Which tractor-trailer condition particularly increases the chance of getting hung up on a raised railroad crossing?
Correct answer: Low clearance under the trailer or its landing gear.
A raised crossing can catch low-slung equipment or landing gear. Check the route and clearance before entering; once stuck, get clear of the tracks and seek emergency help.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Low-clearance rail crossing; physical PDF page 80 (printed 6-2); reviewed September 15, 2026. ↗
You are driving a tractor without its semitrailer. What should you remember about braking?
Correct answer: Low weight on the rear drive axles can make smooth stopping more difficult.
A bobtail tractor can have poor drive-axle traction and be difficult to stop smoothly. Allow adequate space; actual stopping performance depends on the vehicle, brakes, load and road.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Bobtail braking; physical PDF page 79 (printed 6-1); reviewed September 15, 2026. ↗
The trailer wheels have locked during braking and the trailer begins to swing. What action addresses the cause of this trailer skid?
Correct answer: Release the braking that caused the lockup so the trailer wheels can regain traction.
A locked trailer wheel cannot provide normal directional grip. More trailer braking worsens this skid. This describes an actual wheel-lock trailer skid, not a general instruction to pump functioning ABS.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Trailer-wheel skid; physical PDF page 81 (printed 6-3); reviewed September 15, 2026. ↗
A curved backing maneuver with a semitrailer cannot be avoided. Which side does the handbook favor when a safe setup permits it?
Correct answer: Back toward the driver’s side for better visibility.
Driver-side backing generally gives a better view of the trailer and path. Straight backing is preferable when possible; inspect the area and stop to look again whenever uncertain.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Backing visibility; physical PDF page 81 (printed 6-3); reviewed September 15, 2026. ↗
During ordinary driving, which control should normally apply the tractor and trailer service brakes together?
Correct answer: The foot brake pedal.
The foot pedal controls service braking throughout the combination. The trailer hand valve is for checking trailer brakes, not routine driving or straightening a skid.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Trailer hand valve in traffic; physical PDF page 82 (printed 6-4); reviewed September 15, 2026. ↗
Why is the trailer hand valve unsuitable as a parking brake?
Correct answer: Air can leak away and release the brakes it applied.
A hand-valve application depends on air pressure and is not a secure parking method. Use the correct parking system and suitable chocks, especially for trailers without spring brakes.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Hand valve is not parking; physical PDF page 82 (printed 6-4); reviewed September 15, 2026. ↗
A trailer air hose tears apart. What is the purpose of the tractor protection valve?
Correct answer: Protect the tractor’s braking-air supply from escaping through the trailer system.
The protection system limits the trailer failure’s effect on tractor air. It cannot repair the damaged connection or authorize continued operation of an unsafe combination.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Tractor air protection; physical PDF page 83 (printed 6-5); reviewed September 15, 2026. ↗
On a newer tractor with the standard red eight-sided trailer-air control, what does pushing it in do?
Correct answer: Supplies air to the trailer system.
The red control is pushed in to supply trailer air and pulled out to shut off supply and activate trailer emergency braking. Know any different controls on the actual vehicle.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Red trailer-supply control; physical PDF page 83 (printed 6-5); reviewed September 15, 2026. ↗
What does the air pressure in the trailer service line communicate?
Correct answer: The amount of braking requested by the pedal or hand control.
The service or control line carries the braking signal to relay valves. It is not the line whose principal job is charging the trailer reservoirs.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Service-line signal; physical PDF page 83 (printed 6-5); reviewed September 15, 2026. ↗
Which pair of functions belongs to the trailer emergency or supply air line?
Correct answer: Charge the trailer tanks and control emergency braking when supply pressure is lost.
The supply line fills trailer reservoirs. Loss of its pressure also activates the trailer emergency-brake system; it has a different job from the service signal line.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Supply-line functions; physical PDF page 83 (printed 6-5); reviewed September 15, 2026. ↗
How do relay valves help the trailer service brakes respond promptly?
Correct answer: They deliver air from nearby trailer reservoirs in response to the service-line signal.
The control signal tells relay valves how much reservoir air to send to the trailer brakes. Supplying that air locally speeds the response.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Trailer relay valves; physical PDF page 84 (printed 6-6); reviewed September 15, 2026. ↗
When standard red and blue air-line markings are used, which connection is correct?
Correct answer: Red to emergency/supply and blue to service.
The usual color code separates the red supply line from the blue service line. Verify markings and the vehicle’s connections rather than assuming every hose is correctly identified.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Identifying air connections; physical PDF page 83 (printed 6-5); reviewed September 15, 2026. ↗
Before joining the tractor and trailer glad hands, what should be checked?
