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Michigan CDL Combination study materials
Michigan CDL Combination study resources

Michigan CDL Combination practice questions

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Question 1Vehicle inspection

During a pre-trip inspection of a combination vehicle, where should you check the trailer's spring brake (emergency) system?

  1. At the trailer wheels with the parking brake released
  2. At the trailer wheels with the parking brake set Correct answer.
  3. At the tractor protection valve with the engine off
  4. At the trailer relay valve with the parking brake released

Explanation

With the parking brake set, the spring brakes are fully applied, so you can inspect the chambers, pushrods, and slack adjusters at the trailer wheels in their applied position. Releasing the brake would retract the pushrods and hide a stroke or leak problem. Always chock the wheels before going under a combination vehicle.

Question 2Vehicle inspection

You are coupling a tractor to a trailer. After backing under the trailer, what should you do immediately before pulling the fifth wheel release handle?

  1. Back under the trailer to apply pressure to the kingpin Correct answer.
  2. Connect the air lines to the trailer
  3. Lower the trailer landing gear to the ground
  4. Raise the landing gear fully

Explanation

Backing firmly under the trailer pushes the fifth wheel jaw against the kingpin so the locking mechanism can close and latch. Without this pressure the jaws may not snap shut, allowing the trailer to separate later. Confirm the latch is closed and pull-test before driving.

Question 3Vehicle inspection

What is the purpose of the glad hands on a combination vehicle?

  1. They lock the fifth wheel jaw around the kingpin
  2. They couple the air lines between tractor and trailer Correct answer.
  3. They secure the trailer landing gear in the raised position
  4. They connect the electrical wiring between tractor and trailer

Explanation

Glad hands are the quick-coupling fittings that join the tractor's air lines to the trailer's service and emergency (supply) lines. They must be sealed with rubber gaskets to prevent air leaks that would weaken the trailer brakes. Electrical connections use a separate cord and plug.

Question 4Vehicle inspection

While inspecting the fifth wheel, you notice the mounting bolts are loose. What is the correct action?

  1. Tighten the bolts by hand and continue
  2. Proceed but recheck the bolts at the next stop
  3. Place the vehicle out of service until repaired Correct answer.
  4. Note the condition and report it after the trip

Explanation

Loose fifth wheel mounting bolts can let the wheel shift or separate from the frame, causing loss of the trailer. This is a critical defect that puts the vehicle out of service until it is properly repaired and torqued. A driver may not simply hand-tighten structural fasteners and drive on.

Question 5Vehicle inspection

What does the tractor protection valve do when air pressure in the tractor system drops too low?

  1. It vents the trailer air tanks to the atmosphere to save the tractor
  2. It opens fully to send all remaining air to the trailer brakes
  3. It disengages the fifth wheel to separate the trailer
  4. It closes and shuts off air to the trailer, applying the trailer spring brakes Correct answer.

Explanation

The tractor protection valve senses low supply pressure and closes, trapping the trailer's air so the trailer spring brakes apply and the trailer stays put. This prevents a total loss of air from both vehicles and keeps a breakaway trailer from rolling. It is a key safety device in the pre-trip air brake check.

Question 6Vehicle inspection

During the air brake leakage test on a combination vehicle, which procedure is correct?

  1. Shut off the engine, release the parking brake, and watch the gauge for one minute with the brakes applied
  2. Pump the brake pedal repeatedly with the engine running and watch for the low-air warning
  3. Run the engine at high idle and watch the gauge for five minutes without applying the brakes
  4. Shut off the engine, hold the brake pedal, and watch the gauge for three minutes with the parking brake released Correct answer.

Explanation

The leakage test is done with the engine off, the parking brake released, and full pressure built up; you hold the brake pedal and watch the gauge for one minute. A loss greater than the allowed rate (commonly 3 psi for a combination) indicates a leak. The parking brake must be released so the test reflects the service brake system.

Question 7Vehicle inspection

You are performing the tug test after coupling. What are you checking?

