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You have a major leak in the service line, and you apply the brakes. Service air pressure will escape and cause the

A trailer emergency brakes to come on.
If air pressure in the emergency air line drops too low (say because of a bad leak in the service line), the trailer emergency brakes will come on. Also, the tractor protection valve will close to keep air from escaping from the tractor.
B tractor spring brakes to lock on.
C trailer tank pressure to be lost.