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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 _____________.

A the tractor protection valve to close.

When the trailer breaks away or develops a serious leak, the tractor protection valve preserves air in the tractor. The tractor protection valve will close if air pressure in the service line drops too low. This prevents air from escaping the tractor and allows air to escape the trailer emergency line, triggering the trailer emergency brakes.