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The tractor protection valve will close and the trailer _______ brakes will come on when there is a major leak in the brake line.

A emergency
The tractor protection valve keeps air in the tractor if the trailer breaks away or develops a bad leak. If air pressure drops too low in the service line, the tractor protection valve will close. This stops air from escaping from the tractor and lets air out of the trailer emergency line, causing the trailer emergency brakes to come on.
B service
C parking