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6.2 Combination Vehicle Air Brakes

In combination vehicles, the braking system has parts to control the trailer brakes, in addition to the parts described in Section 5. These parts are described below.

Trailer Hand Valve

The trailer hand valve (also called the “trolley valve” or “Johnson bar”) works the trailer brakes. The trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid. The foot brake sends air to all the brakes on the vehicle (including the trailer(s). There is much less danger of causing a skid or jackknife when using just the foot brake.

Never use the hand valve for parking because all the air might leak out, unlocking the brakes (in trailers that do not have spring brakes.) Always use the parking brakes when parking. If the trailer does not have spring brakes, use wheel chocks to keep the trailer from moving.

Tractor Protection Valve

The tractor protection valve keeps air in the tractor or truck should the trailer break away or develop a bad leak. The tractor protection valve is controlled by the “trailer air supply” control valve in the cab. The control valve allows you to open and shut the tractor protection valve. The tractor protection valve will close automatically if air pressure is low (in the range of 20 to 45 psi). When the tractor protection valve closes, it stops any air from going out of the tractor. It also lets the air out of the trailer emergency line. This causes the trailer emergency brakes to come on. (Emergency brakes are covered later.)

Trailer Air Supply Control

The trailer air supply control on newer vehicles is a red 8-sided knob used to control the tractor protection valve. Push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes. The valve will pop out, thus closing the tractor protection valve, when the air pressure drops into the range 20 to 45 psi. Tractor protection valve controls or “emergency” valves on older vehicles may not operate automatically. There may be a lever rather than a knob. The “normal” position is used for pulling a trailer. The “emergency” position is used to shut the air off and put on the trailer emergency brakes.

Trailer Air Lines (see Figure 6-6)

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 (also called the “control line” or “signal line”) carries air 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 (also called the supply line) has two purposes: (1) To supply air to the trailer air tanks, and (2) to control 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 that breaks, letting the air out. When the emergency line loses pressure, it also causes the tractor protection 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.

A question may come up about the proper use of the trailer hand valve (trolley valve). Remember, the trailer hand valve should never be used when driving and should only be used to test the trailer service brakes.

Remember: The tractor protection valve, which controls the trailer emergency brakes will will pop out when air pressure drops between 20 and 45 psi.

The tractor protection valve is designed to activate the trailer emergency brakes before activating the tractor emergency brakes when air pressure drops too low.

It is very important you understand what the service line does in an air brake system. The service line controls the "regular" brakes when you use the brake pedal. Make sure you fully understand the explanation below.
Just as important as the service line you should fully understand the below explanation of how the emergency line works on an air brake system.
When the tractor protection valve closes, the red "emergency brake" valve in the cab of the truck will pop out. This will stop any air from flowing out of the tractor and into the trailer. That way, if there is a major air leak leading to air pressure loss in the system, the trailer brakes will activate before the tractor brakes, helping you keep some control of the vehicle.

CDL:

Commercial Driver's License (CDL)

A CDL is required to drive any of the following vehicles:

  • Any combination of vehicles with a gross combined weight rating (GCWR) of 26,001 or more pounds, providing the gross vehicle weight rating (GVWR) of the vehicle being towed is in excess of 10,000 pounds.
  • Any single vehicle with a GVWR of 26,001 or more pounds, or any such vehicle towing another not in excess of 10,000 pounds.
  • Any vehicle, regardless of size, designed to transport 16 or more persons, including the driver.
  • Any vehicle required by federal regulations to be placarded while transporting hazardous materials.

Combination Vehicle:

A vehicle with two separate parts - the power unit (tractor) and the trailer. Tractor-trailers are considered combination vehicles.

HOS:

Hours Of Service

HOS refers to the logbook hours of service regulations.

OWI:

Operating While Intoxicated

OOS:

When a violation by either a driver or company is confirmed, an out-of-service order removes either the driver or the vehicle from the roadway until the violation is corrected.

Review Questions - Click On The Picture To Begin...

Every combination vehicle has two air lines which are called:
  • Primary Line and Secondary Line
  • Service Line and Emergency Line
  • Red Line and Blue Line
  • Parent Line and Child Line

Quote From The CDL Manual:

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.).