Correct answer: That the rubber seals and mating surfaces are clean and sound.
Damaged or dirty seals can cause air leaks. Properly matched, clean couplers form the sealed connection needed for brake operation.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Glad-hand seals; physical PDF page 83 (printed 6-5); reviewed September 15, 2026. ↗
After supplying trailer air, the trailer spring brakes will not release. What connection error should you investigate?
Correct answer: The service and supply air lines may be crossed.
Crossed lines can send supply air into the service circuit instead of filling the trailer reservoirs. Diagnose the fault before moving; do not force the vehicle against the brakes.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Crossed lines and spring brakes; physical PDF page 84 (printed 6-6); reviewed September 15, 2026. ↗
Why is being able to pull an older trailer without spring brakes not proof that its air lines are connected correctly?
Correct answer: It may roll even though crossed lines have left it without working trailer brakes.
An older trailer with depleted stored air may roll freely. Perform the required trailer-brake checks rather than using movement as evidence of a safe connection.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Older trailer moves without braking; physical PDF page 84 (printed 6-6); reviewed September 15, 2026. ↗
After disconnecting a trailer, why should unused air hoses be attached to suitable dummy couplers?
Correct answer: To keep water and dirt out of the air connections.
Dummy couplers protect the disconnected lines from contamination. Secure the hoses too, so they cannot drag or strike moving parts.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Unused glad hands; physical PDF page 84 (printed 6-6); reviewed September 15, 2026. ↗
Why might a large leak in a trailer service line become obvious when you first apply the brakes?
Correct answer: That line is pressurized by a service-brake application.
A service-line leak may not show as an obvious pressure loss until braking is requested. A sudden pressure drop during the check must be investigated.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Service-line leak timing; physical PDF page 84 (printed 6-6); reviewed September 15, 2026. ↗
Before normal travel, how should unused service and supply shut-off valves at the back of the last trailer be set?
Correct answer: Closed.
Close the lines where no following trailer is connected. Intermediate connections supplying another trailer must instead allow air through.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Last trailer shut-offs; physical PDF page 84 (printed 6-6); reviewed September 15, 2026. ↗
What problem can insufficient lubrication of a conventional fifth-wheel plate cause?
Correct answer: Excess friction that interferes with steering.
The plate needs the lubrication specified for that equipment. Excess friction between tractor and trailer can hinder steering; lubrication does not replace correct locking.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Fifth-wheel lubrication; physical PDF page 85 (printed 6-7); reviewed September 15, 2026. ↗
For the conventional fifth wheel described in the handbook, what position is needed before backing under the trailer?
Correct answer: Plate tilted down toward the tractor’s rear, with the jaws open.
The described preparation lets the trailer enter and the kingpin engage the fifth wheel. Follow the actual fifth-wheel manufacturer’s coupling procedure.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Fifth-wheel coupling position; physical PDF page 85 (printed 6-7); reviewed September 15, 2026. The open-jaws check continues on physical page 86 (printed 6-8). ↗
Before coupling with a sliding fifth wheel, what should you confirm about the slide?
Correct answer: Its locking mechanism is engaged.
A sliding fifth wheel must be locked in its intended position. The trailer connection does not substitute for the slide’s own locks.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Sliding fifth-wheel security; physical PDF page 86 (printed 6-8); reviewed September 15, 2026. ↗
A pre-coupling inspection shows a bent or broken trailer kingpin. What should you do?
Correct answer: Do not couple and operate it until the defect is properly corrected.
The kingpin is a critical coupling part. A low load or a forceful coupling attempt does not make a damaged pin safe.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Damaged kingpin; physical PDF page 86 (printed 6-8); reviewed September 15, 2026. ↗
Why should the tractor be lined up directly with the semitrailer before backing underneath?
Correct answer: An angled approach can push the trailer sideways and damage its landing gear.
Approach straight and slowly. Side loading during coupling can damage supports or move the trailer unexpectedly.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Straight coupling approach; physical PDF page 86 (printed 6-8); reviewed September 15, 2026. ↗
For the handbook’s conventional fifth-wheel coupling, how high should the trailer be?
Correct answer: Low enough for the tractor to lift it slightly while backing underneath.
A trailer set too high can miss proper engagement; too low can damage the nose. Adjust height and alignment before completing the coupling.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Trailer height for coupling; physical PDF page 86 (printed 6-8); reviewed September 15, 2026. ↗
How should connected air hoses be positioned before the tractor backs fully under a semitrailer?
Correct answer: Supported clear of places where they could be crushed or caught.
