Study guide
CDL combination vehicles: air lines, coupling, rollover
A trailer has its own brakes, its own air and its own way of tipping over. Learn how it is joined to the tractor, and what it does when the tractor moves.
- Who takes it
- Anyone who will drive a combination vehicle, such as a tractor-trailer.
- To pass
- 80 percent.
- Questions
- 20 in Michigan, inside its 70-question Class A test. Check your state's manual.
- Read first
- Air Brakes. This test builds on it.
On this page
The two air lines and the valve between them
Two lines run from the tractor to the trailer. Know which one does what.
- Blue
- The service line, also called the control or signal line, often marked blue. It carries the air you control with the foot brake or the trailer hand valve. Manual 6.2
- Red
- The emergency line, also called the supply line, often marked red. It fills the trailer air tanks and controls the trailer emergency brakes. If it loses pressure, those brakes come on. Manual 6.2
- 20 to 45 psi
- The tractor protection valve closes by itself in this range and, on newer vehicles, the red, eight-sided trailer air supply knob pops out. The valve keeps air in the tractor if the trailer breaks away or leaks badly. On older vehicles the control may be a lever and may not work automatically. Manual 6.2
- 90 degrees
- The angle at which you press two glad hands together, seal to seal, before a turn locks them. Manual 6.2
Why trailers roll over
- 10 times
- A fully loaded rig is ten times more likely to roll over in a crash than an empty one. Keep the cargo low and centered, and take turns slowly. Manual 6.1
- 2.0
- Rearward amplification: a quick lane change at the front becomes a hard snap at the back. At 2.0 the rear trailer is twice as likely to turn over as the tractor. Triples are at 3.5. Manual 6.1
- +1 second
- Following distance is at least one second for each 10 feet of vehicle length, plus one second above 40 mph. Space is what lets you steer gently. Manual 6.1
Coupling a tractor and semitrailer: the 16 steps
The order has a logic. The air is connected and the trailer brakes are locked before the tractor backs under, and nothing is trusted until you have pulled against it and looked at it. These are the manual's general steps: rigs differ, so learn the details of the one you will drive. Manual 6.4
Inspect the fifth wheel
Greased, tilted down toward the rear, jaws open, no damaged parts. Check that the kingpin is not bent or broken.
Inspect the area and chock the wheels
Or be sure the trailer spring brakes are on.
Position the tractor
Directly in front of the trailer. Never back under at an angle.
Back slowly
Until the fifth wheel just touches the trailer.
Secure the tractor
Parking brake on, transmission in neutral.
Check trailer height
Low enough that the tractor raises it slightly as it backs under.
Connect the air lines
Emergency to emergency, service to service. Check the glad hand seals.
Supply air to the trailer
Push in the air supply knob and wait until the air pressure is normal. Shut the engine off, apply and release the trailer brakes and listen: this is your check for crossed lines.
Lock the trailer brakes
Pull the air supply knob out.
Back under the trailer
Lowest reverse gear, slowly. Stop when the kingpin locks into the fifth wheel.
Check the connection
Raise the landing gear slightly and pull gently forward with the trailer brakes still locked.
Secure the vehicle
Neutral, parking brakes on, engine off, key in your pocket.
Inspect the coupling
No space between the upper and lower fifth wheel. Jaws closed around the shank of the kingpin, locking lever locked, safety latch over it.
Connect the electrical cord
Fasten the safety catch and check the air lines for damage.
Raise the landing gear
All the way up. Never drive with it part way up.
Remove the wheel chocks
And store them.
Worked example: three faults and what they mean
Start from what you see, hear or feel, and work out what is wrong.
You push in the trailer air supply knob and the trailer spring brakes do not release.
Crossed air lines. Supply air is going into the service line and not into the trailer tanks, so nothing releases the spring brakes. Check the glad hand connections.
After backing under, you see a gap between the upper and lower fifth wheel.
Not coupled. Space means something is wrong: the kingpin may be sitting on top of the closed jaws. Fix it before you move.
You brake hard and the mirror shows the trailer swinging out of your lane.
Release the brakes. Locked trailer wheels caused the skid. Letting go gives them traction back and the trailer follows the tractor again. Do not reach for the trailer hand valve.
Five common mistakes
Common answerAn empty rig stops quicker than a loaded one.
The manual 6.1It takes longer. With a light load the stiff springs and strong brakes give poor traction and the wheels lock easily. A bobtail tractor takes longer still than a rig at full weight.
Common answerThe trailer hand valve is for straightening out a skid, or for parking.
The manual 6.2It is for testing the trailer brakes. In a skid it makes things worse, and when parked its air can leak away and unlock the brakes.
Common answerStuck on the tracks: phone for help, then get out.
The manual 6.1Get out and away from the tracks first. Then look for the emergency notification sign at the crossing and call.
Common answerA parked trailer without spring brakes will stay put.
The manual 6.2Its emergency brakes hold only as long as air stays in the trailer tank. Chock the wheels.
Common answerABS on the trailer lets me follow closer.
The manual 6.3ABS only keeps the wheels from locking, so the trailer is less likely to swing out. It does not necessarily shorten the stop. Brake as you always have.
Three practice questions
Pick an answer. The reason and the manual's own sentence follow.
You have backed under the trailer and the kingpin has locked into the fifth wheel. How do you check that the trailer is locked on?
Show the answer
The answer is B. With the trailer brakes locked, a coupled trailer holds the tractor back. The landing gear is only just off the ground, so the trailer cannot fall if the coupling fails.
Commercial driver manual, section 6.4Raise trailer landing gear slightly off ground. Pull tractor gently forward while the trailer brakes are still locked to check that the trailer is locked onto the tractor.
Which is true about stopping an empty combination vehicle?
Show the answer
The answer is B. An empty trailer presses its tires lightly on the road. The wheels lock up easily, the trailer can swing out, and the stop gets longer.
Commercial driver manual, section 6.1Large combination vehicles take longer to stop when they are empty than when they are fully loaded.
The air pressure in your tractor-trailer keeps falling. At about what pressure does the tractor protection valve close by itself?
Show the answer
The answer is C. The valve closes to keep the remaining air in the tractor. The trailer air supply knob pops out and the trailer emergency brakes come on.
Commercial driver manual, section 6.2The tractor protection valve will close automatically if air pressure is low (in the range of 20 to 45 psi).
Sources
- Oregon Commercial Driver Manual, section 6: Combination VehiclesThe edition this page is checked against. It carries the national model text that state manuals share.
- 49 CFR 383.135: the passing score of 80 percent
- Michigan Commercial Driver License Manual: number of questions
Reviewed by CDL Greenlight on October 13, 2026. CDL Greenlight writes and checks this page. Every figure is the one the commercial driver manual prints, with its section named beside it. Where your state has a rule of its own, your state's manual decides.
Found a mistake? Write to support@cdlgreenlight.com. Corrections we have published