TruckingTruth's Advice:

Be sure you understand the difference between the Service Air Line and the Emergency Air Line.

  • The Service Air Line controls the service brakes when you use the brake pedal.
  • The Emergency Air Line supplies air to the trailer air tanks and the emergency brakes.
Next
What does the Trailer Air Supply Control do?
  • Supplies the tractor with air from the trailer
  • Activates the air compressor located on the trailer
  • Controls the parking brake on the tractor
  • Supplies the trailer with air from the tractor

Quote From The CDL Manual:

The trailer air supply control on newer vehicles is a red 8-sided knob used to control the tractor protection valve. Push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes. The valve will pop out, thus closing the tractor protection valve, when the air pressure drops into the range 20 to 45 psi. Tractor protection valve controls or "emergency" valves on older vehicles may not operate automatically. There may be a lever rather than a knob. The "normal" position is used for pulling a trailer. The "emergency" position is used to shut the air off and put on the trailer emergency brakes.

Prev
Next
What does the Service Air Line do?
  • Supplies air to the trailer air tanks
  • Controls the parking brakes
  • Carries air controlled by the foot brake or the trailer hand brake
  • Controls the emergency brakes

Quote From The CDL Manual:

The service line (also called the "control line" or "signal line") carries air 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.

Prev
Next
What is the usual color for the Service Line?
  • Red
  • Blue
  • Green
  • Orange

Quote From The CDL Manual:

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.

TruckingTruth's Advice:

While not an actual regulation, the normal color scheme is usually as follows:

  • Emergency Line: Red
  • Service Line: Blue
  • Electrical Line: Green

Again, these are not actual regulations so colors may vary from truck to truck, but these colors have become the industry norm.

Prev
Next
What does the Tractor Protection Valve do?
  • All of these are functions of the Tractor Protection Valve
  • Keeps air in the tractor or truck should the trailer break away or develop a bad leak
  • When the valve closes, it stops any air from going out of the tractor
  • Lets the air out of the trailer emergency line, causing the trailer emergency brakes to come on

Quote From The CDL Manual:

The tractor protection valve keeps air in the tractor or truck should the trailer break away or develop a bad leak. The tractor protection valve is controlled by the "trailer air supply" control valve in the cab. The control valve allows you to open and shut the tractor protection valve. The tractor protection valve will close automatically if air pressure is low (in the range of 20 to 45 psi). When the tractor protection valve closes, it stops any air from going out of the tractor. It also lets the air out of the trailer emergency line. This causes the trailer emergency brakes to come on.

Prev
Next
What is the Trailer Hand Valve used for?
  • Should be used to recover a trailer that has begun to jackknife
  • During long or steep declines, the trailer hand valve should be used to protect against tractor brake fade
  • It is only used to test the trailer service brakes
  • Can be used in place of the parking brake

Quote From The CDL Manual:

The trailer hand valve (also called the "trolley valve" or "Johnson bar") works the trailer brakes. The trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid. The foot brake sends air to all the brakes on the vehicle (including the trailer(s'). There is much less danger of causing a skid or jackknife when using just the foot brake.

Never use the hand valve for parking because all the air might leak out, unlocking the brakes (in trailers that do not have spring brakes.) Always use the parking brakes when parking. If the trailer does not have spring brakes, use wheel chocks to keep the trailer from moving.

TruckingTruth's Advice:

Many trucks no longer come with a trailer hand valve, but if your truck has one, you should use it only for testing the trailer service brakes and nothing more.

Prev
Next
What is the usual color for the Emergency Line?
  • Black
  • Green
  • Blue
  • Red

Quote From The CDL Manual:

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.

TruckingTruth's Advice:

While the emergency line will almost always be red, the color is not a regulation so some trucks may use other colors.

Prev
Next
What is the Trailer Hand Valve?
  • A lever which only controls the trailer service brakes
  • A lever which only controls the tractor service brakes
  • A valve used to release the air lines from the tractor to the trailer
  • The trailer release mechanism

Quote From The CDL Manual:

The trailer hand valve (also called the "trolley valve" or "Johnson bar") works the trailer brakes. The trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid. The foot brake sends air to all the brakes on the vehicle (including the trailer(s'). There is much less danger of causing a skid or jackknife when using just the foot brake.

