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Massachusetts CDL Combination study materials
Massachusetts CDL Combination study resources

Massachusetts CDL Combination practice questions

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Question 1Combination Vehicle Basics and Inspection

Before driving a combination vehicle, the air pressure in the tractor's service brake system must build to at least what level before you release the parking brake and move?

  1. 80 psi Correct answer.
  2. 125 psi
  3. 100 psi
  4. 60 psi

Explanation

Massachusetts CDL pre-trip guidance and federal air brake standards require at least 80 psi in the service system before you release the parking brake and drive. Below that, the low-pressure warning has already sounded and braking performance is not assured. Always allow the compressor to build full pressure and let the warning buzzer stop before moving.

Question 2Combination Vehicle Basics and Inspection

A tractor-trailer combination has three air lines connecting the tractor to the trailer. Which line supplies air to the trailer's spring parking brakes to release them?

  1. The service (blue) line
  2. The electrical cord
  3. The supply (green) line
  4. The emergency (red) line Correct answer.

Explanation

The emergency (red) line charges the trailer's air tanks and supplies the pressure that releases the spring parking brakes. The service (blue) line carries the signal that applies the service brakes. The green line is a supply line on some systems, but the red emergency line is the one that releases the springs when charged.

Question 3Combination Vehicle Basics and Inspection

When coupling a tractor to a semi-trailer, you should back up until the tractor's fifth wheel does what?

  1. Slides completely under the trailer nose Correct answer.
  2. Touches the trailer kingpin plate with the trailer raised
  3. Rests against the trailer landing gear
  4. Locks the trailer's glad hands together

Explanation

You back the tractor slowly so the fifth wheel slides fully under the trailer until the kingpin engages the jaws. Backing too far or too short can cause the kingpin to miss the locking jaws. Stop when the fifth wheel is centered under the trailer, then set the tractor parking brake before getting out.

Question 4Combination Vehicle Basics and Inspection

After coupling a trailer, you perform a pull test. What does a successful pull test show?

  1. The trailer brakes hold the combination in place
  2. The kingpin is locked in the fifth wheel Correct answer.
  3. The air lines are connected in the correct order
  4. The landing gear is fully raised

Explanation

A pull test with the trailer brakes set and the tractor in a low gear confirms the kingpin is locked in the fifth wheel; if the coupling is not secure, the tractor will pull out from under the trailer. This is the most reliable way to verify a locked coupling before you drive. Always tug gently with the trailer brakes applied.

Question 5Combination Vehicle Basics and Inspection

What is the correct order for uncoupling a tractor from a trailer?

  1. Disconnect air lines, pull the fifth wheel release, lower landing gear, pull forward
  2. Lower landing gear, disconnect air lines, pull the fifth wheel release, pull forward Correct answer.
  3. Lower landing gear, pull the fifth wheel release, disconnect air lines, pull forward
  4. Pull the fifth wheel release, lower landing gear, disconnect air lines, pull forward

Explanation

You must lower the landing gear and disconnect the air lines before releasing the fifth wheel and pulling forward, so the trailer is supported and the lines are not torn. Pulling the release first risks dropping the trailer nose. Disconnecting the lines last risks damaging them when the tractor moves.

Question 6Combination Vehicle Basics and Inspection

When you make a right turn in a combination vehicle, why must you swing wide?

  1. To keep the tractor's front wheels from rubbing the trailer
  2. To allow the trailer brakes to release
  3. To keep the trailer's rear wheels from hitting the curb Correct answer.
  4. To shorten the turning radius

Explanation

The rear wheels of a trailer track inside the path of the tractor's front wheels, a behavior called off-tracking. On a right turn this causes the trailer to cut the corner and strike the curb or a pedestrian. Swinging wide and checking the right mirror keeps the trailer clear of the curb.

Question 7Combination Vehicle Basics and Inspection

What is the main reason a combination vehicle needs more stopping distance than a single-unit truck?

  1. The combination is heavier and the air system takes longer to apply Correct answer.
  2. The tractor's engine brake is disabled
  3. The trailer has no brakes
  4. The trailer tires are smaller

Explanation

A loaded combination is much heavier, so it carries more momentum, and the air brake system takes time to build and transmit pressure to the trailer brakes. Both factors increase stopping distance. Drivers must leave a larger following gap and brake earlier.

