Final answer:
The air in a trailer air brake system with a spring brake priority is directed first to the service reservoirs when coupled to a tractor air supply.
Step-by-step explanation:
When a trailer air brake system compliant with a spring brake priority system is coupled to a tractor air supply with no air in its on-board air tanks, the air is directed first to the service reservoirs. The reason for this is to ensure there is a supply of air available for operating the service brakes as soon as possible.
This is a critical safety feature because it allows the truck's normal brake functions to become operational quickly, providing the driver with the necessary braking capability. Filling the service reservoirs first ensures that even if there is a failure in the air supply later, the spring brakes, which are a safety backup, can be released, allowing the vehicle to be moved safely to a location for repair.