What must a driver do immediately after coupling a trailer?

After coupling a trailer, it is essential for a driver to test the brakes at low speed to ensure the braking system is functioning correctly. This step is crucial for safety because the braking system of the combination vehicle may have different dynamics compared to a single vehicle. When the driver applies the brakes, they can assess whether the trailer is responding as expected and ensure that both the tractor and trailer are operating in unison.

Testing the brakes helps confirm that the air or hydraulic systems are correctly connected and that there are no leaks or malfunctions that could lead to dangerous situations on the road. It is a critical safety check before proceeding further.

Other suggested actions, while important in their own right, do not directly address the immediate need to ensure the vehicle's braking system is operational after coupling. Checking mirrors or inspecting load distribution are necessary steps but are typically performed as part of pre-trip or post-coupling checks, rather than immediately after the coupling process. Similarly, performing a visual inspection of the vehicle is also important, but testing the brakes takes precedence in ensuring safety and functionality prior to embarking on a journey.