  1. That the trailer lights are working
  2. That the landing gear is fully raised
  3. That the trailer air lines are connected
  4. That the fifth wheel jaws have locked around the kingpin Correct answer.

Explanation

After coupling you pull forward gently with the trailer brakes set; if the kingpin is locked, the tractor will not move away from the trailer. This confirms the fifth wheel jaws engaged the kingpin. A failed tug test means the trailer is not safely coupled.

Question 8Vehicle inspection

Which item must be checked on the trailer during a pre-trip inspection to confirm the brakes will hold when parked?

  1. That the trailer air tank drains are open
  2. That the trailer slack adjusters are fully extended
  3. That the trailer brakes release when the tractor protection valve closes
  4. That the trailer spring brakes apply when the trailer air supply is disconnected Correct answer.

Explanation

Disconnecting the trailer emergency (supply) line should cause the trailer spring brakes to apply automatically. This verifies the emergency braking feature works if the trailer separates from the tractor. If the brakes do not apply, the trailer is unsafe to tow.

Question 9Vehicle inspection

What should you check regarding the trailer landing gear before driving?

  1. That it is lowered to touch the ground
  2. That the crank is removed and stored in the cab
  3. That it is fully raised and the crank is secured Correct answer.
  4. That it is greased and left in the down position

Explanation

The landing gear must be fully raised and its crank secured so it does not drag or strike the road or railroad crossings. A lowered or partially raised gear can catch on pavement and damage the trailer or cause loss of control.

Question 10Vehicle inspection

When inspecting the coupling system, how do you check the safety latch on the fifth wheel?

  1. Push the kingpin out of the jaw by hand
  2. Check that the air lines hang free of the jaw
  3. Look to confirm the latch is over the closed jaw Correct answer.
  4. Pull the release handle and watch the latch open

Explanation

The safety latch is a visual check: the latch must be over the closed jaw to keep the locking mechanism from opening. You should also confirm the release handle is in its locked position. Pulling the handle would actually release the coupling.

Question 11Vehicle inspection

During the pre-trip inspection you find a trailer tire with a deep cut exposing the cord. What is the proper response?

  1. Inflate the tire to the maximum sidewall pressure
  2. Replace or repair the tire before operating Correct answer.
  3. Record the defect and continue the trip
  4. Drive at reduced speed to the nearest terminal

Explanation

A tire with cord exposed is structurally damaged and can fail, especially under the load of a combination vehicle. It must be replaced or repaired before the vehicle is operated. Reduced speed or extra inflation does not make a damaged tire safe.

Question 12Vehicle inspection

What is the correct way to check the slack adjusters on a combination vehicle during the pre-trip inspection?

  1. With the brakes applied, note the angle of the slack adjuster to the pushrod Correct answer.
  2. With the brakes released, push each pushrod in by hand
  3. With the engine running, listen for air leaks at the chambers
  4. With the wheels raised, spin each tire and count the revolutions

Explanation

Slack adjusters are checked with the brakes applied by observing the angle between the slack adjuster and the pushrod; too much travel means the brakes are out of adjustment. This angle check is the standard visual method. You should not attempt to push pushrods in by hand.

Question 13Vehicle inspection

Why must you drain the air tanks during the pre-trip inspection?

  1. To test the low-air warning buzzer
  2. To remove moisture and oil that can damage the brake system Correct answer.
  3. To release the spring brakes for inspection
  4. To increase air pressure in the system

Explanation

Compressed air carries moisture and oil that condense in the tanks; draining them removes this contamination. Water in the system can freeze or corrode valves and cause brake failure. Drains should be opened until clean, dry air escapes, then closed.

Question 14Vehicle inspection

You are inspecting the trailer's electrical connection. Which condition requires the vehicle to be placed out of service?

  1. The plug has a light coating of dust
  2. The cord is secured with a spring clamp
  3. The cord has a small amount of slack
  4. The seven-way plug is cracked and the cord is frayed Correct answer.

