CDL Combination Vehicles Practice Test 2026: Practice test 2

Practice test 2 of 3 for the CDL Combination Vehicles Practice Test: 25 more questions with answers and explanations.

Commercial vehicle
United States
70 questions
Checked October 10, 2026

Question 1 of 25
What does the tractor protection valve do?
- Raises the governor's cut-out pressure
- Keeps air in the tractor if the trailer breaks away
- Drains water from the trailer air tanks
- Locks the fifth wheel jaws around the kingpin
Show answer
Answer: B. Keeps air in the tractor if the trailer breaks away
The tractor protection valve closes automatically when pressure gets low, keeping air in the tractor's brake system.
Question 2 of 25
At about what pressure will the tractor protection valve close automatically?
- At 150 psi
- Between 100 and 125 psi
- Between 60 and 85 psi
- Between 20 and 45 psi
Show answer
Answer: D. Between 20 and 45 psi
The tractor protection valve closes when air pressure falls into the range of 20 to 45 psi.
Question 3 of 25
What does the trailer air supply control look like on newer vehicles?
- A yellow, diamond-shaped knob
- A green lever
- A blue square knob
- A red, eight-sided knob
Show answer
Answer: D. A red, eight-sided knob
The trailer air supply control is a red eight-sided knob. The yellow diamond knob is the parking brake.
Question 4 of 25
How do you use the red trailer air supply knob?
- Turn it while driving to adjust how hard the trailer brakes work
- Use it only to drain water from the trailer air tanks each day
- Push in to supply trailer air; pull out to set trailer emergency brakes
- Pull out to supply trailer air; push in to release the trailer brakes
Show answer
Answer: C. Push in to supply trailer air; pull out to set trailer emergency brakes
Push in to supply air to the trailer, and pull out to shut off the air and put on the trailer emergency brakes.
Question 5 of 25
Which two air lines connect a tractor to a trailer?
- The service line and the emergency line
- The supply line and the return line
- The fuel line and the oil line
- The hydraulic line and the vacuum line
Show answer
Answer: A. The service line and the emergency line
Every combination vehicle has two air lines: the service line and the emergency (supply) line.
Question 6 of 25
What does the service line do?
- Releases the trailer spring brakes when parked
- Carries power to the trailer lights and signals
- Carries pedal or hand valve air to the trailer brakes
- Fills the trailer air tanks while you drive
Show answer
Answer: C. Carries pedal or hand valve air to the trailer brakes
The service line carries air that goes up and down with how hard you press the brake pedal or hand valve.
Question 7 of 25
What does the emergency (supply) line do?
- Carries electric power to the trailer lights
- Carries air only when you press the brake pedal
- Fills the trailer tanks and controls its emergency brakes
- Drains water from the trailer tanks each day
Show answer
Answer: C. Fills the trailer tanks and controls its emergency brakes
The emergency line fills the trailer tanks. If it loses pressure, the trailer emergency brakes come on.
Question 8 of 25
What color is often used for the service air line?
- Green
- Blue
- Yellow
- Red
Show answer
Answer: B. Blue
Service lines are often blue and emergency lines red. Some use metal tags marked "service" and "emergency".
Question 9 of 25
What happens if the emergency line loses pressure?
- Nothing happens until you press the brake
- The trailer emergency brakes come on
- The trailer brakes release completely
- The tractor's engine stops
Show answer
Answer: B. The trailer emergency brakes come on
Loss of pressure in the emergency line sets the trailer emergency brakes and closes the tractor protection valve.
Question 10 of 25
What are glad hands?
- Hand signals a helper uses for backing
- Gloves worn when coupling a trailer
- Couplers that join the air lines to the trailer
- Handles used to crank up the landing gear
Show answer
Answer: C. Couplers that join the air lines to the trailer
Glad hands are the coupling devices that join the service and emergency air lines between tractor and trailer.
Question 11 of 25
How do you connect two glad hands?
- Push them straight together like an electrical plug
- Screw them together like a garden hose
- Tie them together tightly with wire
- Put the seals together at a 90-degree angle, then turn to lock
Show answer
Answer: D. Put the seals together at a 90-degree angle, then turn to lock
Put the seals together with the couplers at a 90-degree angle to each other and turn them so they lock.
Question 12 of 25
You crossed the air lines when coupling. What will happen to a trailer with spring brakes?
- The trailer brakes will work normally
- The spring brakes will not release
- The tractor brakes will fail
- The trailer lights will go out
Show answer
Answer: B. The spring brakes will not release
