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6.2.4 – Trailer Air Lines

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.

Multiple-Choice Questions:

Question #331 (1 of 7)

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Loss of air pressure in the emergency line causes what?

  • The trailer emergency brakes to come on
  • The tractor emergency brakes to release
  • The trailer emergency brakes to release
  • The tractor emergency brakes to come on
Loss of air pressure in the emergency line causes the trailer emergency brakes to come on
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Question #330 (2 of 7)

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The emergency line has two purposes. What are they?

  • It supplies air to the trailer air tanks and provides an emergency air supply to the tractor
  • It supplies air to the tractor auxiliary air tanks and controls the service brakes on combination vehicles.
  • It supplies air to the tractor air tanks and controls the emergency brakes on the tractor
  • It supplies air to the trailer air tanks and controls the emergency brakes on combination vehicles.
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.
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Question #333 (3 of 7)

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What colors are the emergency lines and service lines?

  • The emergency lines are red, the service lines are blue
  • The emergency lines are blue, the service lines are red
  • The emergency lines are red, the service lines are yellow
  • The emergency lines are yellow, the service lines are blue
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.
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Question #328 (4 of 7)

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How do you control airflow in the service line to the trailer?

  • The red eight-sided knob
  • The foot brake or trailer hand brake
  • It's automatic and not in control of the driver
  • The round yellow knob
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.
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Question #329 (5 of 7)

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Which of the following describes the air pressure in the service air line?

  • If you pull out the round yellow knob you will increase the air pressure in the service air line. If you push in the knob you will decrease pressure in the service air line
  • When you apply the trailer hand brake, all of the air to the service line is released. When you release the trailer hand brake, the air pressure in the line is decreased.
  • If you pull out the red eight-sided knob you will increase the air pressure in the service air line. If you push in the knob you will decrease pressure in the service air line
  • Depending on how hard you press the foot brake or hand valve, the pressure in the service line will similarly change
Depending on how hard you press the foot brake or hand valve, the pressure in the service line will similarly change
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Question #327 (6 of 7)

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Every combination vehicle has two air lines that run between each vehicle (tractor to trailer, trailer to dolly, dolly to the second trailer, etc.):

  • The service line and the emergency line
  • The trailer line and the dump line
  • The chamber line and the brake evacuation line
  • The brake line and the tractor line
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.)
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Question #332 (7 of 7)

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When the emergency line loses pressure, it causes the trailer emergency brakes to come on. What else happens?

  • The tractor service brake will be applied
  • The emergency air chambers will pressurize
  • The tractor parking valve will close (the air supply knob will pop out)
  • The trailer parking valve will close (the air supply knob will pop out)
When the emergency line loses pressure, it also causes the tractor parking valve to close (the air supply knob will pop out)
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