Final answer:
Air is supplied to the trailer air tank by pushing in the trailer air supply knob/valve, which controls the air flow from the truck's air compressor to the trailer's brake system. Connecting the emergency line glad hand is also essential to establish the air supply connection.
Step-by-step explanation:
You supply air to the trailer air tank by pushing in the trailer air supply knob/valve. This control is commonly found in the cab of the truck and is part of the tractor-trailer air brake system. Pushing in the knob/valve supplies air from the truck's compressor through the air lines to the trailer's brake system, ensuring that there is sufficient air pressure to operate the trailer brakes effectively. It’s important to check the air pressure gauge to make sure that the air pressure is at the correct level and that there are no leaks in the system. Pulling the knob out typically sets the trailer brakes (as with a parking brake function). Connecting the emergency line glad hand is also an integral step in the process, as it establishes the necessary connection between the truck and the trailer for the air supply system to operate correctly.