With crossed lines, supply air goes to the service line instead of the tanks, so the spring brakes stay on.
Question 13 of 25
What are dummy couplers for?
- Testing the trailer brakes before a trip
- Locking the fifth wheel in place
- Connecting a second trailer to the first
- Keeping water and dirt out of unused air lines
Show answer
Answer: D. Keeping water and dirt out of unused air lines
Use dummy couplers or plugs when the air lines are not connected to keep dirt and water out.
Question 14 of 25
How often should you drain the trailer air tanks?
- Once a week
- Only in winter
- Every day
- Once a month
Show answer
Answer: C. Every day
Water and oil in the trailer tanks can keep the brakes from working right, so drain each tank every day.
Question 15 of 25
In a combination with shut-off valves (cut-out cocks), which valves should be closed?
- The ones at the front of every trailer
- All of them
- Only the ones on the tractor
- Only the ones at the back of the last trailer
Show answer
Answer: D. Only the ones at the back of the last trailer
All shut-off valves are open except those at the back of the last trailer, which must be closed.
Question 16 of 25
Which trailers are not required to have spring brakes?
- Trailers built after 2000
- Trailers under 40 feet
- Trailers built before 1975
- Refrigerated trailers
Show answer
Answer: C. Trailers built before 1975
Trailers built before 1975 do not have to have spring brakes. Their emergency brakes depend on air stored in the tanks.
Question 17 of 25
You park a trailer that has no spring brakes. What should you do?
- Chock the wheels
- Leave the air lines connected
- Rely on the trailer emergency brakes overnight
- Set the trailer hand valve
Show answer
Answer: A. Chock the wheels
Without spring brakes, the trailer's emergency brakes use stored air, which can leak away. Use wheel chocks.
Question 18 of 25
Trailers and converter dollies built on or after what date must have antilock brakes?
- July 1, 2015
- March 1, 2008
- January 1, 1985
- March 1, 1998
Show answer
Answer: D. March 1, 1998
Trailers and converter dollies built on or after March 1, 1998 must have ABS.
Question 19 of 25
Where is the ABS malfunction lamp on a trailer?
- Underneath the trailer, near the axles
- On the right side, near the rear door
- On the left side, at the front or rear corner
- On the roof of the tractor's cab
Show answer
Answer: C. On the left side, at the front or rear corner
Trailers with ABS have a yellow malfunction lamp on the left side, near the front or rear corner.
Question 20 of 25
The air leakage limit for a combination with the brakes released is less than how much per minute?
- 6 psi
- 1 psi
- 10 psi
- 3 psi
Show answer
Answer: D. 3 psi
With brakes released, a combination vehicle should lose less than 3 psi per minute. With brakes applied, no more than 4 psi.
Question 21 of 25
Before coupling, how should the fifth wheel be positioned?
- Tilted down toward the cab, with the jaws closed
- Tilted up toward the cab, with the jaws open
- Tilted down toward the rear, with the jaws open
- Level, with the jaws closed and locked
Show answer
Answer: C. Tilted down toward the rear, with the jaws open
The fifth wheel should tilt down toward the rear, the jaws should be open, and the safety unlocking handle should be in the automatic lock position.
Question 22 of 25
How should you line up the tractor before backing under a trailer?
- Any way, as long as the jaws are open
- Beside the trailer
- At a slight angle to the trailer
- Directly in front of the trailer
Show answer
Answer: D. Directly in front of the trailer
Never back under the trailer at an angle. It can push the trailer sideways and break the landing gear.
Question 23 of 25
When you first back toward the trailer to couple, when do you stop?
- About 10 feet before the trailer
- When the kingpin locks
- When the fifth wheel just touches the trailer
- When the tractor hits the landing gear
Show answer
Answer: C. When the fifth wheel just touches the trailer
Back slowly until the fifth wheel just touches the trailer. Do not hit it.
Question 24 of 25
The diagram shows coupling. How high should the trailer be before you back under it?
- Resting on the ground with the landing gear up
- Exactly level with the top of the tractor's cab
- Much higher than the fifth wheel so they don't touch
- Low enough that it is raised slightly as you back under
Show answer
Answer: D. Low enough that it is raised slightly as you back under
If the trailer is too low, the tractor may hit and damage the trailer nose. If it is too high, it may not couple correctly.
Question 25 of 25
When coupling, which air line goes to which glad hand?
- Either line can go to either glad hand
- Service to emergency, emergency to service
- Emergency to emergency, service to service
- Both lines go to the emergency glad hand
Show answer
Answer: C. Emergency to emergency, service to service
Connect each line to its matching glad hand and support the lines so they won't be crushed.

Related tests