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Pennsylvania CDL Combination study materials
Pennsylvania CDL Combination study resources

Pennsylvania CDL Combination practice questions

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Question 1Combination Vehicles

Before backing a tractor-trailer, the safest first step is to:

  1. Sound the horn twice and back immediately
  2. Rely on the convex mirrors only
  3. Have a passenger watch from inside the cab
  4. Get out and check behind the trailer for people and obstacles Correct answer.

Explanation

A driver should walk around and inspect the area behind and around the trailer before backing, because large blind spots make mirrors unreliable near the rear. Getting out and checking removes the guesswork and prevents crushing pedestrians or hitting objects.

Question 2Combination Vehicles

The air line that supplies air to the trailer's emergency (spring) brakes is the:

  1. Service line
  2. Glad hand seal
  3. Supply line Correct answer.
  4. Electrical cord

Explanation

The supply line (emergency line) charges the trailer air tanks and controls the spring brakes; if it loses pressure the trailer brakes apply automatically. The service line carries the signal that applies the brakes during normal stopping.

Question 3Combination Vehicles

When coupling a tractor to a trailer, you should check that the fifth wheel is:

  1. Locked in the closed position
  2. Greased heavily on the jaws
  3. Raised above the kingpin height
  4. Tilted down toward the rear Correct answer.

Explanation

Before backing under the trailer, the fifth wheel should be tilted down toward the rear so the kingpin can slide under and trip the locking jaws. A fifth wheel that is not tilted can be struck by the kingpin and prevent proper coupling.

Question 4Combination Vehicles

The trailer hand valve (trolley valve) should be used:

  1. Only to test the trailer brakes Correct answer.
  2. For all normal stops
  3. To hold the unit on a hill
  4. To release the spring brakes

Explanation

The trailer hand valve should be used only to test that the trailer brakes work, never for normal stopping or parking. Using it while driving can cause a skid, and it does not supply air to release spring brakes.

Question 5Combination Vehicles

A combination vehicle with a heavy load has a longer stopping distance mainly because:

  1. The trailer pushes the tractor straight
  2. The brakes are smaller than on a car
  3. Air pressure builds slowly
  4. Total weight increases and weight shifts forward Correct answer.

Explanation

A loaded combination weighs far more than a car, and during braking the weight shifts forward onto the steering axle while the trailer pushes against the tractor. Both effects increase the distance needed to stop.

Question 6Combination Vehicles

The main purpose of an anti-lock braking system (ABS) on a combination vehicle is to:

  1. Prevent wheel lockup so you can steer while braking Correct answer.
  2. Replace the need for spring brakes
  3. Automatically apply the trailer brakes
  4. Shorten stopping distance on all surfaces

Explanation

ABS prevents wheels from locking during hard braking, which lets the driver keep steering control. It does not guarantee a shorter stop on every surface and does not replace other brake systems.

Question 7Combination Vehicles

When uncoupling, you should first:

  1. Open the trailer doors
  2. Lower the landing gear and chock the trailer wheels Correct answer.
  3. Pull the release handle and drive away
  4. Disconnect the electrical cord

Explanation

Before uncoupling, lower the landing gear so the trailer is supported and chock the wheels so it cannot roll. Only then should you release the fifth wheel and separate the air lines.

Question 8Combination Vehicles

The tractor protection valve is designed to:

  1. Keep air in the tractor if the trailer breaks away Correct answer.
  2. Release the tractor spring brakes
  3. Increase trailer air pressure
  4. Balance air between the tires

Explanation

The tractor protection valve closes when trailer air pressure drops, keeping enough air in the tractor to stop it if the trailer separates. It protects the tractor's air supply rather than increasing pressure.

Question 9Combination Vehicles

You are backing a trailer and want it to turn to the left. You should turn the steering wheel:

  1. To the right Correct answer.
  2. To the left
  3. Straight and let it drift
  4. Rapidly back and forth

Explanation

When backing, turning the wheel to the right makes the rear of the trailer move left. Steering opposite the desired trailer direction is the standard technique for backing a combination vehicle.

