Final answer:
The tractor should not be moved until the air system is fully charged after pushing in the trailer supply valve, ensuring adequate air pressure for the brake system to function effectively.
Step-by-step explanation:
After pushing in the trailer supply valve, you should not move the tractor until the air system is fully charged. This is because air brake systems require adequate air pressure to operate effectively. Air, being a compressible gas, will first compress and warm up when force is applied, which can result in inefficient braking if the system is not fully pressurized.
Therefore, for the brakes to work properly, it's crucial that the air in the brake lines is not partially filled or empty, but fully charged to the proper levels as indicated by the system's gauges.