Going west to or east from Vancouver on the Trans-Canada with a big vehicle. Learn about the Coquihalla summit.
As part of your CDL air brake inspection, you must test the tractor protection valve & system.
Independently applies and releases the trailer service brakes.
Knowledge Test Question: This air brake control is not to be used for parking.
Although most tractors are still equipped with hand valves, beyond a trailer service tug test, these air brake controls are redundant - no longer used.
In the ole' days of trucking before pervasive use of spring parking brakes, the driver had to hook the air lines to the semi-trailer and than apply the trailer service brakes. This action held the trailer so the truck could be backed under and hooked to the trailer.
The system consists of the tractor protection valve and the trailer supply valve.