I85PREP
⌘ K
Nebraska CDL Combination Vehicles study materials
Nebraska CDL Combination Vehicles study resources

Nebraska CDL Combination Vehicles practice questions

One question at a time. Build your Nebraska CDL Combination Vehicles understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Combination vehicles

When inspecting a combination vehicle before a trip, what is the correct order for checking the coupling system?

  1. Check the kingpin and jaws are locked, then check the safety latch, then check the air lines and electrical cord Correct answer.
  2. Check the trailer brakes are released, then check the jaws, then check the kingpin
  3. Check the fifth wheel is greased, then connect the air lines, then check the jaws
  4. Check the fifth wheel jaws are open, then back under the trailer, then check the kingpin

Explanation

The pre-trip inspection of a tractor-trailer follows a set order: confirm the kingpin is seated in the closed fifth wheel jaws, verify the locking lever or safety latch is engaged, then connect and check the air lines and electrical cord. Doing the coupling check first ensures the trailer will not separate before you pressurize the system.

Question 2Combination vehicles

While driving a combination vehicle, you must make a quick stop on a wet road. What is the most important reason to brake gently and progressively?

  1. Gentle braking prevents the fifth wheel from unlocking
  2. Gentle braking keeps the trailer air supply from being cut off
  3. Hard braking causes the tractor protection valve to close
  4. Hard braking can cause the trailer wheels to lock and the trailer to swing out of line Correct answer.

Explanation

In a combination vehicle the trailer has no steering control of its own, so if its wheels lock during hard braking the trailer can jackknife or swing sideways. Progressive braking keeps the trailer tracking behind the tractor, especially on wet or slippery pavement.

Question 3Combination vehicles

What is the purpose of the tractor protection valve on a combination vehicle?

  1. It regulates the trailer's spring brakes during normal driving
  2. It keeps air in the tractor if the trailer breaks away or the air lines separate Correct answer.
  3. It limits the top speed of the combination
  4. It controls the trailer's electrical turn signals

Explanation

The tractor protection valve is designed to close off the tractor's air supply if the trailer breaks away or an air line separates. This preserves enough air pressure in the tractor to allow the driver to stop safely.

Question 4Combination vehicles

You are backing a tractor-trailer into a loading dock on the right side. Which technique gives you the best control?

  1. Pull past the dock, then back with the trailer angled so it swings toward the dock as you steer Correct answer.
  2. Keep the steering wheel straight and let the trailer follow the tractor
  3. Back straight in from a long distance to avoid turning
  4. Turn the steering wheel toward the left while backing to swing the trailer right

Explanation

Backing a combination vehicle requires setting up the angle before you reverse. Pulling past the dock and angling the trailer lets you steer the trailer toward the dock while keeping the tractor clear of obstacles.

Question 5Combination vehicles

When uncoupling a tractor from a trailer, what should you do first?

  1. Release the trailer spring brakes
  2. Pull the fifth wheel release handle
  3. Disconnect the air lines and electrical cord
  4. Lower the landing gear Correct answer.

Explanation

The landing gear must be lowered and secured before the trailer is released so it will be supported when the tractor pulls away. Only after the gear is down should you disconnect the lines and release the fifth wheel.

Question 6Combination vehicles

During a pre-trip inspection, how much air pressure loss is allowed in a combination vehicle with the engine off and the brakes released?

  1. No more than 2 psi in one minute
  2. No more than 3 psi in one minute Correct answer.
  3. No more than 10 psi in one minute
  4. No more than 5 psi in one minute

Explanation

For a combination vehicle, the air brake leak test allows a loss of no more than 3 psi in one minute with the engine off and the brakes released. A larger drop indicates a leak that must be repaired before driving.

Question 7Combination vehicles

What does the trailer hand valve (trolley valve) do, and when should it be used?

  1. It controls the trailer air supply during normal driving
  2. It applies the tractor brakes only and is used for parking
  3. It releases the trailer spring brakes during a pre-trip inspection
  4. It applies the trailer brakes only and is used for testing the trailer brakes Correct answer.

