7CDLFree CDL permit study for every state
Content preview, before public launch
Course › Combination Vehicles

Combination air brakes: valves, air lines and trailer brakes

Combination Vehicles7 minManual section 6.2

A combination has all the air brake parts of a single truck (study the Air Brakes section first), plus extra parts that send air to the trailer and control its brakes. This lesson covers every one of those parts and what happens when air is lost.

Trailer hand valve (6.2.1)

The trailer hand valve (other names: trolley valve, Johnson bar) works the trailer brakes only.

  • Use it only to test the trailer brakes.
  • Do not use it while driving: braking only the trailer can make the trailer skid. The foot brake sends air to all brakes on the rig, including the trailer(s), so it is much less likely to cause a skid or jackknife.
  • Never use the hand valve for parking. The air can leak out and the brakes will release (on trailers without spring brakes). Always use the parking brakes to park. If the trailer has no spring brakes, use wheel chocks so it cannot roll.

Tractor protection valve and trailer air supply control (6.2.2–6.2.3)

The tractor protection valve saves the tractor's (or truck's) air if the trailer breaks loose or springs a bad leak. You work it from the cab with the trailer air supply control.

  • It closes automatically when air pressure falls low — in the range of 20 to 45 psi.
  • When it closes, it stops air from leaving the tractor and lets the air out of the trailer emergency line. That makes the trailer emergency brakes come on, with possible loss of control.

On newer vehicles the trailer air supply control is a red, eight-sided (octagon) knob:

  • Push it in to supply air to the trailer.
  • Pull it out to shut off the air and apply the trailer emergency brakes.
  • It pops out by itself (closing the tractor protection valve) when pressure drops to 20–45 psi.

On older vehicles the control may be a lever and may not work automatically. The "normal" position is for pulling a trailer; the "emergency" position shuts off the air and applies the trailer emergency brakes.

The two trailer air lines (6.2.4)

Every combination has two air lines running between each unit (tractor to trailer, trailer to dolly, dolly to second trailer, and so on):

  • Service line — also called the control line or signal line. Color: usually blue. It carries air controlled by the foot brake or trailer hand brake. The harder you press, the higher the pressure in this line. It connects to relay valves, which let the trailer brakes apply faster than they could otherwise.
  • Emergency line — also called the supply line. Color: usually red. It has two jobs: (1) it supplies air to the trailer air tanks, and (2) it controls the trailer emergency brakes. If the emergency line loses pressure, the trailer emergency brakes come on.

Emergency line pressure can be lost if the trailer breaks loose and tears the hose apart, or if a hose, metal tube or other part breaks. A loss of emergency line pressure also makes the tractor protection valve close (the air supply knob pops out).

Red parts (hoses, couplers) mark the emergency line so it is not confused with the blue service line.

Glad hands, crossed lines and dummy couplers (6.2.5)

Glad hands are the couplers that connect the service and emergency lines from the tractor to the trailer. Each has a rubber seal that keeps air in.

  • Clean the couplers and rubber seals before connecting.
  • Put the two rubber seals face to face with the couplers held at a 90-degree angle, then twist the glad hand on the hose so the couplers join and lock.
  • Connect the right pair: blue = service, red = emergency (supply). Some lines have metal tags stamped "service" and "emergency".

If you cross the air lines, supply air goes into the service line instead of filling the trailer tanks. There is no air to release the trailer spring brakes. If the spring brakes do not release when you push in the air supply control, check the air line connections.

Older trailers have no spring brakes. If their tank air has leaked away and the lines are crossed, the wheels turn freely — you could drive away with no trailer brakes at all. That is very dangerous. Before driving, always test the trailer brakes with the hand valve or by pulling the air supply control, and pull gently against them in a low gear.

Dummy couplers ("dead-end" couplers) hold the hoses when no trailer is connected. They keep water and dirt out of the couplers and lines. If there are none, the glad hands can sometimes be locked together. Keeping the air supply clean is very important.

Trailer air tanks and shut-off valves (6.2.6–6.2.7)

Every trailer and converter dolly carries one or more air tanks. The emergency (supply) line from the tractor fills them, and they supply the pressure that works the trailer brakes. Relay valves pass air from the tanks to the brakes. Service line pressure, which you set with the brake pedal (and the trailer hand brake), tells the relay valves how much air to send.