Question 10Combination Vehicles

During a pre-trip inspection of a combination vehicle, the trailer air supply should be checked by:

  1. Testing the turn signals
  2. Listening for leaks at the exhaust
  3. Charging the system and confirming the trailer brakes release and apply Correct answer.
  4. Checking tire tread depth

Explanation

A trailer air supply check involves charging the system to confirm the spring brakes release and then applying the brakes to confirm they set. This verifies the supply and service lines are working together.

Question 11Combination Vehicles

If the trailer starts to swing out (trailer sway) at highway speed, the safest response is to:

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

Explanation

Easing off the accelerator and making small, smooth steering corrections lets the combination settle. Hard braking or the trailer hand valve can worsen a sway and cause a jackknife.

Question 12Combination Vehicles

A driver must make a right turn in a combination vehicle. To avoid hitting the curb with the trailer, the driver should:

  1. Swing wide to the left before turning right Correct answer.
  2. Use the trailer hand valve to pivot
  3. Turn from the far left lane
  4. Turn sharply from the right lane only

Explanation

Trailers track inside the path of the tractor, so a right turn requires swinging wide to the left first to keep the rear wheels off the curb. The turn should still be completed from the proper lane without cutting the corner.

Question 13Combination Vehicles

Before descending a long steep grade with a loaded trailer, the correct practice is to:

  1. Use the foot brake continuously the whole way down
  2. Rely on the trailer hand valve
  3. Coast in neutral to save fuel
  4. Shift to a lower gear before the descent begins Correct answer.

Explanation

Selecting a lower gear before the grade lets engine braking control speed without overheating the brakes. Coasting or continuous braking can cause brake fade and loss of control.

Question 14Combination Vehicles

After coupling, you should do a pull test by:

  1. Releasing the fifth wheel
  2. Driving at highway speed
  3. Gently pulling forward with the trailer brakes set Correct answer.
  4. Backing under the trailer again

Explanation

A pull test with the trailer brakes set confirms the kingpin is locked in the fifth wheel; if the trailer holds the tractor, coupling is secure. Driving off without this check risks a trailer separation.

Question 15Combination Vehicles

The air pressure in a combination vehicle should build to a normal operating range before driving because:

  1. The horn requires high pressure
  2. The trailer lights run on air
  3. Spring brakes need air to release and brakes need pressure to work Correct answer.
  4. The tires need air pressure

Explanation

Air pressure releases the spring brakes and provides the force to apply the service brakes. Driving before pressure builds can leave the vehicle with weak or applied brakes.

Question 16Combination Vehicles

When driving a combination vehicle, you should check your mirrors frequently because:

  1. Mirrors keep the cab cool
  2. The trailer may drift out of its lane or develop a problem Correct answer.
  3. The trailer brakes need mirror checks
  4. It is required only at night

Explanation

Frequent mirror checks let you catch trailer drift, sway, or a shifting load before it becomes a serious problem. The trailer follows the tractor, so monitoring it is essential.

Question 17Combination Vehicles

If the trailer breaks away from the tractor, the trailer's emergency brakes will apply because:

  1. The electrical cord pulls a switch
  2. The ABS engages automatically
  3. The supply line separates and releases spring brake air Correct answer.
  4. The driver presses the hand valve

Explanation

When the supply line separates, air pressure is lost and the trailer's spring brakes apply automatically. This is the emergency braking feature built into the air system.

Question 18Combination Vehicles

A driver notices the trailer brakes are not releasing after coupling. The most likely cause is:

  1. The supply line is not connected or the trailer air tank is empty Correct answer.
  2. The landing gear is down
  3. The mirrors are misaligned
  4. The turn signals are on

Explanation

Spring brakes stay applied until they receive air through the supply line. If the line is disconnected or the tank is empty, the trailer brakes will not release.

Question 19Combination Vehicles

When making a lane change in a combination vehicle, you should:

  1. Signal, check mirrors, and confirm the trailer has cleared the lane Correct answer.
  2. Use the trailer hand valve to slow down
  3. Change lanes quickly to avoid traffic
  4. Rely on the convex mirror only

Explanation

A lane change requires signaling, checking mirrors, and confirming the full length of the trailer has cleared before moving over. The trailer is longer than the tractor and needs extra clearance.