Explanation

The trailer hand valve applies only the trailer brakes and is intended for testing the trailer brakes, not for normal stopping. Using it while driving can cause the trailer to swing out of line.

Question 8Combination vehicles

A combination vehicle is more likely to roll over in a curve than a single vehicle because:

  1. The center of gravity is higher and the vehicle is longer Correct answer.
  2. The trailer tires carry less weight
  3. The trailer brakes apply more slowly than the tractor brakes
  4. The tractor has a shorter wheelbase

Explanation

Combination vehicles have a higher center of gravity and greater length, which raises the rollover risk in curves. Drivers must slow down before entering a curve and avoid sudden steering.

Question 9Combination vehicles

You are driving a loaded tractor-trailer down a long steep downgrade. What is the safest braking technique?

  1. Use the trailer hand valve to control speed
  2. Apply the service brakes hard and continuously to hold speed
  3. Select a lower gear and use the engine brake or light steady braking to control speed Correct answer.
  4. Shift to neutral and rely on the service brakes

Explanation

On a long downgrade, selecting a lower gear and using engine braking or light steady brake applications keeps the brakes from overheating. Continuous hard braking can cause brake fade and loss of control.

Question 10Combination vehicles

What is the proper way to check that the trailer spring brakes apply automatically when air pressure is lost?

  1. Pull the trailer air supply valve to close it and watch for the trailer brakes to set Correct answer.
  2. Shut off the engine and listen for the low-air warning buzzer
  3. Press the foot brake and check the trailer wheels for drag
  4. Release the parking brake and roll forward slowly

Explanation

Closing the trailer air supply valve simulates a loss of air to the trailer, which should cause the trailer spring brakes to apply automatically. This confirms the emergency braking system works.

Question 11Combination vehicles

When a combination vehicle starts to skid on a slippery road, what should the driver do?

  1. Release the steering wheel and let the vehicle straighten
  2. Accelerate to pull the trailer straight
  3. Apply the brakes hard and steer against the skid
  4. Stop braking, steer in the direction you want to go, and avoid sudden movements Correct answer.

Explanation

The correct response to a skid is to stop braking and steer in the direction you want the vehicle to go, while avoiding sudden movements. This helps the wheels regain traction and keeps the trailer behind the tractor.

Question 12Combination vehicles

Which coupling component is the strongest connection point between the tractor and trailer?

  1. The electrical cord
  2. The kingpin locked in the fifth wheel jaws Correct answer.
  3. The trailer air lines
  4. The safety latch on the fifth wheel

Explanation

The kingpin and the closed fifth wheel jaws form the primary structural connection that transfers the trailer's weight and pulling force to the tractor. All other connections are secondary systems.

Question 13Combination vehicles

What is the recommended following distance for a combination vehicle traveling at highway speed in good conditions?

  1. At least one second per ten feet of vehicle length Correct answer.
  2. At least one second per 20 feet of vehicle length
  3. At least one second total
  4. At least three car lengths

Explanation

A combination vehicle should follow at least one second for every ten feet of its length, plus an extra second for speeds over 40 mph. This gives the driver enough time to react and stop the longer, heavier vehicle.

Question 14Combination vehicles

You are driving a combination vehicle and need to make a right turn at an intersection with a tight corner. What is the correct technique?

  1. Swing wide to the left before turning to keep the trailer off the curb Correct answer.
  2. Turn sharply and accelerate through the corner
  3. Turn from the left lane to give the trailer room
  4. Use the trailer hand valve to slow the trailer during the turn

Explanation

Because the trailer cuts the corner, the driver must swing wide to the left before turning right so the trailer clears the curb and any objects. Turning too tightly can cause the trailer to ride over the curb or strike pedestrians.

Question 15Combination vehicles

What is off-tracking, and how does it affect a combination vehicle in a turn?