Question 8Combination Vehicle Basics and Inspection

What does the trailer protection valve do when a trailer air line is severed?

  1. It closes to keep the trailer from losing all air Correct answer.
  2. It sounds the low-pressure buzzer
  3. It applies only the trailer service brakes
  4. It locks the trailer's kingpin

Explanation

The trailer protection valve closes when it detects a sudden loss of air, trapping the remaining air in the trailer tanks so the spring brakes can apply. Without it, a severed line would dump all trailer air and leave the trailer without braking. This is one reason a pre-trip check of the valve is important.

Question 9Combination Vehicle Basics and Inspection

When checking the air lines during a pre-trip inspection, what should you look for?

  1. Painted fittings and new grommets
  2. Couplers wrapped in tape
  3. Lines routed over the landing gear
  4. Cracks, chafing, and secure glad hand connections Correct answer.

Explanation

Air lines must be free of cracks, chafing, and leaks, and glad hands must be securely connected with good seals. Chafed or cracked lines can fail under pressure and cause brake loss. Taped or improvised repairs are not acceptable on a commercial vehicle.

Question 10Combination Vehicle Basics and Inspection

You are inspecting the fifth wheel. Which condition requires the vehicle to be placed out of service?

  1. A new kingpin plate
  2. Light grease on the plate
  3. A slightly worn release handle
  4. A missing or broken locking jaw Correct answer.

Explanation

A missing or broken locking jaw means the kingpin cannot be secured, so the trailer could separate from the tractor. That is an out-of-service defect. Grease, a worn handle, or a new plate are normal or minor conditions.

Question 11Combination Vehicle Basics and Inspection

What is the purpose of the trailer's spring brakes?

  1. They hold the glad hands together
  2. They lock the landing gear
  3. They provide the trailer's only service braking
  4. They apply automatically when air pressure is lost Correct answer.

Explanation

Spring brakes are held applied by heavy springs and released by air pressure, so they apply automatically when air pressure drops. This provides emergency and parking braking for the trailer. They are not the service brakes, which are applied by air pressure.

Question 12Combination Vehicle Basics and Inspection

During a pre-trip inspection of a combination vehicle, you check the landing gear. What are you verifying?

  1. That the crank is removed
  2. That the feet are buried in gravel
  3. That the legs are fully extended
  4. That the crank turns freely and the legs are not bent Correct answer.

Explanation

Landing gear must crank freely, with straight legs, intact feet, and no cracks or bends, so the trailer can be supported safely when uncoupled. Bent legs or a frozen crank can fail when the trailer is dropped. The gear should be raised during normal driving.

Question 13Combination Vehicle Basics and Inspection

You are driving a combination vehicle and the trailer begins to swing side to side. What is the best initial response?

  1. Release the accelerator and steer straight Correct answer.
  2. Apply the trailer hand valve
  3. Brake hard immediately
  4. Accelerate to pull the trailer straight

Explanation

Releasing the accelerator and holding the steering straight lets the combination slow and the trailer settle. Hard braking or acceleration can worsen a swing or cause a jackknife. The trailer hand valve should not be used while driving.

Question 14Combination Vehicle Basics and Inspection

What is the proper way to test the trailer service brakes from the cab before a trip?

  1. Release the tractor parking brake and roll forward
  2. Pull the trailer hand valve and listen for air
  3. Press the brake pedal and have someone watch the trailer chambers Correct answer.
  4. Set the trailer brakes and pull against them

Explanation

With a helper watching the trailer brake chambers, you press the brake pedal and confirm the pushrods extend, showing the service brakes apply. This verifies the service line and chambers work. The other methods test parking or coupling, not service brake application.

Question 15Combination Vehicle Basics and Inspection

Which of these is a sign that the trailer's ABS is working during a pre-trip check?

  1. The ABS lamp stays on after the vehicle reaches 5 mph
  2. The ABS lamp flickers with the turn signal
  3. The ABS lamp comes on briefly at start-up and then goes out Correct answer.
  4. The ABS lamp never illuminates

Explanation

The trailer ABS lamp should illuminate briefly at power-up and then go out once the system self-tests, indicating the ABS is functioning. A lamp that stays on indicates a fault. A lamp that never lights may mean a burned-out bulb or disconnected power.