Question 20Combination Vehicles

The most important reason to chock the trailer wheels during uncoupling is to:

  1. Help the landing gear extend
  2. Keep the tires from wearing
  3. Reduce air pressure
  4. Prevent the trailer from rolling when the tractor is removed Correct answer.

Explanation

Once the tractor is removed, nothing holds the trailer in place; chocks prevent it from rolling. This is a critical safety step during uncoupling.

Question 21Combination Vehicles

A driver is descending a grade and the brakes begin to fade. The best immediate action is to:

  1. Accelerate to cool the brakes
  2. Apply the trailer hand valve
  3. Downshift and use engine braking while slowing gradually Correct answer.
  4. Pump the foot brake rapidly

Explanation

Brake fade means the brakes are overheating and losing effectiveness. Downshifting to use engine braking and slowing gradually reduces reliance on the brakes before they fail completely.

Question 22Combination Vehicles

A driver must back a trailer into a loading dock on the right. The correct technique is to:

  1. Back from the right, turning the wheel left
  2. Back straight in from the center of the lot
  3. Back from the left, turning the wheel right to move the trailer rear right Correct answer.
  4. Use the hand valve to steer the trailer

Explanation

Backing from the left gives the driver a better view and more room, and turning the wheel right moves the trailer rear to the right toward the dock. This is the standard sight-side backing technique.

Question 23Combination Vehicles

A driver is checking the air system and notes that pressure drops faster than normal when the engine is off. The most likely problem is:

  1. Low tire tread
  2. A leak in the air system Correct answer.
  3. A weak battery
  4. A loose mirror

Explanation

A rapid pressure drop with the engine off indicates an air leak somewhere in the system, such as a line or fitting. Leaks must be repaired before driving because they can reduce braking ability.

Question 24Combination Vehicles

When turning a combination vehicle at an intersection, the driver should account for off-tracking, which means:

  1. The tractor pulls ahead of the trailer
  2. The trailer wheels follow a shorter path than the tractor wheels Correct answer.
  3. The brakes apply unevenly
  4. The mirrors show a false image

Explanation

Off-tracking is the tendency of the trailer's rear wheels to cut inside the path of the tractor's front wheels. Drivers must swing wide enough to keep the trailer off curbs and out of other lanes.

Question 25Combination Vehicles

A driver is preparing to couple at night and cannot see the kingpin clearly. The safest action is to:

  1. Use a flashlight or work light to inspect the coupling area Correct answer.
  2. Back up slowly by feel
  3. Rely on the backup alarm
  4. Assume the fifth wheel is aligned

Explanation

A flashlight or work light lets the driver confirm the kingpin and fifth wheel are aligned before backing. Coupling by feel or assumption risks a missed connection and a later separation.

Question 26Combination Vehicles

In a combination vehicle, the fifth wheel plate is mounted on the tractor and serves to:

  1. Control the air pressure supplied to the trailer brakes
  2. Lock onto the trailer kingpin and carry the trailer's weight Correct answer.
  3. Steer the trailer's rear axles during a turn
  4. Provide the electrical connection between the tractor and the trailer

Explanation

The fifth wheel is the heavy coupling plate on the tractor frame. Its jaws close around the trailer kingpin, and the plate supports the weight of the trailer's front end while allowing the combination to pivot. Electrical and air connections are made by separate lines and couplers.

Question 27Combination Vehicles

The trailer air supply control (red knob) in the cab is pushed in. This means:

  1. The tractor parking brake is set and the service brakes are disabled
  2. The trailer air tanks are being charged and the trailer spring brakes are released Correct answer.
  3. The trailer brakes are locked on until the knob is pulled out
  4. The trailer emergency brakes are applied and the supply line is closed

Explanation

Pushing the red trailer air supply knob in opens the supply line so the tractor can charge the trailer tanks and release the trailer spring brakes. Pulling the knob out closes the line, which exhausts the trailer supply line and applies the trailer emergency brakes.

