What is the primary function of the trailer air supply valve?

The primary function of the trailer air supply valve is to release the trailer spring park brakes. This valve is a crucial component in the air brake system of truck-trailer combinations, as it controls the flow of air to the trailer brakes. When the driver activates the valve, it allows compressed air to enter the trailer's air brake system, which releases the spring brakes that are applied while the vehicle is parked.

Understanding this function is essential for safe vehicle operation, as it ensures that a driver can efficiently engage the trailer’s braking system when needed. The spring brakes are typically self-engaging and designed to maintain the trailer in a stationary position; therefore, releasing them is necessary before the vehicle can begin moving.

Other choices involve functions that do not accurately represent the trailer air supply valve's role, as filling trailer tanks, maintaining air pressure in the tractor, and adjusting brake sensitivity are managed by different systems and components within the overall air brake setup.