Question 16Combination Vehicle Basics and Inspection

What is the correct way to connect the glad hands when coupling?

  1. Connect both lines to the same trailer port
  2. Match the colored couplers and lock them with the seals in place Correct answer.
  3. Leave one line disconnected for ventilation
  4. Connect the blue line to the red coupler

Explanation

Glad hands are color-coded so the service (blue) and emergency (red) lines match their couplers, with good rubber seals to prevent leaks. Cross-connecting or leaving a line off will cause brake or supply failure. Always check for leaks after connecting.

Question 17Combination Vehicle Basics and Inspection

You are backing a tractor-trailer in a straight line. Which technique helps keep the trailer tracking straight?

  1. Back quickly to reduce drift
  2. Use small steering corrections and watch both mirrors Correct answer.
  3. Rely only on the right mirror
  4. Turn the wheel sharply and hold it

Explanation

Small steering corrections, watching both mirrors, and backing slowly keep the trailer tracking straight. Sharp or held steering inputs cause the trailer to swing and can lead to a jackknife. Mirror checks on both sides reveal drift early.

Question 18Combination Vehicle Basics and Inspection

What is off-tracking in a combination vehicle?

  1. The tractor pulling to one side under braking
  2. The ABS sensor losing signal
  3. The air lines trailing behind the cab
  4. The trailer wheels following inside the tractor's path Correct answer.

Explanation

Off-tracking is the tendency of a trailer's rear wheels to follow a path inside the tractor's front wheels, which is greatest in tight turns. Drivers must allow extra room on the inside of turns to avoid hitting curbs, poles, or pedestrians. Longer wheelbases increase off-tracking.

Question 19Combination Vehicle Basics and Inspection

During a pre-trip inspection, you find a crack in the trailer's frame rail near the fifth wheel plate. What should you do?

  1. Report it and place the vehicle out of service Correct answer.
  2. Note it and drive if the crack is short
  3. Weld it yourself before driving
  4. Cover it with grease and continue

Explanation

A cracked frame rail is a structural defect that can lead to trailer separation or collapse, so the vehicle must be placed out of service and the defect reported. Field welding or covering the crack does not restore structural integrity. Only qualified repair restores the vehicle to service.

Question 20Combination Vehicle Basics and Inspection

What should you do before backing under a trailer to couple?

  1. Check that the trailer brakes are set and the kingpin area is clear Correct answer.
  2. Raise the landing gear
  3. Disconnect the glad hands
  4. Release the tractor parking brake

Explanation

Before backing under, confirm the trailer spring brakes are set so it will not roll, and check that the kingpin area is clear and aligned with the fifth wheel. This prevents the trailer from moving during coupling. The landing gear stays down until the coupling is complete.

Question 21Combination Vehicle Basics and Inspection

How does a heavy load on a combination vehicle affect its center of gravity and handling?

  1. It lowers the center of gravity and improves rollover resistance
  2. It raises the center of gravity and increases rollover risk Correct answer.
  3. It reduces stopping distance
  4. It has no effect on handling

Explanation

A tall or poorly distributed heavy load raises the center of gravity, making the combination more likely to roll over in turns or evasive maneuvers. Drivers must slow down and steer smoothly. Load height and placement matter as much as weight.

Question 22Combination Vehicle Basics and Inspection

What is the purpose of the tractor protection valve?

  1. It closes the tractor air supply to the trailer if pressure drops Correct answer.
  2. It releases the trailer spring brakes
  3. It regulates the trailer's electrical voltage
  4. It locks the fifth wheel jaws

Explanation

The tractor protection valve closes the tractor-to-trailer air supply when system pressure drops, preserving tractor air for braking and helping to apply the trailer spring brakes. This prevents a major air loss from disabling the whole combination. It is checked during the pre-trip air loss test.

Question 23Combination Vehicle Basics and Inspection

After completing the coupling procedure, which check confirms the trailer is properly secured to the tractor?

  1. Verify the trailer lights are off
  2. Confirm the landing gear is down
  3. Visually confirm the locking jaws are closed around the kingpin Correct answer.
  4. Check that the glad hands are warm

Explanation

Looking under the trailer to confirm the locking jaws are closed around the kingpin verifies a secure mechanical connection. This visual check, combined with a pull test, is the standard confirmation. Neither lights nor landing gear position confirms coupling.

