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Question:

If the service line comes apart while you are driving a combination vehicle, but the emergency line stays together, what will happen right away?

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

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.