When you open the emergency shut-off valve at the back of the last trailer after supplying air to trailer(s) _________.

Check that air flows to all trailers use the tractor parking brake and/or chock the wheels to hold the vehicle. Wait for air pressure to reach normal, then push in the red "trailer air supply" knob. This will supply air to the emergency (supply) lines. Use the trailer hand brake 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. Close the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer hand brake or the service brake pedal is on), then close the valve. If you do NOT hear air escaping from both lines, check that the shut-off valves on the other trailer(s) and dolly(s) are in the OPEN position. You MUST have air all the way to the back for all the brakes to work.