  1. The tractor's rear wheels lift off the ground in a sharp turn
  2. The trailer's wheels follow a wider path than the tractor's wheels, so the trailer swings wide
  3. The trailer's brakes apply later than the tractor's brakes
  4. The trailer's wheels follow a shorter path than the tractor's wheels, so the trailer cuts the corner Correct answer.

Explanation

Off-tracking means the rear wheels of the trailer follow a shorter path than the front wheels of the tractor, so the trailer cuts inside the corner. Drivers must allow extra room on the inside of a turn to keep the trailer clear of curbs and objects.

Question 16Combination vehicles

Before backing a tractor-trailer, what should the driver do if visibility behind the trailer is limited?

  1. Get out and check the area, or use a helper to guide the backing Correct answer.
  2. Back quickly to reduce the time spent in reverse
  3. Sound the horn continuously while backing
  4. Rely on the mirrors only and back slowly

Explanation

When visibility behind the trailer is limited, the driver should get out and check the area or use a helper to guide the backing. This prevents striking objects or pedestrians that cannot be seen in the mirrors.

Question 17Combination vehicles

What is the purpose of the relay valve in a trailer air brake system?

  1. It prevents the trailer from being coupled to the wrong tractor
  2. It converts electrical signals into air pressure
  3. It speeds up the application and release of the trailer brakes by using air from a nearby reservoir Correct answer.
  4. It releases the trailer spring brakes during normal operation

Explanation

The relay valve uses air from a reservoir near the trailer brakes to apply and release them quickly, reducing the lag that would occur if air had to travel all the way from the tractor. This shortens stopping distance.

Question 18Combination vehicles

When connecting the air lines between the tractor and trailer, which color line is the emergency (supply) line?

  1. Green
  2. Blue
  3. Red Correct answer.
  4. Yellow

Explanation

The red line is the emergency or supply line that charges the trailer air tanks. The blue line is the service line that carries the brake application signal.

Question 19Combination vehicles

You are coupling to a trailer and notice the fifth wheel jaws will not close around the kingpin. What is the most likely cause?

  1. The trailer air lines are connected
  2. The landing gear is too high
  3. The kingpin is too far forward or the tractor is not backed in far enough Correct answer.
  4. The trailer spring brakes are released

Explanation

If the jaws will not close, the kingpin is usually not positioned deep enough in the fifth wheel, meaning the tractor was not backed in far enough or is misaligned. The driver should pull forward, realign, and back in again.

Question 20Combination vehicles

What should you check to confirm the trailer is properly supported before pulling the tractor out from under it during uncoupling?

  1. That the landing gear is down and the ground can support it Correct answer.
  2. That the fifth wheel jaws are still locked
  3. That the trailer spring brakes are released
  4. That the trailer air lines are still connected

Explanation

Before pulling out, confirm the landing gear is fully down and resting on solid ground that can support the trailer's weight. Otherwise the trailer can drop and be damaged or cause injury.

Question 21Combination vehicles

How does a combination vehicle's stopping distance compare with that of a single vehicle of similar weight?

  1. It is about the same because the total weight is similar
  2. It is unpredictable and cannot be estimated
  3. It is longer because of the greater weight and the time needed for trailer brakes to apply Correct answer.
  4. It is shorter because the trailer brakes help stop the load

Explanation

A combination vehicle takes longer to stop because of its greater weight and the lag before the trailer brakes fully apply. Drivers must allow more following distance and plan stops earlier.

Question 22Combination vehicles

What is the correct action if the low-air warning buzzer and light come on while driving a combination vehicle?

  1. Shift to a lower gear and accelerate to build air pressure
  2. Release the trailer air supply valve to reset the system
  3. Move to a safe place and stop as soon as possible Correct answer.
  4. Continue driving until you reach a service station

Explanation

The low-air warning signals that air pressure is dropping toward the level at which spring brakes will apply. The driver should move to a safe location and stop as soon as possible to avoid a loss of braking control.

