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.