Final answer:
To test a trailer air brake system's air supply control, charge the system and press in the air supply knob while the engine is running and observe if the trailer brakes release properly.
Step-by-step explanation:
In the process of testing a trailer air brake system, one should charge the trailer air brake system and then push in the air supply knob to ensure that the trailer air supply control functions properly. This action will allow pressurized air to flow into the trailer's brake system, which will then engage the brakes. If the air supply control is functioning correctly, the trailer brakes will release. Remember that during the test, the engine should be running to maintain air pressure, but you shouldn't pump the brake pedal continuously, as that could deplete air in the system, causing the trailer brakes to engage. Disconnecting the air supply knob is part of a different test to ensure that the emergency brake system functions when air pressure is lost.