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Every combination vehicle has two air lines: the service line and the emergency line. They run between each vehicle (tractor to trailer, trailer to dolly, dolly to second trailer, etc.).
Service Air Line. The service line carries air, which is controlled by the foot brake or the trailer hand brake. Depending on how hard you press the foot brake or hand valve, the pressure in the service line will similarly change. The service line is connected to relay valves. These valves allow the trailer brakes to be applied more quickly than would otherwise be possible.
Emergency Air Line. The emergency line has two purposes. First, it supplies air to the trailer air tanks. Second, the emergency line controls the emergency brakes on combination vehicles. Loss of air pressure in the emergency line causes the trailer emergency brakes to come on. The pressure loss could be caused by a trailer breaking loose, thus tearing apart the emergency air hose. Or it could be caused by a hose, metal tubing or other part breaking, letting the air out. When the emergency line loses pressure, it also causes the tractor parking valve to close (the air supply knob will pop out).
Emergency lines are often coded with the color red (red hose, red couplers or other parts) to keep from getting them mixed up with the blue service line.
What colors are the emergency lines and service lines?
Every combination vehicle has two air lines that run between each vehicle (tractor to trailer, trailer to dolly, dolly to the second trailer, etc.):
Loss of air pressure in the emergency line causes what?
The emergency line has two purposes. What are they?
How do you control airflow in the service line to the trailer?
When the emergency line loses pressure, it causes the trailer emergency brakes to come on. What else happens?
Which of the following describes the air pressure in the service air line?
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