Final answer:
Before moving the tractor, it is imperative to wait until the air system reaches normal pressure after engaging the trailer supply valve, ensuring the brakes function properly and safely.
Step-by-step explanation:
After pushing in the trailer supply valve, you should not move the tractor until the whole air system is at normal pressure. It is essential that the air pressure is sufficient for the brakes to operate effectively. In pneumatic brake systems, air pressure must build up to the manufacturer's specified level to ensure that the brake components have enough force to safely control the vehicle. If the system is not at normal pressure, the brakes might not function correctly, potentially leading to unsafe driving conditions.
It's important to understand that, similar to filling a tire, if air continues to be pumped into an air brake system without allowing for expansion or release, the pressure increases. Therefore, allowing the air system to reach and maintain the manufacturer-specified pressure is crucial for safe operation. Once the normal operating pressure has been reached, as indicated by the pressure gauge, it is safe to move the tractor.