Missouri Combination Vehicles practice test
25 questions · 80% to pass · Required for Class A combination drivers.
How to use this practice test
Read each question, click an answer, and the correct choice is highlighted with a short explanation referencing the underlying CDL Manual concept. Your live score appears at the top of the page. Refresh to reset.
This test runs entirely in your browser. Nothing is uploaded; no account is required. Pages refresh to a new randomized cut from the bank.
-
1. According to the FMCSA CDL Manual, trailer hand valve (trolley valve) should NOT be used:
Explanation. Never use the trolley valve for parking, and avoid using it for service braking; it may cause skids. (Reinforcement variant — same underlying CDL Manual concept.)
-
2. In real-world commercial driving, when backing a trailer, the trailer turns:
Explanation. When you back, turning the steering wheel one way causes the trailer to go the opposite way at first. (Reinforcement variant — same underlying CDL Manual concept.)
-
3. In real-world commercial driving, trailers are most likely to swing out (jackknife) when:
Explanation. Locked trailer wheels with a turning tractor lead to a trailer swing or jackknife. (Reinforcement variant — same underlying CDL Manual concept.)
-
4. Glad hands are used to connect:
Explanation. Glad hands are the air-line couplers between tractor and trailer.
-
5. The emergency air line is usually:
Explanation. Red is the emergency line; blue is the service (control) line.
-
6. In real-world commercial driving, when coupling, you should back under the trailer slowly and:
Explanation. Stop just before contact, then back firmly to lock the kingpin. (Reinforcement variant — same underlying CDL Manual concept.)
-
7. When uncoupling, you should lower the landing gear until:
Explanation. Land gear should support some weight before pulling out, but not lift the trailer off the fifth wheel.
-
8. When backing a trailer, the trailer turns:
Explanation. When you back, turning the steering wheel one way causes the trailer to go the opposite way at first.
-
9. When operating a CMV in interstate commerce, trailer hand valve (trolley valve) should NOT be used:
Explanation. Never use the trolley valve for parking, and avoid using it for service braking; it may cause skids. (Reinforcement variant — same underlying CDL Manual concept.)
-
10. During a pre-trip inspection, trailer hand valve (trolley valve) should NOT be used:
Explanation. Never use the trolley valve for parking, and avoid using it for service braking; it may cause skids. (Reinforcement variant — same underlying CDL Manual concept.)
-
11. When coupling, you should back under the trailer slowly and:
Explanation. Stop just before contact, then back firmly to lock the kingpin.
-
12. In real-world commercial driving, off-tracking happens because:
Explanation. Trailer wheels cut inside the tractor wheels in a turn — the longer the trailer, the more pronounced the effect. (Reinforcement variant — same underlying CDL Manual concept.)
-
13. Per federal regulations, when coupling, you should back under the trailer slowly and:
Explanation. Stop just before contact, then back firmly to lock the kingpin. (Reinforcement variant — same underlying CDL Manual concept.)
-
14. Most state DMV CDL handbooks state that after coupling, you should test the connection by:
Explanation. A "tug test" with trailer brakes set verifies the kingpin is locked in the fifth wheel. (Reinforcement variant — same underlying CDL Manual concept.)
-
15. Per federal regulations, when backing a trailer, the trailer turns:
Explanation. When you back, turning the steering wheel one way causes the trailer to go the opposite way at first. (Reinforcement variant — same underlying CDL Manual concept.)
-
16. When operating a CMV in interstate commerce, when uncoupling, you should lower the landing gear until:
Explanation. Land gear should support some weight before pulling out, but not lift the trailer off the fifth wheel. (Reinforcement variant — same underlying CDL Manual concept.)
-
17. When operating a CMV in interstate commerce, crossing the air lines (red to blue) will cause:
Explanation. Crossed glad hands disable the service brakes on the trailer and may cause emergency application failure. (Reinforcement variant — same underlying CDL Manual concept.)
-
18. Experienced commercial drivers know that glad hands are used to connect:
Explanation. Glad hands are the air-line couplers between tractor and trailer. (Reinforcement variant — same underlying CDL Manual concept.)
-
19. Experienced commercial drivers know that empty trailers:
Explanation. Empty trailers bounce, can lose traction, and may lock up faster than loaded trailers. (Reinforcement variant — same underlying CDL Manual concept.)
-
20. Per federal regulations, after coupling, you should test the connection by:
Explanation. A "tug test" with trailer brakes set verifies the kingpin is locked in the fifth wheel. (Reinforcement variant — same underlying CDL Manual concept.)
-
21. Experienced commercial drivers know that the emergency air line is usually:
Explanation. Red is the emergency line; blue is the service (control) line. (Reinforcement variant — same underlying CDL Manual concept.)
-
22. According to the FMCSA CDL Manual, off-tracking happens because:
Explanation. Trailer wheels cut inside the tractor wheels in a turn — the longer the trailer, the more pronounced the effect. (Reinforcement variant — same underlying CDL Manual concept.)
-
23. In real-world commercial driving, after coupling, you should test the connection by:
Explanation. A "tug test" with trailer brakes set verifies the kingpin is locked in the fifth wheel. (Reinforcement variant — same underlying CDL Manual concept.)
-
24. Experienced commercial drivers know that crossing the air lines (red to blue) will cause:
Explanation. Crossed glad hands disable the service brakes on the trailer and may cause emergency application failure. (Reinforcement variant — same underlying CDL Manual concept.)
-
25. During a pre-trip inspection, crossing the air lines (red to blue) will cause:
Explanation. Crossed glad hands disable the service brakes on the trailer and may cause emergency application failure. (Reinforcement variant — same underlying CDL Manual concept.)
About the Combination Vehicles exam
Combination Vehicles knowledge is required for any driver pulling a trailer with a Class A vehicle. It covers off-tracking, rollover prevention, fifth-wheel coupling and uncoupling, trailer brake operation, antilock braking on trailers, and managing space and stopping distance with a loaded trailer.
This Missouri-specific edition uses the same federal source material every U.S. state adopts. The Missouri DOR administers the actual exam at its service centers. Bring your CDL handbook for any last-minute reference; it’s free at every Missouri DOR office.
Once you’re consistently scoring above 90% on this practice set, you’re well above the 80% required to pass the official Missouri exam. Drill the questions you miss most often, then take the test cold once a day for a week leading up to your appointment.
Tips that actually work
- Read every answer choice before clicking. CDL questions are famous for "best answer" wording where two choices look right.
- Don’t memorize question text — learn the underlying rule. The DMV reshuffles wording constantly.
- Keep a list of the questions you miss. Re-drill them in isolation until you can’t miss them.
- Practice in short, frequent sessions. Two 20-minute sessions per day beats one two-hour cram.
What happens after I pass?
Passing the knowledge test earns you a Commercial Learner’s Permit (CLP). You must hold the CLP for at least 14 days before you can take the road skills test, and you must drive with a CDL-holding instructor in the cab during that period. Once you pass the road skills test, your CDL is issued.