To test the air leakage rate, what should you do first?

Answer: B — Release the service brake

To test the air leakage rate, it is essential first to release the service brake. This step is crucial because it ensures that the air system is not under any pressure and allows you to accurately measure how much air is leaking from the system. Once the service brake is released, any air loss can be observed as the vehicle's air pressure stabilizes. Performing this action before checking the air gauge or engaging the brakes is necessary. If the brakes are engaged, it could lead to a false assessment of the leakage rate since the brake system will retain pressure and mask any significant leaks. Running the engine is also not required at this initial stage, as the focus should be solely on assessing the leakage rate without additional variables impacting the pressure readings. This method ensures a clear and straightforward process to effectively identify any potential issues with the air brake system.