CDL Vehicle Combination Practice Test

What happens when you apply the brakes with a major leak in the service line?

1. The tractor emergency brakes engage

2. The trailer emergency brakes come on

3. The service air is compressed

4. The braking system is disabled

The correct answer is: The trailer emergency brakes come on

When there is a major leak in the service line and the brakes are applied, the trailer emergency brakes will come on. This is an important feature of combination vehicles equipped with air brake systems. The design includes a safety mechanism to help ensure that the trailer remains under control even if there is a failure in the service braking system. When the service line experiences a significant leak, air pressure is lost. In response to this loss of pressure, the system is designed to automatically engage the trailer's emergency brakes to prevent the trailer from rolling or causing an accident. This automatic engagement occurs because the emergency brakes rely on a different part of the air supply system, which is isolated from the service brakes. Therefore, this feature enhances safety, allowing the driver to have more control over the vehicle and trailer during a potential emergency. In contrast, if we consider the other options, they either imply that the braking system continues to function normally or that it becomes entirely disabled, which is not the case when the emergency brakes are engaged correctly.