CDL Vehicle Combination Practice Test

What action should you take before backing a tractor under a trailer?

1. Ensure the air system is at normal pressure

2. Check the cargo load

3. Inspect the tires

4. Remove any obstructions

The correct answer is: Ensure the air system is at normal pressure

Before backing a tractor under a trailer, ensuring that the air system is at normal pressure is crucial because it directly affects the function of the vehicle's braking systems and other pneumatic operations. The air brake system needs to be fully pressurized to ensure that the brakes will function correctly during coupling and operation. If the air pressure is too low, it can lead to brake failure or other malfunctions, posing a significant safety risk while maneuvering and coupling the tractor and trailer. Checking the cargo load, inspecting the tires, and removing any obstructions are also important safety practices, but they relate to different aspects of vehicle safety and readiness. For example, ensuring the cargo load is balanced is more relevant for safe travel than for the act of backing under the trailer. Similarly, inspecting the tires is essential for road safety, while removing obstructions is crucial for maintaining a clear path during backing. However, these actions do not directly impact the immediate process of coupling in the same way that the air system pressure does, making the air pressure check the priority action before backing under a trailer.