Explanation

A cracked plug and a frayed cord can cause loss of trailer lights, turn signals, and brake lights, and may short or arc. This is a safety defect that must be repaired before operating. Minor dust or normal slack is acceptable.

Question 15Vehicle inspection

When checking the coupling during a pre-trip inspection, you notice the gap between the apron of the trailer and the fifth wheel. What does a visible gap indicate?

  1. The trailer is properly seated on the fifth wheel
  2. The landing gear is fully raised
  3. The trailer may not be fully coupled and could separate Correct answer.
  4. The air lines are connected correctly

Explanation

When the trailer is properly seated, the apron rests flat against the fifth wheel with no gap. A visible gap suggests the kingpin is not fully locked or the trailer is riding high, which risks separation. You should not drive until the coupling is corrected.

Question 16Vehicle inspection

During the low-air warning test, at what point should the warning signal activate?

  1. When pressure drops below about 60 psi Correct answer.
  2. When the trailer supply line is disconnected
  3. When pressure drops below about 100 psi
  4. When pressure reaches the governor cut-out level

Explanation

The low-air warning buzzer and light must activate before pressure falls below about 60 psi, warning the driver before spring brakes apply. On many vehicles the warning comes on around 60 psi. Testing this ensures the driver gets adequate warning of a pressure loss.

Question 17Vehicle inspection

What is the correct sequence for the air brake system check on a combination vehicle after building full pressure?

  1. Leakage test, then low-pressure warning test, then check that spring brakes apply Correct answer.
  2. Leakage test, then release the parking brake, then the low-pressure warning test
  3. Low-pressure warning test, then leakage test, then release the parking brake
  4. Check spring brakes, then leakage test, then the low-pressure warning test

Explanation

The standard order is to build pressure, test for leakage with the brakes applied, then bleed pressure down to verify the warning activates, then confirm the spring brakes apply. Following this order ensures each function is verified under the right conditions. Rushing the sequence can mask a defect.

Question 18Vehicle inspection

You are inspecting the trailer frame and notice a cracked cross member. What should you do?

  1. Continue if the crack is shorter than six inches
  2. Place the vehicle out of service until repaired Correct answer.
  3. Weld the crack yourself before the trip
  4. Wrap the member with tape for temporary support

Explanation

A cracked frame cross member weakens the trailer structure and can lead to failure under load, so the vehicle must be placed out of service until properly repaired. Field welding or taping is not an acceptable repair. Structural defects are not excused by crack length.

Question 19Vehicle inspection

Why is it important to check that the trailer's spring brakes are applied when the trailer is parked during inspection?

  1. To confirm the trailer will not roll if it separates from the tractor Correct answer.
  2. To reduce air pressure in the trailer tanks
  3. To allow the landing gear to be raised
  4. To make the trailer lights easier to see

Explanation

The spring brakes are the trailer's automatic parking and emergency brake; if they apply when air is lost, a separated or parked trailer will not roll away. Verifying this during inspection confirms the emergency braking function. This is a critical safety check for combination vehicles.

Question 20Vehicle inspection

During the pre-trip inspection, how should you check the trailer's service brakes?

  1. By listening for the compressor to cycle
  2. With the wheels raised off the ground
  3. With the engine running, the parking brake released, and a helper watching the wheels as you apply the pedal Correct answer.
  4. With the engine off and the trailer disconnected

Explanation

With air pressure built up, the parking brake released, and a helper at the wheels, you apply the brake pedal and confirm the trailer brake chambers move and the brakes engage. This verifies the service brakes actually work. The engine should be running to maintain pressure during the check.

Question 21Vehicle inspection

You must inspect the trailer's air line connections. Which condition indicates the glad hands are improperly connected?

  1. The lines have rubber gaskets in place
  2. The lines are color-coded red and blue
  3. The lines cross or kink when the trailer turns Correct answer.
  4. The lines hang with slight slack

Explanation

Air lines that cross or kink can pinch off air flow or be damaged when the trailer turns, leading to brake problems. Properly routed lines should not cross or kink. Color coding, gaskets, and slight slack are all normal and desirable.