Question 28Combination Vehicles

You are inspecting the coupling after backing under a trailer. The gap between the trailer apron and the fifth wheel is too small to see daylight, and the release handle is in. What should you do before raising the landing gear?

  1. Pull the tractor forward slightly and recheck the gap Correct answer.
  2. Raise the landing gear first, then check the coupling
  3. Release the trailer spring brakes and listen for a leak
  4. Get under the trailer and inspect the jaws by hand

Explanation

A proper coupling leaves a small gap between the trailer apron and the fifth wheel, and the jaws must be closed around the kingpin. If the gap is wrong or the handle position is uncertain, pull forward gently and recheck rather than trusting the coupling. Never put your body under a trailer to inspect the jaws.

Question 29Combination Vehicles

When you lower the landing gear after coupling, the correct practice is to:

  1. Crank the legs down until they touch the ground, then crank two extra turns
  2. Raise the legs completely so the trailer weight rests on the fifth wheel Correct answer.
  3. Leave the legs partly down so the trailer cannot tip forward
  4. Crank the legs fully down and leave them supporting part of the trailer weight

Explanation

After the coupling is confirmed and the air lines are connected, the landing gear must be raised all the way so the full trailer weight rides on the fifth wheel and trailer axles. Leaving the legs down can damage them and interfere with steering and suspension travel.

Question 30Combination Vehicles

The trailer hand valve (trolley valve) should be used for which purpose?

  1. Holding the vehicle at a stoplight on a hill
  2. Applying the trailer brakes during normal driving to slow down
  3. Parking the combination on a grade
  4. Testing the trailer brakes during a pre-trip inspection Correct answer.

Explanation

The hand valve is useful for checking that the trailer brakes apply and release during inspection. It should not be used to park or to hold the vehicle, and using it while driving can cause the trailer to swing. Normal braking is done with the foot brake, which applies both tractor and trailer brakes.

Question 31Combination Vehicles

You are about to back a combination vehicle under a low trailer, and the tractor's exhaust stack is tall. What is the main concern?

  1. The stack will prevent the fifth wheel jaws from closing
  2. The stack will reduce air pressure to the trailer brakes
  3. The stack may contact the trailer nose and damage it or the tractor Correct answer.
  4. The stack may block the view through the right mirror

Explanation

On tractors with tall exhaust stacks, backing under a low trailer can drive the stack into the trailer nose or the underside of the body. Check clearance before backing and stop if the stack will contact the trailer. The other items are not effects of a tall stack.

Question 32Combination Vehicles

When you uncouple a trailer, the correct order of steps begins with:

  1. Pulling the fifth wheel release handle
  2. Disconnecting the air lines and electrical cord
  3. Pulling the tractor slowly forward
  4. Lowering the landing gear to the ground Correct answer.

Explanation

Uncoupling starts by lowering the landing gear until it firmly contacts the ground and supports the trailer. Only then are the air lines and electrical cord disconnected, the release handle pulled, and the tractor driven out from under the trailer.

Question 33Combination Vehicles

The gladhands on a combination vehicle connect the trailer to the tractor's:

  1. Service and emergency air lines Correct answer.
  2. Electrical system and the trailer lights
  3. Hydraulic brake lines and the parking brake
  4. Fuel supply and the trailer heater

Explanation

The two gladhands couple the service line (blue) and the emergency or supply line (red) between tractor and trailer. They carry air for braking, not electricity or fuel. The electrical cord is a separate connection.

Question 34Combination Vehicles

You are checking the air lines before a trip. Which condition indicates the emergency (supply) line is properly connected?

  1. The gladhand seals are missing and air escapes
  2. The couplers are locked and the line does not leak when charged Correct answer.
  3. The line is taped to the service line to keep it out of the way
  4. The line hangs loosely with no coupler seal

Explanation

Properly connected gladhands are locked together with good seals and no audible leak when the system is charged. Missing seals, loose couplers, or taped lines indicate a defect that must be corrected before driving.

Question 35Combination Vehicles

During a pre-trip inspection you apply the trailer hand valve and the trailer brakes do not apply. What is the most likely explanation?

