What is the primary purpose of pulling the tractor ahead slightly with the trailer brakes locked?

Pulling the tractor ahead slightly while the trailer brakes are locked serves the primary purpose of ensuring a tight coupling between the tractor and trailer. This maneuver allows the driver to confirm that the kingpin of the trailer has properly engaged with the fifth wheel of the tractor. A secure connection is vital for safe operation, as it minimizes the risk of the trailer detaching while in transit. Performing this action helps identify any potential issues with the connection before the vehicle is put on the road.

While adjusting the trailer lights, testing the air lines, or checking tire pressure are important vehicle maintenance tasks, they are not directly related to the process of confirming that the tractor and trailer are securely coupled. Therefore, focusing on achieving a tight coupling is the key reason for pulling the tractor slightly ahead with the trailer brakes locked.