Question 22Vehicle inspection

What should you verify about the trailer's mud flaps and splash guards during inspection?

  1. That they are secure and not damaged Correct answer.
  2. That they are removed for weight savings
  3. That they are made of metal
  4. That they touch the ground at all times

Explanation

Mud flaps must be securely mounted and undamaged so they deflect debris and spray from the tires. Missing or damaged flaps can throw objects at other vehicles and are a defect. They should hang at the proper height, not drag on the ground.

Question 23Vehicle inspection

During inspection you find the trailer's ABS warning light stays on after the ignition check. What does this indicate?

  1. The ABS system has a fault and may not work in a hard stop Correct answer.
  2. The trailer brakes are fully applied
  3. The ABS system is functioning normally
  4. The air pressure is too high

Explanation

An ABS lamp that remains illuminated after the bulb check signals a fault in the anti-lock system, meaning the trailer may lose ABS function during a hard stop. The vehicle should be repaired before operating. A working system's light should go out shortly after the check.

Question 24Vehicle inspection

Why should you check the kingpin for wear and damage during a pre-trip inspection?

  1. It controls the trailer's air pressure
  2. It secures the mud flaps
  3. It holds the landing gear in place
  4. A worn kingpin can allow the trailer to shift or separate from the fifth wheel Correct answer.

Explanation

The kingpin is the pivot that locks the trailer to the fifth wheel; wear or damage can let the trailer shift, wobble, or separate. Inspecting it for cracks, excessive wear, and secure mounting prevents a serious coupling failure. It has no role in air pressure or accessories.

Question 25Vehicle inspection

When performing a pre-trip inspection of a combination vehicle, what is the correct way to check the trailer's lights?

  1. Have a helper operate the switches while you watch the trailer lights Correct answer.
  2. Check the lights after the trip begins
  3. Rely on the dashboard indicator only
  4. Turn on the headlights and check only the front

Explanation

Trailer lights must be checked from outside the vehicle, so a helper operates the switches while you observe the clearance, brake, turn, and marker lights. Dashboard indicators alone do not confirm the trailer bulbs are working. This check should be done before departure.

Question 26Basic control of combination vehicles

When you are coupling a tractor to a semitrailer, at what point should you back under the trailer until you hear the tractor's fifth wheel click?

  1. After you have locked the trailer brakes
  2. Before the trailer kingpin enters the fifth wheel Correct answer.
  3. After connecting the air lines and electrical cord
  4. After raising the trailer landing gear fully

Explanation

The coupling procedure calls for backing slowly under the trailer until the fifth wheel contacts the kingpin and you hear the click of the locking jaws engaging. This happens while the trailer is still supported by its landing gear, before you raise the gear or hook up lines.

Question 27Basic control of combination vehicles

After coupling, you tug-test the tractor against the trailer with the trailer brakes set. What is this test checking?

  1. That the landing gear has been raised
  2. That the trailer brakes will hold on a grade
  3. That the fifth wheel jaws have locked around the kingpin Correct answer.
  4. That the air lines are free of leaks

Explanation

The tug test pulls the tractor forward with the trailer brakes locked, confirming the fifth wheel jaws have captured the kingpin. If the jaws are not locked, the trailer stays put while the tractor pulls out from under it.

Question 28Basic control of combination vehicles

Before backing a tractor-semitrailer, why should you set the trailer brakes and leave the tractor brakes released?

  1. To keep the trailer from rolling while the tractor pivots under it Correct answer.
  2. To make the tractor steer more sharply in reverse
  3. To prevent the fifth wheel from unlocking
  4. To reduce air pressure needed for the brakes

Explanation

With trailer brakes set, the trailer stays fixed while the tractor moves and pivots during backing. Releasing the tractor brakes lets you maneuver the power unit without dragging the trailer.

Question 29Basic control of combination vehicles

When making a right turn in a combination vehicle, what must you do to keep the trailer's rear wheels from striking the curb?

