Review Questions - Click On The Picture To Begin...
- Service Line and Emergency Line
- Red Line and Blue Line
- Parent Line and Child Line
- Primary Line and Secondary 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.
- Green
- Orange
- Red
- Blue
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.
- It supplies air to the trailer air tanks
- When the line loses air pressure, it causes the tractor protection valve to close
- It controls the emergency brakes on combination vehicles
- It carries air controlled by the foot brake or trailer hand brake
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).
- There is no control for the trailer air supply on newer vehicles as it's all done automatically
- A yellow diamond shaped knob used to control the tractor parking brake
- A lever on the side of the steering wheel used to control the trailer hand valve
- A red 8-sided knob used to control the tractor protection valve
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.
- Carries air controlled by the foot brake or the trailer hand brake
- Supplies air to the trailer air tanks
- Controls the parking brakes
- 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.
- 0 to 25 psi
- 20 to 45 psi
- 90 to 110 psi
- 50 to 75 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.
- It is only used to test the trailer service brakes
- 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
- 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.
- A valve used to release the air lines from the tractor to the trailer
- A lever which only controls the trailer service brakes
- The trailer release mechanism
- A lever which only controls the tractor service brakes
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.
- Green
- Red
- Black
- Blue
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.
- 20 to 45 psi
- 100 to 125 psi
- 50 to 70 psi
- 0 to 20 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.
- The trailer hand valve
- The brake pedal
- The red "trailer air supply" control valve
- The yellow "parking brake" control valve
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.
- You may wear out the tractor brakes more quickly
- There is an increased danger of making the trailer skid
- It only applies braking force to the tractor brakes
- It can throw your brakes out of adjustment
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.
- When the valve closes, it stops any air from going out of the tractor
- 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
- 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.
- Controls the parking brake on the tractor
- Activates the air compressor located on the trailer
- Supplies the tractor with air from the trailer
- 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.
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