Before supplying air to the air tanks of a second trailer, what is the first step?

The first step before supplying air to the air tanks of a second trailer is to open the shutoff valves at the rear of the first trailer. This action ensures that the air system of the first trailer is fully operational and can provide the necessary compressed air to the second trailer. When the shutoff valves are opened, airflow from the first trailer's air system can pass through to the air tanks of the second trailer. This is essential for the functioning of the braking system of both trailers, allowing for safe operation on the road.

It's crucial to perform this step correctly because if the valves are closed, the second trailer will not receive the necessary air supply, leading to a malfunctioning brake system, which can compromise safety while driving. Proper sequence in managing air supply is fundamental in maintaining control and ensuring the integrity of the coupling between multiple trailers.