Keep water and oil from collecting in the tanks; otherwise the brakes may not work right. Each tank has a drain valve — drain all of them daily. Automatic drains remove most moisture, but still open the drains to check.

Shut-off valves (also called cut-out cocks) are in the service and supply lines at the back of trailers used to tow other trailers. They close off the lines when no other trailer is being towed.

Rule: every shut-off valve stays OPEN, except the pair at the rear of the LAST trailer, which stay CLOSED.

Trailer service, parking and emergency brakes (6.2.8)

Newer trailers have spring brakes, the same as trucks and tractors. Converter dollies, and trailers made before 1975, do not have to have spring brakes.

A trailer with no spring brakes has emergency brakes powered by air stored in its own tank. They apply when the emergency line loses pressure — for example, when you pull out the air supply knob or the trailer comes uncoupled. Such a trailer has no parking brake.

A major leak in the emergency line closes the tractor protection valve and sets the trailer emergency brakes. But those brakes hold only as long as there is air in the trailer tank. The air eventually leaks away and then there are no brakes. That is why you must use wheel chocks when you park a trailer without spring brakes.

A major leak in the service line may not show up until you brake. Then the leak drains tank pressure fast. If it drops low enough, the trailer emergency brakes come on.

References and scope

The topic reference is listed below. Sources support verification; the lesson itself is here. The current content needs technical and language review before public launch.

Manual reference: 6.2

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

Educational questions with explanations. The answer review also works without the interactive quiz.

Preparing your questions…

Review questions with answers

  1. What is the trailer hand valve (trolley valve) meant to be used for?
    1. Parking the trailer
    2. Testing the trailer brakes
    3. Slowing down on hills
    4. Straightening a skidding trailer
    Show answer
    Testing the trailer brakes

    Use the trailer hand valve only to test the trailer brakes.

  2. Why should you not use the trailer hand valve while driving?
    1. It wears out the tractor brakes
    2. It turns off the ABS
    3. It drains the air tanks
    4. It can make the trailer skid
    Show answer
    It can make the trailer skid

    Braking only the trailer can cause a skid; the foot brake applies all brakes and is much safer.

  3. Why should you never use the trailer hand valve for parking?
    1. The air might leak out and release the brakes
    2. It locks the fifth wheel
    3. It sets the tractor spring brakes too hard
    4. It is illegal only in some states
    Show answer
    The air might leak out and release the brakes

    Air can leak away and unlock the brakes on trailers without spring brakes. Use the parking brakes.

  4. Which brake control sends air to ALL the brakes on the rig, including the trailers?
    1. The trailer hand valve
    2. The trailer air supply knob
    3. The foot brake
    4. The dolly petcock
    Show answer
    The foot brake

    The foot brake applies air to every brake on the vehicle, so it is less likely to cause a skid.

  5. What does the tractor protection valve do?
    1. Supplies air to the trailer tanks
    2. Keeps air in the tractor brake system if the trailer breaks away or leaks badly
    3. Controls the ABS lamp
    4. Releases the fifth wheel jaws
    Show answer
    Keeps air in the tractor brake system if the trailer breaks away or leaks badly

    It protects the tractor's air supply if the trailer breaks away or develops a bad leak.

  6. The tractor protection valve closes automatically when air pressure drops to about:
    1. 60 to 80 psi
    2. 100 to 125 psi
    3. 20 to 45 psi
    4. 5 to 10 psi
    Show answer
    20 to 45 psi

    It closes automatically when pressure is low, in the 20 to 45 psi range.

  7. When the tractor protection valve closes, what happens to the trailer?
    1. Air leaves the emergency line and the trailer emergency brakes come on
    2. The trailer brakes fully release
    3. The trailer service brakes get more air
    4. Nothing changes on the trailer
    Show answer
    Air leaves the emergency line and the trailer emergency brakes come on

    Closing lets air out of the trailer emergency line, so the trailer emergency brakes apply, with possible loss of control.

