical supply loss and separation of the trailer from the tractor may happen, but the immediate and significant response is the engagement of the trailer brakes. Recognizing these responses is vital for any commercial driver in understanding their vehicle's systems and ensuring operational safety.