CDL Vehicle Combination Practice Test

Which of the following is crucial after supplying air to the trailer?

1. Check for visible wiring defects

2. Check if the airlines are crossed

3. Ensure the tires are aligned

4. Measure the cargo weight

The correct answer is: Check if the airlines are crossed

When supplying air to the trailer, checking if the airlines are crossed is essential for safety and functionality. Crossed airlines can lead to improper braking system operation, which is particularly critical in a vehicle combination. If the service and emergency lines are reversed, braking could fail to happen in the expected way, which may endanger the driver, the load, and others on the road. Ensuring that the airlines are correctly connected allows the air braking system to function properly. This means that the trailer will respond accurately when the brakes are applied, maintaining control and safety during operation. It also helps prevent issues such as air leaks or malfunctioning brakes, which can arise from improperly routed airlines. While checking for wiring defects, aligning tires, and measuring cargo weight are important maintenance and operational checks, they do not directly impact the immediate function of the braking system in the same critical way that verifying the connection of the airlines does right after supplying air to the trailer.