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Question:

With the hand valve on, you should test the trailer brakes by opening the service line valve at the rear of the trailer. When you do this, you should hear:

A Air escape from the open valve.
Explaination

Use the trailer handbrake to provide air to the service line. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged.