You put on the brakes because there is a significant leak in the service line. What will happen?

A Service air will escape and cause the trailer emergency brakes to come on.

explanation

You may not notice a major leak in the service line until you try to put the brakes on. Then, the air loss from the leak will lower the air tank pressure quickly. If it goes low enough, the trailer emergency brakes will come on. [California Commercial Driver Handbook]