  8. On newer vehicles, what does the trailer air supply control look like?
    1. A blue round knob
    2. A yellow square button
    3. A gray lever on the steering column
    4. A red eight-sided knob
    Show answer
    A red eight-sided knob

    It is a red, eight-sided knob that controls the tractor protection valve.

  9. Pulling OUT the trailer air supply knob will:
    1. Supply air to the trailer
    2. Shut off air to the trailer and apply the trailer emergency brakes
    3. Release the tractor parking brakes
    4. Drain the trailer tanks
    Show answer
    Shut off air to the trailer and apply the trailer emergency brakes

    Push in to supply air; pull out to shut off air and put on the trailer emergency brakes.

  10. On older vehicles with a lever for the trailer air supply, which position is used to pull a trailer?
    1. Emergency
    2. Park
    3. Normal
    4. Off
    Show answer
    Normal

    The "normal" position is for pulling a trailer; "emergency" shuts the air off and sets the trailer emergency brakes.

  11. True or False: Tractor protection valve controls on older vehicles may not operate automatically.
    1. True
    2. False
    Show answer
    True

    True. Older controls may be a lever and may not pop out by themselves.

  12. The service air line is also called the:
    1. Supply line
    2. Emergency line
    3. Tow line
    4. Control or signal line
    Show answer
    Control or signal line

    The service line is also called the control line or signal line.

  13. What controls the air pressure in the service line?
    1. The foot brake or trailer hand brake
    2. The trailer air supply knob
    3. The governor only
    4. The dolly petcock
    Show answer
    The foot brake or trailer hand brake

    The service line carries air controlled by the foot brake or trailer hand brake; harder pressure means more air.

  14. The service line connects to relay valves. What do relay valves do?
    1. Release the spring brakes
    2. Let the trailer brakes apply more quickly
    3. Fill the tractor tanks
    4. Close the tractor protection valve
    Show answer
    Let the trailer brakes apply more quickly

    Relay valves allow the trailer brakes to be applied faster than would otherwise be possible.

  15. What are the two jobs of the emergency (supply) air line?
    1. Control the ABS and the lights
    2. Release the fifth wheel and raise the landing gear
    3. Supply air to the trailer tanks and control the trailer emergency brakes
    4. Carry the service brake signal and drain the tanks
    Show answer
    Supply air to the trailer tanks and control the trailer emergency brakes

    The emergency line fills the trailer air tanks and controls the emergency brakes.

  16. If the trailer breaks loose and tears the emergency air line apart, what happens?
    1. The trailer emergency brakes come on
    2. The trailer brakes release
    3. The ABS lamp turns off
    4. The service brakes get stronger
    Show answer
    The trailer emergency brakes come on

    Loss of emergency line pressure applies the trailer emergency brakes and closes the tractor protection valve.

  17. Glad hands are usually color coded. Which is correct?
    1. Red = service, blue = emergency
    2. Yellow = service, green = emergency
    3. Black = both lines
    4. Blue = service, red = emergency
    Show answer
    Blue = service, red = emergency

    Blue is used for the service line and red for the emergency (supply) line.

  18. When connecting glad hands, the couplers should be pressed together at what angle?
    1. 45 degrees
    2. 90 degrees
    3. 180 degrees
    4. Straight in line (0 degrees)
    Show answer
    90 degrees

    Hold the couplers at a 90-degree angle with the seals together, then twist to lock.

  19. What should you do with the glad hands and rubber seals before connecting them?
    1. Grease them heavily
    2. Paint them
    3. Clean them
    4. Soak them in water
    Show answer
    Clean them

    Clean the couplers and rubber seals before making a connection.

  20. If you cross the air lines on a trailer with spring brakes, what will happen when you push in the air supply knob?
    1. The trailer spring brakes will not release
    2. The trailer will roll freely with no brakes
    3. The ABS lamp will flash
    4. The tractor protection valve will be damaged
    Show answer
    The trailer spring brakes will not release

    Supply air goes to the service line instead of the tanks, so there is no air to release the spring brakes.

  21. The trailer spring brakes do not release when you push in the trailer air supply control. What should you check first?
    1. The fifth wheel grease
    2. The ABS lamp
    3. The landing gear
    4. The air line connections
    Show answer
    The air line connections

    If the spring brakes don't release, check whether the air lines are crossed.