Question 24Combination Vehicle Basics and Inspection

You are descending a long grade with a loaded combination. What is the recommended braking technique?

  1. Shift to neutral and coast
  2. Use only the trailer hand valve
  3. Ride the brakes continuously
  4. Use a low gear and apply brakes in short, firm applications Correct answer.

Explanation

Selecting a low gear and using short, firm brake applications lets the engine and brakes share the load and prevents overheating. Continuous light braking overheats the drums and can cause brake fade. Coasting in neutral removes engine braking and is unsafe.

Question 25Combination Vehicle Basics and Inspection

What should you check about the trailer's rear impact guard (ICC bumper) during a pre-trip inspection?

  1. That it drags on the ground
  2. That it is removable for loading
  3. That it is securely attached and not bent or missing Correct answer.
  4. That it is painted a bright color

Explanation

The rear impact guard must be securely attached and undamaged so it can prevent a smaller vehicle from riding under the trailer in a rear-end collision. A bent, loose, or missing guard is a safety defect. Paint color and removability are not the inspection criteria.

Question 26Combination Vehicle Basics and Inspection

In a combination vehicle, what does the fifth wheel coupling device do?

  1. It applies the trailer spring brakes whenever the service brake pedal is released.
  2. It locks the kingpin of the trailer to the tractor so the two units pivot and move as one. Correct answer.
  3. It controls the tractor's rear suspension height when the trailer is loaded.
  4. It keeps the trailer's electrical circuits grounded through the frame.

Explanation

The fifth wheel is the horseshoe-shaped plate mounted on the tractor that captures and locks the trailer's kingpin. Once locked, it transfers the trailer's weight to the tractor and lets the combination pivot at the coupling point while pulling as a unit.

Question 27Combination Vehicle Basics and Inspection

For a combination vehicle, which statement about the tractor protection valve is accurate?

  1. It regulates the trailer's air pressure so the trailer brakes apply more slowly than the tractor's.
  2. It releases air from the trailer tanks to keep the compressor from cycling too often.
  3. It prevents the trailer from rolling when the tractor is parked without the parking brake set.
  4. It closes the tractor's air supply to the trailer if air pressure drops to an unsafe level. Correct answer.

Explanation

The tractor protection valve is a safety device that cuts off the tractor's air supply to the trailer when pressure falls too low, which also helps protect the tractor's own air. This keeps the tractor from losing all its air through a trailer leak or a broken line, and it causes the trailer spring brakes to set.

Question 28Combination Vehicle Basics and Inspection

A combination vehicle has a shorter wheelbase tractor and a long trailer. What does this configuration do to the vehicle's turning behavior?

  1. The trailer's wheels follow exactly the same path as the tractor's front wheels.
  2. The trailer's wheels pivot around the tractor's drive axles without cutting inward.
  3. The trailer's rear wheels track wider than the tractor's front wheels in every turn.
  4. The trailer's rear wheels cut the corner more sharply than the tractor's rear wheels. Correct answer.

Explanation

Because the trailer pivots at the fifth wheel, its rear wheels follow a path inside the path of the tractor's front wheels. The longer the trailer and the tighter the turn, the more the trailer's rear wheels cut the corner, so you must allow extra room on the inside of the turn.

Question 29Combination Vehicle Basics and Inspection

You are pre-tripping a combination vehicle and notice the trailer's air line glad hands. Which condition should be corrected before you drive?

  1. The glad hand seals are missing and the couplers are joined without rubber seals. Correct answer.
  2. The lines are color-coded so the service and emergency lines are distinguishable.
  3. The glad hands are connected with the couplers locked and the lines supported.
  4. The lines are routed with enough slack to allow turning without tension.

Explanation

Glad hand couplers depend on rubber seals to keep the connection airtight. If the seals are missing or damaged, the connection will leak air, which can cause the tractor protection valve to shut off supply and set the trailer spring brakes.

Question 30Combination Vehicle Basics and Inspection

During a pre-trip inspection of a combination vehicle, you find the fifth wheel jaws are not fully closed around the kingpin. What is the correct action?

  1. Chock the trailer wheels and continue because the trailer landing gear will hold the load.
  2. Do not move the vehicle until the coupling is corrected and the jaws are locked around the kingpin. Correct answer.
  3. Tap the kingpin with a hammer until the jaws close on their own.
  4. Drive slowly to the shop so the trailer weight seats the jaws as you go.

