CDL Vehicle Combination Practice Test

What happens if the trailer's air supply is lost while driving?

1. Trailer brakes will disengage

2. Trailer brakes will apply automatically

3. Vehicle will come to a sudden stop

4. Nothing will happen

The correct answer is: Trailer brakes will apply automatically

When the trailer's air supply is lost while driving, the trailer brakes will apply automatically. This safety feature is designed to prevent the trailer from rolling uncontrollably if the air supply is compromised. In a typical air brake system, if air pressure drops below a certain level, the brakes are designed to spring into action. This application of the brakes helps to slow down the vehicle safely and minimize the risk of an accident by ensuring that the trailer remains under control. The automatic application of brakes when air supply is lost is a critical safety measure because it helps to manage the braking action evenly between the tractor and the trailer. This prevents situations where the trailer might push against the towing vehicle, which can lead to loss of control. The system typically relies on spring brakes, which are held off by air pressure, and when that pressure decreases, the springs engage, applying the brakes. Understanding this mechanism is important for drivers as it emphasizes the need for maintaining proper air pressure in the system and recognizing that a drop in air pressure means the trailer will automatically engage its brakes to ensure safety on the road.