How do you supply air to the trailer tanks?

To supply air to the trailer tanks, pushing in the trailer air supply valve is the correct method. This action allows air from the tractor's air system to flow into the trailer's air tanks. When the valve is pushed in, it opens the connection between the tractor and trailer, enabling the transmission of air necessary for the trailer's braking system to operate effectively.

Turning the air supply knob typically doesn't actively engage the system to supply air; instead, it is often a control for a specific system or indication rather than connecting air directly. Pulling out the trailer air supply valve generally serves the opposite function by closing the connection, which prevents air from flowing into the trailer tanks, making it unsuitable for supplying air. Checking the pressure gauge is a necessary step for monitoring the air system's status but does not actively supply air to the tanks. Thus, pushing in the trailer air supply valve is the action required to ensure proper air supply to the trailer.