  1. The electrical cord is unplugged
  2. The trailer air tanks are not charged or the service line is blocked Correct answer.
  3. The trailer spring brakes are released
  4. The tractor parking brake is set

Explanation

The hand valve sends air through the service line to apply the trailer brakes, so it needs charged trailer tanks and an open service line. If the trailer brakes do not respond, check that the trailer is charged and that the service gladhand is connected and not blocked. Electrical and parking brake issues do not explain this symptom.

Question 36Combination Vehicles

A combination vehicle has a trailer with a single set of rear axles. Compared with a tractor-semitrailer with tandem trailer axles, the driver should expect:

  1. Less off-tracking because the trailer is shorter Correct answer.
  2. No off-tracking because the trailer axles steer
  3. The same off-tracking regardless of trailer length
  4. More off-tracking because the trailer is longer

Explanation

Off-tracking is the tendency of the rear wheels to follow a path inside the front wheels during a turn. A shorter trailer with fewer rear axles cuts the corner less than a longer trailer, so it off-tracks less. Trailer axles do not steer.

Question 37Combination Vehicles

When driving a combination vehicle, the safest following distance compared with a single vehicle should be:

  1. Shorter, because the combination is heavier
  2. Longer, because of the greater weight and stopping distance Correct answer.
  3. Determined only by the speed limit sign
  4. The same, because the brakes are identical

Explanation

A loaded combination vehicle is heavier and needs more distance to stop, so following distance should be increased. The one-second-per-ten-feet rule and additional space for the trailer's length help ensure a safe gap.

Question 38Combination Vehicles

You are driving a combination vehicle and the trailer begins to skid on a wet road. The best response is to:

  1. Turn the steering wheel sharply away from the skid
  2. Release the brakes to let the wheels roll and steer in the direction you want to go Correct answer.
  3. Apply more brake pressure to stop the skid
  4. Accelerate to pull the trailer straight

Explanation

When wheels lock and skid, releasing the brakes lets them roll again so steering control returns. Then steer where you want to go. Hard braking, sharp steering, or accelerating can make the skid worse.

Question 39Combination Vehicles

A combination vehicle's air compressor is not building pressure normally. Before driving, the driver should:

  1. Not move the vehicle until the low-pressure warning and buildup are corrected Correct answer.
  2. Drive slowly and check the gauge later
  3. Release the trailer spring brakes manually and proceed
  4. Pump the brakes to build pressure

Explanation

The air system must build to normal operating pressure and the low-pressure warning must work before the vehicle is moved. A compressor or leak problem means the brakes may not function, so the defect must be corrected first.

Question 40Combination Vehicles

During a pre-trip inspection, the low air pressure warning must activate before the pressure drops below:

  1. 100 psi
  2. 45 psi
  3. 20 psi
  4. 60 psi Correct answer.

Explanation

The low air pressure warning device must come on before the pressure falls below 60 psi. This gives the driver warning while enough pressure remains to stop safely. A warning that does not activate is a defect.

Question 41Combination Vehicles

A driver must make a sharp right turn in a combination vehicle and the trailer's rear wheels are cutting the corner. The driver should:

  1. Signal and change lanes to the left instead
  2. Turn tightly to the right and accelerate through the turn
  3. Brake hard in the middle of the turn
  4. Swing wide to the left before the turn and watch the trailer in the right mirror Correct answer.

Explanation

Off-tracking makes the trailer's rear wheels follow inside the tractor's path, so the driver swings wide before the turn and watches the trailer in the right mirror. Turning tightly or accelerating can put the trailer over the curb or into objects.

Question 42Combination Vehicles

A combination vehicle's trailer has a broken spring brake. The driver should:

  1. Continue the trip and repair it later
  2. Place the trailer out of service until the brake is repaired Correct answer.
  3. Use only the tractor brakes for the trip
  4. Disconnect the trailer and tow it with a pickup

Explanation

A broken spring brake means the trailer's emergency and parking braking is compromised, so the trailer must be placed out of service until repaired. Continuing the trip or relying on tractor brakes alone is unsafe.