Explanation

The fifth wheel jaws must fully close and lock around the kingpin to secure the trailer. A partially closed jaw can release the trailer under load or during a turn, so the vehicle must not be moved until the coupling is confirmed locked and the pull test is passed.

Question 31Combination Vehicle Basics and Inspection

You are inspecting the coupling on a combination vehicle. Which check best confirms the trailer is fully and properly attached to the tractor?

  1. Pull gently forward against the trailer with the trailer brakes set and confirm the tractor does not move away. Correct answer.
  2. Confirm the trailer is level and the air bags are inflated.
  3. Check that the trailer's electrical cord is plugged in and the lights work.
  4. Look under the trailer and confirm the landing gear is raised off the ground.

Explanation

The pull test confirms the fifth wheel jaws have captured the kingpin. With the trailer brakes set, pulling gently against the trailer should show no separation; if the tractor moves forward freely, the coupling is not secure.

Question 32Combination Vehicle Basics and Inspection

When you are coupling a tractor to a trailer, at what point should you raise the trailer landing gear?

  1. Only after you have driven a short distance to confirm the trailer tows well.
  2. After the coupling is complete and the pull test confirms the trailer is locked to the tractor. Correct answer.
  3. While the tractor is still backing under the trailer to help align the kingpin.
  4. Before backing under the trailer so the trailer settles onto the fifth wheel.

Explanation

Landing gear should stay on the ground to support the trailer until the kingpin is locked and the pull test passes. Raising it before the coupling is secure could let the trailer drop or shift.

Question 33Combination Vehicle Basics and Inspection

You are driving a combination vehicle and need to make a lane change. How should you account for the trailer in your mirror checks?

  1. Check only the mirror on the side you are moving toward, then complete the change.
  2. Signal and change lanes promptly so other drivers see the trailer move as a unit.
  3. Rely on the tractor's mirrors only, since the trailer follows the tractor's path.
  4. Check the mirrors for the trailer's rear and confirm the full length of the vehicle is clear before moving over. Correct answer.

Explanation

A combination vehicle is much longer than a single unit, and the trailer occupies space behind you that the tractor mirrors may not fully show. You must confirm the entire vehicle, including the trailer's rear, is clear before changing lanes.

Question 34Combination Vehicle Basics and Inspection

A combination vehicle's trailer has a spring brake system. If air pressure in the trailer system drops below the safe level, what happens?

  1. The trailer spring brakes apply automatically and the trailer stops. Correct answer.
  2. The trailer's service brakes modulate to maintain a steady speed.
  3. The trailer's brakes are unaffected because they operate on hydraulic pressure.
  4. The trailer brakes release so the wheels roll freely.

Explanation

Spring brakes are held off by air pressure and apply by spring force when air pressure is lost. A drop in trailer air pressure therefore sets the trailer brakes automatically, which is a key safety feature of combination vehicles.

Question 35Combination Vehicle Basics and Inspection

During a pre-trip inspection of a combination vehicle, you check the trailer's brake chambers and pushrods. Which finding indicates a problem that should be corrected before driving?

  1. A pushrod that moves freely and returns when the brake is released.
  2. A brake chamber that is securely mounted with tight clamps.
  3. A slack adjuster that is properly lubricated and moves with the pushrod.
  4. A pushrod that is bent and does not return fully when the brake is released. Correct answer.

Explanation

A bent pushrod that does not return indicates a brake chamber or linkage problem that can prevent the brake from releasing or applying correctly. Brake components must move freely and return properly, so this condition must be fixed before the vehicle is driven.

Question 36Combination Vehicle Basics and Inspection

You are inspecting the trailer's air tank and drain valves during a pre-trip. What should you do with the drain valves?

  1. Drain any accumulated moisture and confirm the valves close and do not leak. Correct answer.
  2. Leave them open so moisture can escape while you drive.
  3. Open them only while the engine is running at high idle.
  4. Seal them closed permanently to prevent air loss.

Explanation

Air tanks collect moisture that can corrode the system and freeze in cold weather. Draining the tanks during the pre-trip and confirming the valves reseal and hold pressure keeps the air system reliable.