  1. Turn from the right lane and accelerate through the turn
  2. Brake hard in the middle of the turn to pivot the trailer
  3. Keep the tractor close to the right curb throughout the turn
  4. Swing wide to the left before the turn and watch the trailer in the mirrors Correct answer.

Explanation

Off-tracking causes the trailer's rear wheels to cut inside the tractor's path, so you swing wide before turning and monitor the trailer in the mirrors. This keeps the trailer clear of the curb and any objects on the inside of the turn.

Question 30Basic control of combination vehicles

What is the purpose of the trailer hand valve (trolley valve) in a combination vehicle?

  1. To adjust the slack adjusters
  2. To apply the trailer brakes independently for testing and control Correct answer.
  3. To release the tractor spring brakes
  4. To set the parking brake on the tractor only

Explanation

The trailer hand valve applies only the trailer brakes, which lets you test trailer brake operation and can help control a trailer in some situations. It is not a parking brake and should not be used for routine stopping.

Question 31Basic control of combination vehicles

What does the term off-tracking describe in a combination vehicle?

  1. The trailer's rear wheels following a path inside the tractor's front wheels in a turn Correct answer.
  2. The tractor drifting sideways on a crowned road
  3. The trailer swaying at highway speed
  4. The front wheels wandering due to loose steering

Explanation

Off-tracking is the tendency of the rear wheels of a long combination to cut inside the path of the front wheels during a turn. The longer the combination, the greater the off-tracking.

Question 32Basic control of combination vehicles

When should you shift gears while driving a combination vehicle through a curve?

  1. Just after the curve to maintain momentum
  2. Before entering the curve, so you can hold a steady speed through it Correct answer.
  3. Anywhere in the curve if traffic is light
  4. At the apex of the curve for maximum control

Explanation

Shifting in a curve can cause the drive wheels to lose traction or the vehicle to lurch, so you select the proper gear before entering. Hold a steady speed through the curve with both hands on the wheel.

Question 33Basic control of combination vehicles

What is the correct way to stop a combination vehicle on a downgrade if you must pull over?

  1. Rely on the trailer hand valve alone to hold the vehicle
  2. Turn the wheels toward the curb and leave the brakes released
  3. Set the parking brake firmly and leave the transmission in neutral
  4. Set the parking brake, leave the transmission in a low gear, and chock the wheels if possible Correct answer.

Explanation

On a grade, the parking brake plus a low gear and wheel chocks provide a backup against rolling. Leaving the transmission in gear and chocking the wheels protects you if the brakes release or fail.

Question 34Basic control of combination vehicles

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

  1. Pull the release handle, lower the landing gear, disconnect lines, then pull forward
  2. Disconnect lines, pull the release handle, lower the landing gear, then pull forward
  3. Lower the landing gear, disconnect lines, pull the release handle, then pull the tractor forward Correct answer.
  4. Lower the landing gear, pull the release handle, disconnect lines, then pull forward

Explanation

You first lower the landing gear to support the trailer, then disconnect the air and electrical lines, then release the fifth wheel and pull the tractor forward slowly. Doing the steps in this order keeps the trailer supported and prevents damage to the lines.

Question 35Basic control of combination vehicles

You are backing a tractor-semitrailer into a loading dock on your right. Where should you position the tractor before starting the maneuver?

  1. As close to the dock as possible with the wheels turned right
  2. Angled so the trailer will pivot toward the dock as you back Correct answer.
  3. Parallel to the dock with the trailer ahead of the tractor
  4. Pointed straight ahead with the trailer in line

Explanation

You set up at an angle so that as you back, the tractor pushes the trailer toward the dock in a controlled arc. Starting straight or too close removes the room needed to pivot the trailer.

Question 36Basic control of combination vehicles

What is a jackknife in a combination vehicle?

