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6.2 Combination Vehicle Air Brakes (continued from previous page)

Hose Couplers (Glad Hands)

"Glad hands" are coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer. The couplers have a rubber seal that prevents air from escaping.

Clean the couplers and rubber seals before a connection is made. When connecting the glad hands, press the two seals together with the couplers at a 90-degree angle to each other. A turn of the glad hand attached to the hose will join and lock the couplers.

Some vehicles have “dead end” or dummy couplers to which the hoses may be attached when they are not in use. This will prevent water and dirt from getting into the coupler and the air lines. Use the dummy couplers when the air lines are not connected to a trailer. If there are no dummy couplers, the glad hands can sometimes be locked together (depending on the couplings). It is very important to keep the air supply clean.

When coupling, be sure to couple the proper glad hands together. To help avoid mistakes, colors are sometimes used. Blue is used for the service lines, and red for the emergency (supply) lines. Sometimes, metal tags are attached to the lines with the words “service” and “emergency” stamped on them.

If you do cross the air lines, supply air will be sent to the service line instead of going to charge the trailer air tanks. Air will not be available to release the trailer spring brakes (parking brakes). If the spring brakes do not release when you push the trailer air supply control, check the air line connections.

Older trailers do not have spring brakes. If the air supply in the trailer air tank has leaked away, there will be no emergency brakes, and the trailer wheels will turn freely. If you crossed the air lines, you could drive away, but you would not have trailer brakes. This would be very dangerous. Always test the trailer brakes before driving with the hand valve or by pulling the air supply (tractor protection valve) control. Pull gently against them in a low gear to make sure the brakes work.

Trailer Air Tanks

Each trailer and converter dolly has one or more air tanks. They are filled by the emergency (supply) line from the tractor. They provide the air pressure used to operate trailer brakes. Air pressure is sent from the air tanks to the brakes by relay valves. The pressure in the service line tells how much pressure the relay valves should send to the trailer brakes. The pressure in the service line is controlled by the brake pedal and the trailer hand brake.

It is important not to let water and oil build up in the air tanks. If you do, the brakes may not work correctly. Each tank has a drain valve on it, and you should drain each tank every day. If your tanks have automatic drains, they will keep most moisture out, but you should still open the drains to make sure.

Shut-off Valves

Shut-off valves (also called “cut-out cocks”) are used in the service and supply air lines at the back of trailers used to tow other trailers. These valves permit closing the air lines off when another trailer is not being towed. You must check that all shut-off valves are in the open position except the ones at the back of the last trailer, which must be closed.

Trailer Service, Parking and Emergency Brakes

Newer trailers have spring brakes just like trucks and truck tractors. However, converter dollies and trailers built before 1975 are not required to have spring brakes. Those that do not have spring brakes have emergency brakes that work from the air stored in the trailer air tank. The emergency brakes come on whenever air pressure in the emergency line is lost. These trailers have no parking brake. The emergency brakes come on whenever the air supply knob is pulled out or the trailer is disconnected. But the brakes will hold only as long as there is air pressure in the trailer air tank. Eventually, the air will leak away, and then there will be no brakes. Therefore, it is very important for safety that you use wheel chocks when you park trailers without spring brakes.

A major leak in the emergency line will cause the tractor protection valve to close and the trailer emergency brakes to come on

You may not notice a major leak in the service line until you try to put the brakes on. Then, the air loss from the leak will lower the air tank pressure quickly. If it goes low enough, the trailer emergency brakes will come on.

Test Your Knowledge

  • Why should you not use the trailer hand valve while driving?
  • Describe what the trailer air supply control does.
  • Describe what the service line is for.
  • What is the emergency air line for?
  • Why should you use chocks when parking a trailer without spring brakes?
  • Where are shut-off valves?

Study section 6.2 if you can't answer all of these questions.

Make sure you're familiar with what glad hands are. Also, be sure you specifically remember that glad hands have rubber seals which prevents air from escaping. The rubber seals must be checked to be sure they are in good condition to prevent air leaks.
There is no requirement or exact standard for color coding. While it is general practice to make blue the service line, red the emergency line, and green the electrical line, these colors are not required by any laws or regulations.
Remember: The trailer air brakes are supplied with air using the emergency line (also called the supply line).
Remember: Air tanks should be drained daily. We've seen this a couple times now and it's something that comes up on the written exam for many people.
Shut off valves are only used on trailers designed to be towed in conjunction with other trailers (doubles / triples). The shut off valves, when in the open position, allows air to flow out to another trailer. If you're only pulling one trailer, the shut off valves should always be closed or else the air will simply escape out the back of the trailer. If you're pulling more than one trailer, all valves should be open except the very last trailer, which should have all valves in the closed position.

Combination Vehicle:

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

Doubles:

Refers to pulling two trailers at the same time, otherwise known as "pups" or "pup trailers" because they're only about 28 feet long. However there are some states that allow doubles that are each 48 feet in length.

HOS:

Hours Of Service

HOS refers to the logbook hours of service regulations.