Never use the hand valve for parking because all the air might leak out, unlocking the brakes (in trailers that do not have spring brakes.) Always use the parking brakes when parking. If the trailer does not have spring brakes, use wheel chocks to keep the trailer from moving.

Prev
Next
On newer vehicles, what controls the trailer air supply?
  • A yellow diamond shaped knob used to control the tractor parking brake
  • A red 8-sided knob used to control the tractor protection valve
  • A lever on the side of the steering wheel used to control the trailer hand valve
  • There is no control for the trailer air supply on newer vehicles as it's all done automatically

Quote From The CDL Manual:

The trailer air supply control on newer vehicles is a red 8-sided knob used to control the tractor protection valve. Push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes. The valve will pop out, thus closing the tractor protection valve, when the air pressure drops into the range 20 to 45 psi. Tractor protection valve controls or "emergency" valves on older vehicles may not operate automatically. There may be a lever rather than a knob. The "normal" position is used for pulling a trailer. The "emergency" position is used to shut the air off and put on the trailer emergency brakes.

Prev
Next
Which of the following statements about the Emergency Air Line is false?
  • It supplies air to the trailer air tanks
  • It controls the emergency brakes on combination vehicles
  • It carries air controlled by the foot brake or trailer hand brake
  • When the line loses air pressure, it causes the tractor protection valve to close

Quote From The CDL Manual:

The emergency line (also called the supply line) has two purposes:

  • (1) To supply air to the trailer air tanks
  • (2) to control 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 that breaks, letting the air out. When the emergency line loses pressure, it also causes the tractor protection valve to close (the air supply knob will pop out).

Prev
Next
Which device inside the cab of a truck controls the Tractor Protection Valve
  • The yellow "parking brake" control valve
  • The trailer hand valve
  • The red "trailer air supply" control valve
  • The brake pedal

Quote From The CDL Manual:

The tractor protection valve keeps air in the tractor or truck should the trailer break away or develop a bad leak. The tractor protection valve is controlled by the "trailer air supply" control valve in the cab. The control valve allows you to open and shut the tractor protection valve. The tractor protection valve will close automatically if air pressure is low (in the range of 20 to 45 psi). When the tractor protection valve closes, it stops any air from going out of the tractor. It also lets the air out of the trailer emergency line. This causes the trailer emergency brakes to come on.

Prev
Next
Why shouldn't you use the trailer hand valve while driving?
  • It only applies braking force to the tractor brakes
  • There is an increased danger of making the trailer skid
  • It can throw your brakes out of adjustment
  • You may wear out the tractor brakes more quickly

Quote From The CDL Manual:

The trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid. The foot brake sends air to all the brakes on the vehicle (including the trailer(s'). There is much less danger of causing a skid or jackknife when using just the foot brake.

Prev
Next
The Tractor Protection Valve will close automatically in what air pressure range
  • 50 to 75 psi
  • 0 to 25 psi
  • 90 to 110 psi
  • 20 to 45 psi

Quote From The CDL Manual:

The tractor protection valve will close automatically if air pressure is low (in the range of 20 to 45 psi).

TruckingTruth's Advice:

It is definitely worth memorizing the psi range the tractor protection valve will automatically close. Not only will you need to know this for the written and pre-trip exams, but you'll definitely want to know this info for real-world driving situations and when testing the air brake system.

Prev
Next
The trailer air supply control valve will pop out, thus closing the tractor protection valve, in what air pressure range?
  • 50 to 70 psi
  • 0 to 20 psi
  • 20 to 45 psi
  • 100 to 125 psi

Quote From The CDL Manual:

The trailer air supply control on newer vehicles is a red 8-sided knob used to control the tractor protection valve. Push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes. The valve will pop out, thus closing the tractor protection valve, when the air pressure drops into the range 20 to 45 psi. Tractor protection valve controls or "emergency" valves on older vehicles may not operate automatically. There may be a lever rather than a knob. The "normal" position is used for pulling a trailer. The "emergency" position is used to shut the air off and put on the trailer emergency brakes.

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