Question 37Combination Vehicle Basics and Inspection

A driver is inspecting the fifth wheel plate on a combination vehicle. Which condition is acceptable for service?

  1. The plate is dry and rusty, and the jaws close only partway.
  2. The plate is cracked near a mounting bolt but the trailer still couples.
  3. The plate is securely mounted, lightly greased, and the jaws lock fully around the kingpin. Correct answer.
  4. The plate is loose on its mounts but the jaws appear to close.

Explanation

A serviceable fifth wheel is firmly mounted, properly lubricated so the trailer can pivot, and has jaws that fully lock the kingpin. Loose mounts, cracks, or partial jaw closure are defects that can cause the trailer to separate or handle poorly.

Question 38Combination Vehicle Basics and Inspection

You are pre-tripping a combination vehicle and find the trailer's rear impact guard is bent upward and does not extend low enough. Why does this matter?

  1. It changes the trailer's turning radius and causes off-tracking.
  2. It may fail to prevent a smaller vehicle from riding under the trailer in a rear-end crash. Correct answer.
  3. It increases the trailer's stopping distance on wet pavement.
  4. It prevents the trailer's spring brakes from applying.

Explanation

The rear impact guard is designed to stop a smaller vehicle from sliding under the trailer in a rear-end collision. If it is bent, too high, or damaged, it cannot provide that protection, so it must be repaired.

Question 39Combination Vehicle Basics and Inspection

You are coupling a tractor to a trailer on uneven ground. What is the safest approach to align the tractor with the trailer?

  1. Back quickly so the trailer cannot shift while the kingpin engages.
  2. Raise the trailer landing gear first so the trailer can be pushed into alignment.
  3. Approach at an angle so the fifth wheel jaws open wider for the kingpin.
  4. Line up the tractor and trailer carefully, then back slowly so the kingpin enters the fifth wheel centered. Correct answer.

Explanation

Coupling requires the tractor and trailer to be aligned so the kingpin enters the fifth wheel squarely. Backing slowly and centered prevents damage to the kingpin, jaws, and trailer, especially on uneven ground where the trailer may shift.

Question 40Combination Vehicle Basics and Inspection

A combination vehicle is equipped with a trailer that has its own ABS. During a pre-trip check, the trailer ABS warning lamp stays on after the ignition is cycled. What does this indicate?

  1. The tractor protection valve has closed and cut off trailer air.
  2. The trailer spring brakes are about to release automatically.
  3. The trailer ABS is functioning normally and the lamp is simply a bulb check.
  4. The trailer ABS has a fault, and the system should be checked before relying on it. Correct answer.

Explanation

A trailer ABS lamp that remains illuminated after the initial bulb check signals a fault in the trailer ABS system. The trailer may still have normal brakes, but the ABS function may be lost, so the fault should be diagnosed and corrected.

Question 41Coupling and Uncoupling

Before backing a tractor under a trailer during coupling, what should you check about the trailer's parking brakes?

  1. They are released so the trailer can roll into alignment
  2. They are set only after the kingpin is locked
  3. They are released only after the air lines are connected
  4. They are set so the trailer cannot roll during coupling Correct answer.

Explanation

The trailer spring brakes must be set (parking brakes engaged) before you back under it, so the trailer stays put while the fifth wheel contacts the kingpin. If the trailer can roll, coupling forces can push it away or damage the landing gear.

Question 42Coupling and Uncoupling

What is the correct order of steps when coupling a tractor to a trailer?

  1. Connect air lines, lock the kingpin, raise the landing gear, then tug test
  2. Inspect the fifth wheel, back under, lock the kingpin, tug test, then connect lines and raise gear Correct answer.
  3. Raise the landing gear, back under, connect lines, then lock the kingpin
  4. Tug test, back under, raise the landing gear, then lock the kingpin

Explanation

Coupling follows a fixed sequence: inspect the fifth wheel and kingpin area, back under the trailer, lock the kingpin, do a tug test, then connect the air lines and raise the landing gear. Doing the tug test before connecting lines confirms the lock without relying on trailer brakes.

Question 43Coupling and Uncoupling

You have backed under a trailer and the fifth wheel appears to have engaged. What does a proper tug test involve?