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

How are the trailer air tanks filled?
  • Using the trailer tank line
  • Using a combination of the emergency air line and the service air line
  • Using the emergency air line
  • Using the service air line

Quote From The CDL Manual:

Each trailer and converter dolly has one or more air tanks. They are filled by the emergency (supply) line from the tractor. They provide the air pressure used to operate trailer brakes.

TruckingTruth's Advice:

The service air line is only used to control the service brakes (when you press on the brake pedal or use the trailer hand-valve). All other trailer air functions use the emergency line.

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What happens if you cross the Emergency Air Line and the Service Air Line?
  • Air will continue filling the air holding tanks until the tanks rupture from excessive pressure
  • Newer vehicles automatically detect if the lines are crossed and will auto-correct itself
  • Supply air will be sent to the service line instead of going to charge the trailer air tanks which means the trailer spring brakes will not release
  • Supply air will cause all brakes to release and the truck will roll freely without any braking ability

Quote From The CDL Manual:

If you do cross the air lines, supply air will be sent to the service line instead of going to charge the trailer air tanks. Air will not be available to release the trailer spring brakes (parking brakes). If the spring brakes do not release when you push the trailer air supply control, check the air line connections.

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Trailers built before what year may not be equipped with trailer spring brakes?
  • 1970
  • 1990
  • 1985
  • 1975

Quote From The CDL Manual:

Newer trailers have spring brakes just like trucks and truck tractors. However, converter dollies and trailers built before 1975 are not required to have spring brakes. Those that do not have spring brakes have emergency brakes that work from the air stored in the trailer air tank. The emergency brakes come on whenever air pressure in the emergency line is lost. These trailers have no parking brake. The emergency brakes come on whenever the air supply knob is pulled out or the trailer is disconnected. But the brakes will hold only as long as there is air pressure in the trailer air tank. Eventually, the air will leak away, and then there will be no brakes. Therefore, it is very important for safety that you use wheel chocks when you park trailers without spring brakes.

TruckingTruth's Advice:

It is extremely unlikely you will drive a trailer built before 1975, but for the written exam, you still may need to know that trailers built before 1975 were not required to have spring brakes.

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What happens when there is a major leak in the emergency air line?
  • It will cause the tractor protection valve to close and the trailer emergency brakes to come on
  • Nothing will happen, but an in-cab warning indicator will let you know about the problem
  • It will cause the tractor protection valve to open and the trailer emergency brakes to come on
  • It will disable the emergency brakes

Quote From The CDL Manual:

A major leak in the emergency line will cause the tractor protection valve to close and the trailer emergency brakes to come on.

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When pulling a single trailer with a Shut-Off Valve, the valves must be in the:
  • Open position
  • Closed position
  • Off position
  • Stopped position

Quote From The CDL Manual:

You must check that all shut-off valves are in the open position except the ones at the back of the last trailer, which must be closed.

TruckingTruth's Advice:

When only pulling one trailer, be sure the shut-off valves are all in the fully closed position.

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How many air tanks do trailers have
  • At least two
  • Trailers typically do not have their own air tanks
  • At least one
  • At least four

Quote From The CDL Manual:

Each trailer and converter dolly has one or more air tanks.

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What are shut-off valves
  • Placed at each trailer spring brake and are designed to engage the emergency brakes when air pressure becomes too high
  • The yellow and red air control knobs located in the cab of a truck
  • Used in the service and supply air lines at the back of trailers used to tow other trailers
  • Designed to turn off the truck engine when a brake failure occurs

Quote From The CDL Manual:

Shut-off valves (also called "cut-out cocks") are used in the service and supply air lines at the back of trailers used to tow other trailers. These valves permit closing the air lines off when another trailer is not being towed. You must check that all shut-off valves are in the open position except the ones at the back of the last trailer, which must be closed.

TruckingTruth's Advice:

Only trailers designed to be pulled with another trailer behind it will have a shut off valve. If you are only pulling one trailer, make certain the valve is fully in the closed position or it will leak air out of the air system.

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What are Glad Hands?
  • Used to secure the trailer kingpin to the tractor 5th wheel
  • A name for the yellow and red air valves in the cab of a truck
  • Caps which cover the valve stems on tires
  • Coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer

Quote From The CDL Manual:

"Glad hands" are coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer.

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What will happen if there is a major leak in the service air line?
  • When you try to put the service brakes on, air will quickly leak and will lower the air tank pressure, possibly activating the trailer emergency brakes
  • The emergency brakes will come on automatically as soon as a major leak occurs
  • Air pressure will quickly leak away, even if you are not using the service brakes
  • Nothing, but braking power will be reduced

Quote From The CDL Manual:

You may not notice a major leak in the service line until you try to put the brakes on. Then, the air loss from the leak will lower the air tank pressure quickly. If it goes low enough, the trailer emergency brakes will come on.

TruckingTruth's Advice:

Remember, the service line is not used unless you depress the brake pedal or use the trailer hand valve. Therefor, if there is a major leak in your service line, you may not know it until you try to use the service brake. A thorough pre-trip inspection should catch any major service line leaks before you begin your trip.

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