What is the purpose of pulling out the trailer air supply control?

Pulling out the trailer air supply control is primarily meant to test the trailer emergency brakes. This control is a crucial part of the air brake system in vehicles with trailers. When the air supply control is pulled out, it allows the air pressure to be released from the trailer's air tanks. This simulates a situation where the trailer brakes can engage, allowing the driver to verify that the emergency braking system is functioning properly.

This is an essential safety check because if the emergency brakes do not operate correctly, it could lead to a dangerous situation while on the road, especially if the trailer were to detach from the towing vehicle. By pulling out the control, drivers can ensure that their trailer will be secure in the event that emergency braking is needed.

The other options relate to various functions in the braking system, but they do not accurately describe the specific purpose of pulling the trailer air supply control. For example, releasing the parking brakes and engaging the service brakes involve different controls, and checking air pressure levels is typically done through dashboard gauges rather than manipulating the air supply control.