  1. Rocking the tractor back and forth in reverse against the kingpin
  2. Driving a short distance and checking the trailer in the mirrors
  3. Easing forward gently with the trailer brakes released while watching for movement
  4. Pulling forward firmly with the trailer brakes set to confirm the kingpin is locked Correct answer.

Explanation

With the trailer spring brakes set, pull forward gently but firmly against them; if the kingpin is not locked, the trailer will stay put and the tractor will pull out from under it. This reveals a high or missed coupling before you raise the gear or drive away.

Question 44Coupling and Uncoupling

After coupling, you raise the landing gear, connect the air lines, and build air pressure. What should you check before releasing the trailer brakes?

  1. That the trailer lights work in all modes
  2. That the trailer has not crept forward
  3. That the air lines are not crossed and the trailer brakes release and hold Correct answer.
  4. That the fifth wheel jaws are painted

Explanation

Supply the trailer with air and confirm the brakes release when the tractor protection valve allows flow, then confirm they set again when you apply the brake. Crossed lines or a closed shutoff valve will leave the trailer without working brakes.

Question 45Coupling and Uncoupling

When uncoupling, why should you lower the landing gear until it firmly contacts the ground before releasing the fifth wheel?

  1. To support the trailer's weight so it does not drop when the tractor pulls out Correct answer.
  2. To keep the trailer at the correct ride height for the next coupling
  3. To align the kingpin with the fifth wheel
  4. To take weight off the tractor's rear axle for better traction

Explanation

The landing gear must carry the trailer's weight before the fifth wheel is released; otherwise the trailer drops when the tractor pulls away, damaging the gear and possibly the tractor. Crank the gear down until it is solidly on the ground.

Question 46Coupling and Uncoupling

What is the correct sequence for uncoupling a tractor from a trailer?

  1. Disconnect lines, pull the release, lower the gear, then pull out
  2. Pull the fifth wheel release, lower the gear, disconnect lines, then pull out
  3. Lower the gear, pull the release, disconnect lines, then pull out
  4. Lower the gear, disconnect lines, pull the fifth wheel release, then pull out Correct answer.

Explanation

Uncoupling order: lower the landing gear to the ground, disconnect the air and electrical lines and secure them, then release the fifth wheel and pull the tractor out slowly. Releasing the fifth wheel before the gear is down can drop the trailer.

Question 47Coupling and Uncoupling

During a pre-trip inspection of the fifth wheel, what should you look for on the locking jaws?

  1. That they are greased heavily and fully closed around nothing
  2. That they are closed around the shank of the kingpin with no gaps Correct answer.
  3. That they are painted to show wear
  4. That they are open and free of grease

Explanation

With the trailer coupled, the locking jaws should be closed around the kingpin shank with no visible gap, and the release handle should be in the locked position. Gaps or an open handle indicate an incomplete coupling.

Question 48Coupling and Uncoupling

You back under a trailer but the fifth wheel rides up over the kingpin instead of locking. What is the most likely cause?

  1. The trailer is too low and the kingpin strikes the fifth wheel plate
  2. The landing gear is cranked too high
  3. The trailer is too high so the kingpin passes above the locking jaws Correct answer.
  4. The tractor protection valve is closed

Explanation

A high coupling happens when the trailer sits too high, letting the kingpin ride over the top of the fifth wheel instead of entering the throat. Always check that the trailer height matches the tractor before backing under.

Question 49Coupling and Uncoupling

After coupling and the tug test, you notice the trailer is sitting noticeably low at the front and the tractor frame is close to the trailer. What should you do?

  1. Raise the landing gear higher
  2. Release air from the trailer to lower it
  3. Drive slowly and monitor it
  4. Check that the kingpin is fully seated and the fifth wheel is locked properly Correct answer.

Explanation

A trailer riding low or a gap at the fifth wheel can indicate the kingpin is not fully seated in the jaws. Stop and verify the coupling before moving; an improper lock can let the trailer separate.

Question 50Coupling and Uncoupling

When should the trailer air lines be connected during the coupling process?

  1. Before backing under the trailer
  2. Immediately after the kingpin is locked and the tug test is done Correct answer.
  3. Before the tug test but after backing under
  4. After the tractor has pulled away from the trailer

Explanation

Connect the air lines after the kingpin is locked and the tug test confirms the coupling. Connecting them earlier can damage the lines or give a false sense that the trailer is secured.