Question 23Combination vehicles

When should the trailer air supply valve (red button) be pushed in during a pre-trip inspection?

  1. After the air pressure has built up to normal operating level Correct answer.
  2. Before starting the engine
  3. Before connecting the air lines
  4. While the trailer spring brakes are still applied

Explanation

The trailer air supply valve should be pushed in only after the system has built up to normal pressure, which charges the trailer tanks and releases the spring brakes. Pushing it in too early can leave the trailer without enough air.

Question 24Combination vehicles

Why must a driver check that the trailer's spring brakes are applied before backing under a trailer to couple?

  1. To raise the landing gear automatically
  2. To allow the air lines to be connected more quickly
  3. To make the fifth wheel jaws easier to close
  4. To keep the trailer from rolling while the tractor backs under it Correct answer.

Explanation

With the trailer spring brakes applied, the trailer stays put while the tractor backs under it. A trailer that rolls during coupling can cause the kingpin to miss the fifth wheel and create a serious hazard.

Question 25Combination vehicles

A driver is inspecting a combination vehicle and finds the fifth wheel mounting bolts are loose. What is the correct response?

  1. Note it on the inspection report and drive normally
  2. Do not drive the vehicle until the mounting is repaired Correct answer.
  3. Tighten them later at the next scheduled maintenance
  4. Drive only at low speeds until the end of the shift

Explanation

Loose fifth wheel mounting bolts compromise the structural connection between the tractor and trailer and can lead to separation. The vehicle must be taken out of service until the mounting is repaired.

Question 26Combination vehicles

In a combination vehicle, what is the primary purpose of the glad hands on the trailer air lines?

  1. To lock the trailer's spring brakes when the supply line loses pressure
  2. To couple the tractor and trailer air lines so air can flow between them Correct answer.
  3. To drain moisture from the trailer air tanks during a walk-around inspection
  4. To release the fifth wheel jaws when uncoupling the trailer

Explanation

Glad hands are the quick-coupling fittings that join the tractor's air lines to the trailer's air lines, allowing supply and control air to pass between the units. They must be sealed with good coupler gaskets so pressure does not leak out at the connection.

Question 27Combination vehicles

When uncoupling a tractor from a trailer on level ground, which step should be completed before pulling the tractor forward?

  1. Lower the landing gear until it firmly contacts the ground Correct answer.
  2. Release the trailer spring brakes with the red button
  3. Chock the tractor's rear wheels and leave the trailer brakes released
  4. Disconnect the electrical cord before lowering the gear

Explanation

The landing gear must be lowered to the ground and secured before the tractor is pulled out, so the trailer is supported when the fifth wheel releases the kingpin. Only after the gear is down should the air lines and electrical cord be disconnected.

Question 28Combination vehicles

What is the purpose of the trailer hand valve during a pre-trip inspection?

  1. To test that the trailer service brakes apply and release properly Correct answer.
  2. To bypass the tractor protection valve during normal driving
  3. To hold the trailer in place while the tractor engine is off
  4. To supply air to the trailer tanks while the engine is running

Explanation

The hand valve (trolley valve) applies only the trailer service brakes, which makes it a convenient way to check that trailer brakes work during inspection. It should never be used for parking or for normal braking on the road.

Question 29Combination vehicles

Which condition indicates that the tractor protection valve is working as designed?

  1. The low-air warning buzzer sounds only when the engine is off
  2. The trailer air supply button pops out automatically at about 20 to 45 psi Correct answer.
  3. Air pressure in the tractor is maintained after a major trailer leak
  4. The trailer brakes release when the red button is pulled out

Explanation

The tractor protection valve closes the tractor-to-trailer air supply when pressure drops into roughly the 20 to 45 psi range, which pops out the trailer air supply button and protects the tractor's remaining air. This lets the driver retain enough pressure to stop the tractor.

Question 30Combination vehicles

What does the yellow (service) air line control on a combination vehicle?

