Final answer:
Pulling the dash-mounted trailer air supply valve (red) outward (closed) results in the application of the trailer parking brakes. This is a safety mechanism in air brake systems on tractor-trailers that ensures the trailer remains stationary when needed.
Step-by-step explanation:
When the dash-mounted trailer air supply valve (red) is pulled outward (closed), it results in the application of the trailer parking brakes. This action is part of the air brake system on a tractor-trailer setup, which is essential for the safe operation of the vehicle. The redundant systems in trucks ensure that if air pressure is lost in the system, the parking brakes will automatically engage due to spring pressure. This is a safety feature to prevent a trailer from moving when it is not supposed to.
The dash-mounted trailer air supply valve is connected to a system of valves and braking mechanisms that includes the tractor protection valve. When the red valve is pulled out and the system is in the closed position, it prevents air from being supplied to the trailer's brakes, causing them to engage due to spring pressure. While the air in the brake lines needs to be properly managed and non-compressible to ensure efficient brake operation, the specific action of pulling out the red valve is designed to apply the trailer parking brakes rather than direct air through the tractor protection valve or charge the supply line.