What does the trailer air supply control (the red control in the cab) do in normal operation?

• A.It applies only the tractor's front brakes
• B.It is the trailer's cargo door control
• C.Pushing it in supplies air to the trailer and charges its tanks; pulling it out shuts off trailer supply and applies the trailer emergency (spring) brakes
• D.Pulling it out charges the trailer tanks faster

Why

Supply air is what releases trailer spring brakes and fills the trailer reservoirs for service braking. Removing that supply is also the parking and emergency command for the trailer brakes.

Manual topic: Air Brakes - Tractor Protection Valve · From the Air Brakes practice bank (AB-232).

Practice this subject as a full test

More Tractor Protection Valve, Trailer Brakes & Stab Braking questions

• A trailer breaks away from the tractor and tears both air lines loose. What happens on the tractor side in the first moments?
• At about what pressure will the trailer air supply control pop out (or the tractor protection valve close) as system pressure falls?
• How do you test the tractor protection valve during an inspection?
• What is the difference between the trailer's emergency (supply) air line and its service air line?
• What commonly identifies the emergency and service glad hands on a combination vehicle?