  22. With an older trailer without spring brakes, crossed air lines and an empty trailer tank, what is the danger?
    1. The trailer will not move at all
    2. You could drive away with no trailer brakes
    3. The tractor brakes will lock
    4. The fifth wheel will unlock
    Show answer
    You could drive away with no trailer brakes

    Without spring brakes and with no tank air, the wheels turn freely, so you could drive off with no trailer brakes.

  23. What is the purpose of dummy couplers?
    1. To connect a second trailer
    2. To test the ABS
    3. To keep water and dirt out of the couplers and lines when not in use
    4. To lock the trailer brakes for parking
    Show answer
    To keep water and dirt out of the couplers and lines when not in use

    Dummy (dead-end) couplers hold the hoses when not connected and keep dirt and water out.

  24. How are the trailer air tanks filled?
    1. By the emergency (supply) line from the tractor
    2. By the service line only
    3. By a compressor on the trailer
    4. By the dolly petcock
    Show answer
    By the emergency (supply) line from the tractor

    The trailer tanks are filled by the emergency (supply) line from the tractor.

  25. How often should you drain the air tanks?
    1. Once a month
    2. Only when the low-air warning comes on
    3. Never, if they have automatic drains
    4. Every day
    Show answer
    Every day

    Drain each tank every day; even with automatic drains, open them to be sure.

  26. Why must you keep water and oil from building up in the air tanks?
    1. It makes the trailer heavier
    2. The brakes may not work correctly
    3. It changes the color of the glad hands
    4. It sets off the ABS lamp
    Show answer
    The brakes may not work correctly

    Water and oil in the tanks can keep the brakes from working right.

  27. Where are shut-off valves (cut-out cocks) located?
    1. On the tractor dashboard
    2. On the fifth wheel
    3. In the service and supply lines at the back of trailers used to tow other trailers
    4. Inside the trailer air tanks
    Show answer
    In the service and supply lines at the back of trailers used to tow other trailers

    Shut-off valves are in the service and supply lines at the rear of trailers that tow other trailers.

  28. Which shut-off valves should be closed?
    1. Only those at the back of the last trailer
    2. All of them
    3. Only those on the first trailer
    4. Those on the converter dolly
    Show answer
    Only those at the back of the last trailer

    All shut-off valves must be open except those at the rear of the last trailer, which must be closed.

  29. Which units are NOT required to have spring brakes?
    1. All trailers built after 2000
    2. Tractors
    3. Converter dollies and trailers built before 1975
    4. Straight trucks
    Show answer
    Converter dollies and trailers built before 1975

    Spring brakes are not required on converter dollies or on trailers made before 1975.

  30. Why should you use wheel chocks when parking a trailer without spring brakes?
    1. Chocks are required only on hills
    2. The air in the trailer tank will eventually leak away and the brakes will release
    3. Chocks keep the ABS working
    4. The fifth wheel may unlock
    Show answer
    The air in the trailer tank will eventually leak away and the brakes will release

    Emergency brakes on these trailers hold only while there is tank air; when it leaks away, there are no brakes.

  31. Trailers without spring brakes:
    1. Have a separate mechanical parking brake
    2. Do not have emergency brakes
    3. Can be parked with the trailer hand valve
    4. Have no parking brake
    Show answer
    Have no parking brake

    These trailers have emergency brakes that work from tank air, but no parking brake.

  32. When might you first notice a major leak in the service line?
    1. When you put on the brakes
    2. When you start the engine
    3. When you couple the trailer
    4. Only during the pre-trip inspection
    Show answer
    When you put on the brakes

    A service line leak may not show until you brake; then the air loss drops tank pressure quickly.

  33. Besides colors, how are service and emergency air lines sometimes marked?
    1. With yellow tape on the service line
    2. By the size of the glad hands only
    3. With a number painted on the trailer
    4. With metal tags stamped "service" and "emergency"
    Show answer
    With metal tags stamped "service" and "emergency"

    Blue (service) and red (emergency) colors are common, and sometimes metal tags stamped with the line names are attached.