  1. The trailer's rear wheels lifting off the ground under braking
  2. The fifth wheel unlocking during a turn
  3. The trailer pushing the tractor's drive wheels sideways until the combination folds Correct answer.
  4. The tractor's front wheels losing contact on a bump

Explanation

A jackknife occurs when the trailer pushes the tractor's drive axles sideways, folding the combination at the fifth wheel. It is most likely when the drive wheels lose traction during braking or on slippery surfaces.

Question 37Basic control of combination vehicles

How can you reduce the risk of a rearward amplification (crack-the-whip) effect in a long combination?

  1. Shift to a higher gear in the curve
  2. Brake sharply just before a curve
  3. Drive at a steady speed and avoid abrupt steering inputs Correct answer.
  4. Accelerate hard through curves to keep the trailer tight

Explanation

Rearward amplification makes the last trailer swing wider than the tractor during sudden maneuvers, so smooth steering and steady speed reduce the effect. Abrupt inputs multiply the swing toward the rear.

Question 38Basic control of combination vehicles

What is the proper technique for stopping a combination vehicle that has begun to skid?

  1. Accelerate to pull the trailer straight
  2. Hold the brakes firmly and steer hard in the direction of the skid
  3. Apply the trailer hand valve fully and hold it
  4. Release the brakes to let the wheels roll, then brake again once traction returns Correct answer.

Explanation

Releasing the brakes lets the wheels roll and regain traction, after which you can reapply braking more gently. Holding the brakes or adding power usually worsens the skid.

Question 39Basic control of combination vehicles

Why should you avoid using the trailer hand valve for normal stopping?

  1. It is not connected to the trailer brake system
  2. It disables the tractor brakes permanently
  3. It uses too much air and drains the tanks quickly
  4. It applies only the trailer brakes and can cause the trailer to skid or swing Correct answer.

Explanation

The hand valve applies only the trailer brakes, so using it for routine stops can lock the trailer wheels and cause a skid or jackknife. Normal stopping uses the foot brake, which applies the whole combination.

Question 40Basic control of combination vehicles

When checking the air brake system on a combination vehicle, what should you do before starting the static leak test?

  1. Build full pressure, shut off the engine, and release the parking brake Correct answer.
  2. Drain all air from the tanks and leave the engine running
  3. Set the trailer brakes and leave the engine at high idle
  4. Pump the brake pedal several times to empty the tanks

Explanation

The static leak test begins with full system pressure, the engine off, and the parking brake released so you can observe pressure drop with the brakes applied and released. This lets you detect leaks in the tractor and trailer systems.

Question 41Basic control of combination vehicles

What does the red trailer air supply control (trailer protection valve) do when pushed in?

  1. It charges the trailer air tanks and releases the trailer spring brakes Correct answer.
  2. It dumps all trailer air to the atmosphere
  3. It applies the trailer service brakes only
  4. It locks the fifth wheel jaws

Explanation

Pushing in the red trailer air supply valve charges the trailer system and releases the trailer spring brakes so the trailer can roll. Pulling it out closes the supply and applies the trailer spring brakes.

Question 42Basic control of combination vehicles

You are driving a loaded combination and need to descend a long steep grade. What is the best practice?

  1. Select a low gear before the grade and use engine braking with light, steady brake applications Correct answer.
  2. Use the trailer hand valve to control speed down the grade
  3. Ride the brakes continuously to hold a constant speed
  4. Descend in neutral to save fuel and use the foot brake as needed

Explanation

You choose a low gear before the descent so engine braking helps control speed, and you use short, steady brake applications to avoid overheating. Coasting in neutral or riding the brakes can lead to brake fade and loss of control.

Question 43Basic control of combination vehicles

What is the purpose of the tractor protection valve?

  1. To release the trailer spring brakes automatically
  2. To increase air pressure to the trailer brakes
  3. To close the air supply to the trailer if pressure drops, protecting the tractor's air Correct answer.
  4. To balance air between tractor and trailer tanks

Explanation

The tractor protection valve closes the trailer air supply line when pressure drops too low, preserving the tractor's air for its own brakes. This prevents a trailer leak from draining the whole system.

