How can you test that air flows to all trailers?

Answer: B — Engage the trailer brake and open valves

To ensure that air flows to all trailers, engaging the trailer brake and opening the valves is a fundamental procedure. This action allows you to activate the air brake system, enabling you to check whether the air pressure reaches each trailer in the setup. By opening the valves, you can also observe if there is adequate air pressure and distribution throughout the entire system, confirming that all trailers are receiving the necessary air supply for their braking systems to function properly. In contrast, engaging the tractor brake alone would not provide information about air flow to the trailers, as it does not test the air supply line to the trailers directly. Checking for leaks is important but doesn't verify air flow to each trailer; it's a separate maintenance check for any potential issues. Testing only one trailer does not give a complete picture of the air system’s functionality, as it neglects the other trailers connected to the system, which may also have their own issues. Thus, engaging the trailer brake and opening the valves is the most effective method to ensure proper air flow in a multi-trailer arrangement.