  1. The tractor's front wheel brakes only
  2. The trailer emergency brakes and spring brake release
  3. The trailer's air tank drain valves
  4. The trailer service brakes, applied by the foot brake or hand valve Correct answer.

Explanation

The yellow line carries the control signal that applies the trailer service brakes when the driver uses the foot brake or the trailer hand valve. The red line, by contrast, supplies air and controls the emergency and spring brakes.

Question 31Combination vehicles

For a combination vehicle, which of these is a required part of the pre-trip inspection of the coupling system?

  1. Checking that the electrical cord is longer than the air lines
  2. Checking that the fifth wheel is greased and the jaws are locked around the kingpin Correct answer.
  3. Checking that the trailer has a spare tire mounted on the landing gear
  4. Checking that the kingpin is painted a bright color

Explanation

A proper coupling inspection includes confirming the fifth wheel jaws are closed and locked around the kingpin and that the fifth wheel is adequately greased. A tug test and visual check of the locking jaws help confirm a secure connection.

Question 32Combination vehicles

Why should you avoid making sudden steering movements while pulling a loaded trailer?

  1. Sudden movements can cause the trailer to swing out or the combination to roll over Correct answer.
  2. Sudden movements increase air pressure in the trailer tanks
  3. Sudden movements reduce the trailer's stopping distance
  4. Sudden movements improve trailer tracking in curves

Explanation

A loaded trailer has a high center of gravity and significant momentum, so abrupt steering can cause the trailer to swing laterally or the whole combination to roll over. Smooth, gradual steering inputs keep the trailer tracking behind the tractor.

Question 33Combination vehicles

When backing a tractor-trailer, which technique helps keep the trailer under control?

  1. Back slowly and use the mirrors, pulling forward to correct if needed Correct answer.
  2. Turn the steering wheel in the direction opposite to where you want the trailer to go
  3. Back at highway speed to maintain momentum
  4. Rely on the backup camera alone and ignore the mirrors

Explanation

Backing a combination vehicle should be done slowly, using both mirrors to monitor the trailer, and pulling forward to reposition whenever the trailer drifts off line. Speed and impatience are the main causes of backing accidents with trailers.

Question 34Combination vehicles

You are driving a combination vehicle and the trailer begins to sway side to side at highway speed. What is the best initial response?

  1. Accelerate to pull the trailer straight
  2. Apply the trailer hand valve firmly
  3. Ease off the accelerator and hold the steering wheel steady Correct answer.
  4. Steer sharply in the opposite direction of the sway

Explanation

Trailer sway is best corrected by reducing speed gradually and keeping the steering steady, which lets the trailer settle behind the tractor. Hard braking or sharp steering can amplify the sway and lead to a jackknife or rollover.

Question 35Combination vehicles

What is the correct sequence for releasing the tractor parking brake and trailer spring brakes when preparing to drive after a pre-trip inspection?

  1. Leave the red button out and drive with trailer spring brakes applied
  2. Release the tractor parking brake first, then push in the red button
  3. Pull out the red button and release the parking brake at the same time
  4. Push in the red trailer air supply button, then release the tractor parking brake Correct answer.

Explanation

Air must first be supplied to the trailer by pushing in the red button so the spring brakes release, and then the tractor parking brake can be released. This order ensures the trailer is charged and the brakes are fully released before movement.

Question 36Combination vehicles

During a pre-trip inspection, you apply the trailer hand valve and hear air escaping near the glad hands. What does this most likely indicate?

  1. The trailer air tanks are overcharged
  2. A defective coupler gasket or a loose glad hand connection Correct answer.
  3. The trailer spring brakes have fully released
  4. The tractor protection valve has failed closed

Explanation

Air escaping at the glad hands usually points to a worn or missing coupler gasket or a glad hand that is not fully locked. This leak must be corrected before driving, because it can prevent the trailer brakes from receiving adequate pressure.

Question 37Combination vehicles

What is the purpose of the trailer spring brakes in a combination vehicle?

