While you are driving a combination vehicle, if the service line comes apart but the emergency line stays together, what will immediately occur?

A Nothing is likely to happen until you try to apply the brakes.

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]