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. ↗