  1. To lock the fifth wheel jaws during coupling
  2. To apply automatically when air pressure is lost, acting as an emergency and parking brake Correct answer.
  3. To provide the trailer's normal service braking during stops
  4. To increase air pressure in the trailer tanks during downhill driving

Explanation

Spring brakes are held off by air pressure and apply mechanically when supply pressure drops, providing emergency and parking braking for the trailer. This fail-safe design is why a trailer will not roll freely when its air is depleted.

Question 38Combination vehicles

You are coupling to a trailer and the trailer is too high for the fifth wheel to pick up the kingpin. What is the correct action?

  1. Back under the trailer at higher speed to force engagement
  2. Leave the trailer as is and rely on the air suspension to lift it
  3. Raise the trailer with the landing gear until the fifth wheel contacts the trailer plate, then back under Correct answer.
  4. Lower the landing gear to raise the trailer

Explanation

The trailer must be raised or lowered with the landing gear so the fifth wheel slides under the trailer plate and the kingpin enters the jaws at the correct height. Forcing the coupling or backing at speed can damage the fifth wheel and kingpin.

Question 39Combination vehicles

After coupling and doing a tug test, what should you check visually to confirm the coupling is secure?

  1. That the landing gear is fully raised and the trailer is level
  2. That the trailer lights are on and the air lines are connected
  3. That the trailer wheels are chocked and the brakes are released
  4. That the fifth wheel jaws are closed around the kingpin and the release handle is in the locked position Correct answer.

Explanation

The visual check confirms the locking jaws have closed around the kingpin and the release handle has returned to its locked position. Combined with the tug test, this verifies the tractor and trailer are securely connected before driving.

Question 40Combination vehicles

Why is a combination vehicle's center of gravity important when loading a trailer?

  1. A higher center of gravity improves cornering stability
  2. Center of gravity affects only fuel economy, not handling
  3. Center of gravity only matters for the tractor, not the trailer
  4. A high center of gravity increases the risk of rollover in curves and evasive maneuvers Correct answer.

Explanation

A high center of gravity raises the rollover threshold risk because the vehicle's weight shifts outward more easily in a turn. Loading heavy cargo low and distributing it evenly helps keep the combination stable.

Question 41Combination vehicles

When driving a combination vehicle through a work zone with narrow lanes, what is the safest approach?

  1. Maintain highway speed to clear the zone quickly
  2. Move to the left lane and increase speed
  3. Hug the right edge to give oncoming traffic more room
  4. Reduce speed and keep the vehicle centered in the lane, watching mirror clearances Correct answer.

Explanation

Reducing speed and staying centered gives the driver the most margin on both sides and keeps the trailer tracking predictably. Hugging an edge or speeding through a narrow zone increases the chance of striking a barrier or another vehicle.

Question 42Combination vehicles

What does the term "crack-the-whip" effect refer to in a combination vehicle?

  1. The vibration caused by unbalanced trailer tires
  2. The jerking motion when the trailer brakes release suddenly
  3. The snapping sound made by loose air lines at highway speed
  4. The tendency of the rear of the trailer to swing outward during a turn Correct answer.

Explanation

Crack-the-whip describes how the rear of a long trailer can swing outward and travel a wider path than the tractor during a turn. Drivers must allow extra room on the inside of turns to keep the trailer from striking curbs or other vehicles.

Question 43Combination vehicles

What is the correct action if the trailer air supply button pops out while you are driving?

  1. Reach back and hold the button in while driving
  2. Continue driving normally until the next stop
  3. Pull off the road safely and investigate the air loss before continuing Correct answer.
  4. Accelerate to build air pressure faster

Explanation

A popping trailer air supply button signals a loss of air pressure that has applied the trailer spring brakes, so the driver should stop safely and find the cause. Continuing to drive risks losing braking ability and damaging the brake system.

Question 44Combination vehicles

Before descending a long grade in a combination vehicle, what should the driver do regarding gear selection?

