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You are driving a combination vehicle when the trailer breaks away, pulling apart both air lines. You would expect the trailer brakes to come on and

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.