Coupling movement can pinch or snag poorly routed hoses. Support them so the connection remains intact as the tractor moves under the trailer.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Air-hose support during coupling; physical PDF page 86 (printed 6-8); reviewed September 15, 2026. ↗
While following the handbook’s coupling checks, you apply and release trailer brakes with the engine off. What should you listen and watch for?
Correct answer: Brake movement and release air, without signs of major pressure loss.
Listening helps confirm trailer application and release, while the pressure gauge can reveal major air loss. Perform the complete procedure with the vehicle secured.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Confirming trailer brake response; physical PDF page 86 (printed 6-8); reviewed September 15, 2026. ↗
How should the tractor be backed under the trailer to engage the kingpin?
Correct answer: Slowly, using the lowest reverse gear.
A controlled low-speed approach avoids striking the kingpin too hard. Stop when the proper connection is made, then verify it.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Coupling speed; physical PDF page 86 (printed 6-8); reviewed September 15, 2026. ↗
The coupling resists a gentle pull against locked trailer brakes. What else is still needed?
Correct answer: A visual check of correct fifth-wheel engagement and locking.
A gentle pull is one check, not a replacement for looking at the plates, jaws, kingpin and locking mechanism. Complete both parts of verification.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Tug test plus inspection; physical PDF page 86 (printed 6-8); reviewed September 15, 2026. The locking-jaw inspection continues on physical page 87 (printed 6-9). ↗
Before entering the coupling inspection area, what does the handbook specify to prevent another person moving the truck?
Correct answer: Secure the vehicle, shut the engine off and take the key.
The handbook specifies neutral, parking brakes, engine off and taking the key. Use the full safe inspection procedure and stay clear of pinch and wheel hazards.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Securing before coupling inspection; physical PDF page 86 (printed 6-8); reviewed September 15, 2026. ↗
A visible gap remains between the upper and lower fifth-wheel plates after attempted coupling. What does that mean?
Correct answer: The coupling may be incorrect and must be corrected before driving.
A gap can indicate a high-hitched or otherwise incorrect connection. Do not drive until the actual cause is resolved and correct engagement is verified.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Gap between fifth-wheel plates; physical PDF page 86 (printed 6-8); reviewed September 15, 2026. ↗
Where should conventional fifth-wheel locking jaws close around the kingpin?
Correct answer: Around the shank, not the head.
The shank is the narrower section between the base and head shown in Figure 6.8. Correct jaw engagement must be visually verified after securing the vehicle.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Kingpin engagement; physical PDF page 91 (printed 6-13); reviewed September 15, 2026. ↗
After the locking lever is in the locked position, what should be checked about the safety latch?
Correct answer: It is properly engaged as required for that fifth wheel.
The latch or catch helps retain the locked mechanism. Some equipment requires placing it by hand; follow the actual manufacturer’s design and instructions.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Coupling safety latch; physical PDF page 87 (printed 6-9); reviewed September 15, 2026. ↗
After inserting the trailer electrical plug, what should you do with its retaining safety catch?
Correct answer: Fasten it and confirm the cord is secure.
A loose plug can disconnect as the rig moves. Secure the connection and route the cord away from damaging contact.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Trailer electrical connection; physical PDF page 87 (printed 6-9); reviewed September 15, 2026. ↗
After the coupling is verified, how far should the landing gear be raised before travel?
Correct answer: All the way into its proper travel position.
Partly raised supports can strike a crossing or other obstruction. Fully raise them and secure the operating handle before moving off.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Landing gear travel position; physical PDF page 87 (printed 6-9); reviewed September 15, 2026. ↗
What should happen to the landing-gear crank after the supports are raised?
Correct answer: Secure the handle in its proper stowed position.
A properly stowed crank cannot swing into hazards or come loose during the trip. Check the manufacturer’s retention arrangement.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Landing-gear crank; physical PDF page 87 (printed 6-9); reviewed September 15, 2026. ↗
After the trailer’s full weight rests on the tractor, which clearance check matters?
Correct answer: Space between tractor components, landing gear and the trailer during turns.
Check frame-to-landing-gear and tire-to-trailer clearance with the trailer’s weight on the tractor. Correct coupling must also leave adequate clearance for turning.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Clearance after load transfer; physical PDF page 87 (printed 6-9); reviewed September 15, 2026. ↗
Before uncoupling a loaded semitrailer, what must be true of the parking surface?
Correct answer: It can support the trailer’s weight on its landing gear.
Once uncoupled, the supports transfer weight to the ground. An unsuitable surface can let the landing gear sink or the trailer fall.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Uncoupling surface; physical PDF page 87 (printed 6-9); reviewed September 15, 2026. ↗
Why should the tractor and trailer be lined up straight before pulling apart?