Question 43Combination Vehicles

When coupling, after the tractor is backed under the trailer and the fifth wheel has locked, what should be done before connecting the air lines?

  1. Raise the landing gear fully
  2. Connect the electrical cord
  3. Set the trailer parking brakes and check the coupling Correct answer.
  4. Pull the tractor forward to test the lock

Explanation

After the jaws lock, set the trailer brakes and visually check that the coupling is secure, including the gap and the release handle. Air lines are connected after the coupling is confirmed, and the pull test checks the lock before driving.

Question 44Combination Vehicles

The trailer's spring brakes apply automatically when:

  1. The driver presses the foot brake
  2. The trailer air supply pressure drops below a safe level Correct answer.
  3. The trailer is loaded to capacity
  4. The tractor engine is started

Explanation

Spring brakes are held off by air pressure. If supply pressure drops, as in a breakaway or a pulled red knob, the springs apply the brakes automatically. Normal foot braking uses service brakes, not the spring brakes.

Question 45Combination Vehicles

A driver is inspecting a combination vehicle and finds the trailer's slack adjusters move more than about one inch when pulled by hand. This indicates:

  1. A properly functioning automatic slack adjuster
  2. That the trailer brakes are fully released
  3. Excessive pushrod travel that requires adjustment or repair Correct answer.
  4. Normal operation that needs no attention

Explanation

Excessive pushrod travel means the brake chamber has to move too far before the brake applies, reducing braking force. The slack adjuster needs adjustment or repair before the vehicle is driven. Normal travel is much shorter.

Question 46Combination Vehicles

You are driving a combination vehicle down a long grade and want to control speed without overheating the brakes. The best technique is to:

  1. Ride the brakes continuously
  2. Coast in neutral to save fuel
  3. Use a low gear and light, steady braking Correct answer.
  4. Use the trailer hand valve to hold speed

Explanation

Selecting a low gear before the grade and using light, steady brake applications keeps speed controlled without overheating the brakes. Continuous braking, coasting in neutral, or using the hand valve can cause fade or loss of control.

Question 47Combination Vehicles

A combination vehicle is equipped with ABS. When the ABS warning light stays on after the vehicle starts moving, the driver should:

  1. Apply the trailer hand valve to test it
  2. Ignore it because ABS is optional
  3. Disconnect the ABS fuse to reset it
  4. Have the system checked, since ABS may not function Correct answer.

Explanation

A warning light that stays on indicates an ABS fault, and the system may not operate during a hard stop. The vehicle should be checked by a qualified technician. Disabling the system or ignoring the light is unsafe.

Question 48Combination Vehicles

A driver is preparing to couple at a loading dock and the trailer is not chocked. The driver should:

  1. Back under the trailer quickly to prevent rolling
  2. Ask someone to stand behind the trailer
  3. Leave the trailer free to roll for alignment
  4. Chock the trailer wheels or confirm the trailer brakes are set Correct answer.

Explanation

Before coupling or uncoupling, the trailer must be prevented from rolling by setting its brakes or chocking the wheels. Backing quickly or leaving it free to roll can cause the trailer to move and injure someone.

Question 49Combination Vehicles

When making a lane change in a combination vehicle, the driver should:

  1. Signal, check mirrors, and allow extra time for the longer vehicle to clear Correct answer.
  2. Change lanes quickly to minimize exposure
  3. Assume other drivers will yield because of the vehicle size
  4. Rely only on the right mirror

Explanation

A combination vehicle is longer and accelerates more slowly, so the driver signals early, checks both mirrors carefully, and allows extra time and space to complete the lane change. Quick moves or reliance on other drivers increase risk.

Question 50Combination Vehicles

A driver has coupled to a trailer and completed the pull test. Before driving away, the driver should also:

  1. Release the trailer spring brakes by pulling the red knob
  2. Lower the landing gear to the ground
  3. Remove the gladhand seals to prevent pressure buildup
  4. Check that the trailer lights and air lines are connected and working Correct answer.

Explanation

After coupling and the pull test, the driver verifies that the electrical cord and air lines are connected and that the trailer lights and brakes work. The landing gear must be raised, and the red knob must be pushed in, not pulled out.