  1. Wait until the brakes begin to fade, then downshift
  2. Select a gear low enough that engine braking controls speed without using the service brakes Correct answer.
  3. Shift into the highest gear to save fuel
  4. Shift to neutral to let the vehicle coast

Explanation

The proper technique is to choose a gear before the descent that lets engine braking limit speed so the service brakes are not overheated. Shifting to neutral or waiting for brake fade removes control and is dangerous on long grades.

Question 45Combination vehicles

How should you check that the trailer's spring brakes will apply if the air supply is lost?

  1. Apply the foot brake and watch for the trailer lights to come on
  2. Open the trailer tank petcocks and watch for water to drain
  3. Pull the red trailer air supply button out, release the tractor parking brakes, and gently pull against the trailer to check that its brakes hold Correct answer.
  4. Push the red button in and check for air leaks at the glad hands

Explanation

Pulling out the red button exhausts the trailer supply line and should apply the trailer emergency spring brakes. With the tractor parking brakes released, gently pull against the trailer to verify that its brakes hold. Listening for escaping air alone does not establish that the brakes work.

Question 46Combination vehicles

What is the main reason a combination vehicle requires a longer distance to stop than a single vehicle of similar weight?

  1. The combination has more mass and momentum, and brake timing across the units adds distance Correct answer.
  2. The tractor's engine brake works only at low speed
  3. The trailer tires have less tread than the tractor tires
  4. The trailer's air brakes are stronger than the tractor's

Explanation

A combination vehicle carries more total mass and momentum, and the trailer brakes may apply slightly after the tractor brakes, which lengthens stopping distance. Drivers must increase following distance to allow for this.

Question 47Combination vehicles

You are inspecting a combination vehicle and find the trailer's spring brake chambers are not releasing even with full air pressure. Which condition should you suspect?

  1. A loose electrical cord connection
  2. An underinflated trailer tire on the left side
  3. A stuck relay valve or a plugged supply line to the spring brake chambers Correct answer.
  4. A clogged fuel filter on the tractor

Explanation

If full air pressure is present but the spring brakes will not release, the problem is likely in the air delivery path, such as a stuck relay valve or a blocked supply line. The vehicle must not be driven until the brake system is repaired.

Question 48Combination vehicles

A driver must couple to a trailer parked on a slope. What is the safest practice?

  1. Back under the trailer quickly to avoid rolling
  2. Couple with the tractor in neutral to avoid jerking the trailer
  3. Chock the trailer wheels and confirm the trailer spring brakes are applied before coupling Correct answer.
  4. Leave the trailer brakes released so it can settle onto the fifth wheel

Explanation

On a slope, the trailer must be secured with chocks and its spring brakes applied so it cannot roll during coupling. Backing under quickly or leaving the brakes released risks the trailer moving and injuring the driver.

Question 49Combination vehicles

What happens to the trailer brakes if the red supply line is cut or disconnected while the vehicle is moving?

  1. The trailer air tanks vent through the yellow line
  2. The trailer spring brakes apply and the tractor protection valve closes Correct answer.
  3. The trailer brakes lock only when the foot brake is pressed
  4. The trailer service brakes release and the trailer rolls freely

Explanation

A severed red supply line causes a rapid loss of trailer air pressure, which applies the trailer spring brakes automatically. The tractor protection valve then closes to preserve the tractor's air supply so the driver can stop safely.

Question 50Combination vehicles

You are backing a tractor-trailer into a loading dock and the trailer begins to drift toward the left. What is the correct corrective action?

  1. Accelerate to straighten the trailer quickly
  2. Pull forward to straighten the combination, then resume backing slowly Correct answer.
  3. Continue backing and steer harder to the right
  4. Apply the trailer hand valve to pivot the trailer

Explanation

When the trailer drifts off line during backing, pulling forward to reset the alignment is the safest correction. Continuing to back while steering harder or accelerating can cause the trailer to jackknife against the tractor.