What is the first step to take before starting the air brake system test?

The first step to take before starting the air brake system test is to ensure the air pressure is at minimum level. This is crucial because the air brake system operates using compressed air, and the system needs to be at a specific pressure threshold to perform accurate testing. When beginning the test, it's essential to have the air pressure at a level that is sufficient to engage and disengage the brakes correctly, as well as to validate the overall functioning of the air brake system.

Checking the air pressure first ensures that you're not testing the system under conditions that may yield incorrect results. If the air pressure is too low, you may not be able to accurately assess the brake operation or the integrity of the system, potentially overlooking issues that could affect safety.

Starting the engine or releasing the parking brake is important in the overall testing process, but these steps come after confirming that the system meets the minimum air pressure requirement. Checking brake fluid levels is generally more relevant for hydraulic brake systems, not for air brakes, making it less applicable here.