Correct answer: Pulling out at an angle can damage the landing gear.
The handbook calls for straight alignment to avoid side loads on the supports. Keep the rig secured while preparing to release the coupling.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Straight uncoupling alignment; physical PDF page 87 (printed 6-9); reviewed September 15, 2026. ↗
During the properly secured uncoupling sequence, why gently back against the kingpin before releasing the fifth wheel?
Correct answer: To ease pressure on the locking jaws.
Relieving tension can make the locking lever easier to release. Follow the complete sequence, including trailer security and tractor parking brakes.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Relieving jaw pressure; physical PDF page 87 (printed 6-9); reviewed September 15, 2026. ↗
When lowering landing gear on a loaded trailer before uncoupling, what does the handbook advise after firm ground contact?
Correct answer: Ease tractor load without lifting the trailer off the fifth wheel.
After firm ground contact, the handbook uses a few additional low-gear turns to take some weight off the tractor and ease release. Do not jack the trailer completely off the fifth wheel.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Loaded-trailer support setting; physical PDF page 87 (printed 6-9); reviewed September 15, 2026. ↗
How does the handbook advise stowing the electrical cable after uncoupling?
Correct answer: Hang it securely with the plug facing down to keep moisture out.
The downward-facing plug helps prevent moisture entry. Support the cable so it cannot be damaged during bobtail travel.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Disconnected electrical cord; physical PDF page 87 (printed 6-9); reviewed September 15, 2026. ↗
Why stop with the tractor frame still under the trailer after the fifth wheel clears it during uncoupling?
Correct answer: The frame offers protection if the landing gear collapses or sinks.
The intermediate stop provides a safeguard before fully leaving the trailer. Secure the tractor and inspect the supports before pulling completely clear.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Partial pull-out safeguard; physical PDF page 87 (printed 6-9); reviewed September 15, 2026. The support inspection continues on physical page 88 (printed 6-10). ↗
Before the tractor moves completely clear of an uncoupled trailer, what should be checked?
Correct answer: That the ground supports the trailer and the landing gear is not damaged.
Confirm the supports are sound and bearing the trailer safely. A failed support can let the trailer fall after the tractor moves away.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Support verification before clearing; physical PDF page 88 (printed 6-10); reviewed September 15, 2026. ↗
A pintle-hook mount has a missing fastener and visible movement. What is the proper response?
Correct answer: Have the unsafe mounting corrected before towing.
The pintle hook and its mounting must be secure and undamaged. A locked latch cannot make an insecure mounting safe.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Pintle-hook mounting; physical PDF page 88 (printed 6-10); reviewed September 15, 2026. ↗
Before towing with a pintle coupling, what must be verified after the drawbar eye is seated?
Correct answer: The latch and any required locking pin are properly secured.
Correct seating must be followed by the equipment’s locking procedure. An unsecured latch can permit separation; do not generalize one manufacturer’s handle geometry to every hook.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Locked pintle coupling; physical PDF page 89 (printed 6-11); reviewed September 15, 2026. Latch seating starts on physical page 88 (printed 6-10); the locking-pin check is on page 89. ↗
Under the cited federal rule for air-braked towing units, in what pressure range must the tractor protection valve or similar device operate automatically?
Correct answer: Between 20 and 45 psi.
49 CFR 393.43(a) specifies 20–45 psi. The exact set point and inspection procedure come from the equipment manufacturer; this is distinct from the low-air warning threshold.
Source: 49 CFR 393.43(a). eCFR title 49 displayed current through September 11, 2026; reviewed September 15, 2026. ↗
For an ordinary two-axle trailer required to have brakes, outside driveaway-towaway operations, how long must its automatically applied breakaway brakes remain applied under the cited federal rule?
Correct answer: At least 15 minutes.
49 CFR 393.43(d) requires immediate automatic breakaway application and at least 15 minutes of holding. The question excludes the driveaway-towaway exception and the separate three-or-more-axle application qualification.
Source: 49 CFR 393.43(d), (f). eCFR title 49 displayed current through September 11, 2026; reviewed September 15, 2026. ↗
During the secured check for service air at the back of the last trailer, what must be applied before opening that service-line shut-off briefly?
Correct answer: The trailer hand brake or service brake pedal.
The service line needs an application signal for this test. Close the valve after checking; all intermediate valves must pass air, and the complete procedure requires rollaway protection.
Source: Missouri Commercial Driver License Manual, revised August 2025; Section 6 (July 2017), Service air through the complete rig; physical PDF page 92 (printed 6-14); reviewed September 15, 2026. ↗