Question 44Basic control of combination vehicles

When turning left in a combination vehicle, why must you watch the trailer's right side in the mirror?

  1. The trailer brakes may release during the turn
  2. The fifth wheel may unlock in a left turn
  3. The trailer can swing wide and strike objects on the right as you complete the turn Correct answer.
  4. The tractor's front wheels may lift off the ground

Explanation

In a left turn the trailer's rear end swings to the right, so you check the right mirror for pedestrians, poles, or vehicles that could be struck. Off-tracking and tail swing both matter in tight turns.

Question 45Basic control of combination vehicles

What should you do if the trailer begins to sway while you are driving at highway speed?

  1. Accelerate to pull the trailer straight
  2. Brake hard to stop the sway
  3. Ease off the accelerator and steer gently to hold your lane Correct answer.
  4. Turn the wheel sharply to counter the sway

Explanation

Easing off the accelerator lets the combination slow gradually and the sway damp out, while gentle steering keeps you in the lane. Hard braking or sharp steering can worsen the sway and cause a rollover.

Question 46Basic control of combination vehicles

You are backing a long combination with two trailers. What is the key difference from backing a single semitrailer?

  1. The rear trailer responds opposite to the steering input, so you must plan and correct carefully Correct answer.
  2. Backing a double is prohibited in all situations
  3. The rear trailer follows the steering input exactly like a single trailer
  4. You must always back with the trailer hand valve applied

Explanation

With two trailers, the rear unit pivots opposite to the direction you steer, making backing unpredictable and requiring small corrections. Many drivers avoid backing doubles unless absolutely necessary.

Question 47Basic control of combination vehicles

What is the correct response if the trailer spring brakes apply unexpectedly while you are driving because of a loss of air pressure?

  1. Continue driving because the trailer brakes will release on their own
  2. Shift to a higher gear and accelerate through the drag
  3. Apply the hand valve to override the spring brakes
  4. Pull off the road safely and stop, since the trailer brakes will not release until pressure is restored Correct answer.

Explanation

Loss of air pressure causes the trailer spring brakes to apply automatically, and they will not release until system pressure is restored. You should move off the road safely and investigate rather than fight the brakes.

Question 48Basic control of combination vehicles

In a combination vehicle with a low center of gravity and a heavy load, why is it important to slow down before a curve rather than braking in it?

  1. The brakes do not work once the vehicle is turning
  2. Braking in a curve shifts weight and can cause the trailer to roll or skid Correct answer.
  3. Slowing before a curve is only needed for empty trailers
  4. Braking in a curve improves trailer tracking

Explanation

Braking in a curve transfers weight forward and sideways, reducing traction and increasing rollover risk, especially with a high or heavy load. Slowing before the curve lets you hold a steady speed and stable line through it.

Question 49Basic control of combination vehicles

What is the proper procedure for checking that the trailer brakes hold after coupling?

  1. Apply the hand valve and release the tractor parking brake
  2. Release all brakes and roll forward to feel the trailer drag
  3. Set the trailer brakes, release the tractor brakes, and try to pull forward slowly Correct answer.
  4. Set the tractor brakes and accelerate against them

Explanation

With the trailer brakes set and the tractor brakes released, a slow pull forward should be resisted if the trailer brakes hold. This verifies the trailer brake system is working before you drive away.

Question 50Basic control of combination vehicles

Why might a combination vehicle require a longer stopping distance than a single-unit truck of similar weight?

  1. Combination vehicles cannot use engine braking
  2. The trailer brakes are always weaker than the tractor brakes
  3. The trailer's brakes may not be properly balanced with the tractor's, reducing the combination's braking effectiveness Correct answer.
  4. The fifth wheel adds friction that delays braking

Explanation

If the trailer brakes provide insufficient braking relative to the tractor brakes, the trailer can push the tractor and lengthen the stop. Similar vehicle weight does not imply extra mass in the combination. Proper brake adjustment and smooth application are essential.