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Georgia CDL Combination Vehicles Test study materials
Georgia CDL Combination Vehicles Test study resources

Georgia CDL Combination Vehicles Test practice questions

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Question 1Combination Vehicle Basics and Risks

In a combination vehicle, what does the term "off-tracking" describe?

  1. The trailer brakes applying slightly after the tractor brakes during a stop
  2. The coupling jaws releasing the kingpin when the tractor is reversed
  3. The air pressure in the trailer tank dropping below the tractor tank pressure
  4. The rear wheels of the trailer follow a path inside the path of the front wheels during a turn Correct answer.

Explanation

Off-tracking is the tendency of the rear axles of a long combination to cut the corner and travel a shorter path than the steering axle. The longer the combination, the wider the sweep of the front and the greater the inward cut of the trailer wheels. Drivers must swing wide and check mirrors to keep trailer tires off curbs and out of adjacent lanes.

Question 2Combination Vehicle Basics and Risks

Why must a driver of a combination vehicle check that the trailer is securely coupled before driving away?

  1. The tractor will not build air pressure without the trailer connected
  2. The trailer lights will not work unless the kingpin is fully seated
  3. A trailer that is not locked to the fifth wheel can separate from the tractor Correct answer.
  4. An uncoupled trailer will slide forward into the cab on the first hard stop

Explanation

The only thing holding the trailer to the tractor is the fifth wheel jaws closed around the kingpin. If the jaws are not locked, the trailer can break loose during a turn or over a bump. A firm pull test with the trailer brakes set confirms the coupling before you commit to the road.

Question 3Combination Vehicle Basics and Risks

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

  1. To apply only the trailer brakes for testing and for holding the trailer during coupling Correct answer.
  2. To release air from the trailer supply line automatically
  3. To adjust the slack adjusters on the trailer axles
  4. To set and release the tractor parking brakes

Explanation

The hand valve supplies air directly to the trailer brakes so the driver can test them or hold the trailer still while coupling. It does not apply the tractor brakes. Many newer tractors omit it, so drivers should know whether their unit has one.

Question 4Combination Vehicle Basics and Risks

A combination vehicle with a fully loaded trailer will generally need a longer stopping distance than a bobtail tractor because:

  1. The trailer tires have a smaller contact patch than tractor tires
  2. The tractor engine produces less power when towing
  3. The trailer brakes are disconnected whenever the trailer is loaded
  4. The added weight increases the momentum that the brakes must overcome Correct answer.

Explanation

Stopping distance depends on total weight and speed, and a loaded trailer can add tens of thousands of pounds to the combination. More mass means more kinetic energy that the brakes must convert to heat. Drivers must plan braking earlier and keep a larger following gap when loaded.

Question 5Combination Vehicle Basics and Risks

Which condition makes a combination vehicle most likely to roll over?

  1. Using the trailer hand valve to test the brakes
  2. Traveling at the posted speed limit on a straight level highway
  3. Taking a curve or ramp faster than the posted advisory speed Correct answer.
  4. Downshifting before a long downgrade

Explanation

Rollovers usually happen when a combination takes a curve or ramp too fast and the centrifugal force lifts the high side of the vehicle. Posted ramp speeds are advisory and are set for safe tracking of large vehicles. Entering too fast leaves no margin to correct.

Question 6Combination Vehicle Basics and Risks

The rearward amplification effect in a combination vehicle means that:

  1. The last trailer in a multi-trailer combination swings wider and faster than the tractor in a quick lane change Correct answer.
  2. The engine rpm rises faster than the road speed
  3. The trailer brakes apply harder than the tractor brakes
  4. The air compressor output increases with trailer length

Explanation

Rearward amplification describes how a quick steering input at the tractor is magnified as it travels back through the combination. The rearmost unit can move sideways much farther and faster than the tractor did. This is why abrupt lane changes are especially dangerous with multiple trailers.

Question 7Combination Vehicle Basics and Risks

You are driving a tractor-trailer and must make a right turn at a city corner. What is the safest technique?

  1. Accelerate through the turn so the trailer tracks outside the tractor path
  2. Stay close to the right curb so cars cannot pass on your right, then turn sharply
  3. Swing wide to the left before the turn, then bring the trailer around while watching the right mirror Correct answer.
  4. Begin the turn early and let the trailer tires ride the curb to save space

Explanation

A combination needs extra room because the trailer wheels cut inside the tractor path. Swinging wide to the left before the turn and monitoring the right mirror keeps the trailer off the curb and away from pedestrians. Turning too tight or too fast is a common cause of curb strikes and right-side crashes.

Question 8Combination Vehicle Basics and Risks

During a pre-trip inspection of a combination vehicle, you find the trailer supply line (emergency line) has a slow leak at the gladhand. What should you do?

  1. Tape the gladhand and continue, since the leak is small
  2. Close the tractor shut-off valve and drive without trailer brakes
  3. Do not move the vehicle until the leak is repaired or the gladhand is replaced Correct answer.
  4. Increase engine rpm to keep the compressor ahead of the leak

Explanation

The supply line charges the trailer air tanks and signals the trailer emergency brakes; a leak can cause the trailer brakes to set unexpectedly or leave insufficient air for braking. Any air leak in the coupling system must be corrected before the vehicle is moved. A minor-looking leak grows worse under vibration and demand.

Question 9Combination Vehicle Basics and Risks

You are backing a tractor-trailer into a loading dock on the driver's side. Which practice gives you the best control?

  1. Use a helper or get out and check, and back slowly using both mirrors with the trailer turning toward the driver's side Correct answer.
  2. Rely only on the right mirror because the left mirror shows the tractor
  3. Turn the steering wheel opposite the direction you want the trailer to go
  4. Back quickly so the trailer has momentum to overcome the dock lip

Explanation

Backing a combination is safest when done slowly with a spotter or a walk-around check, using both mirrors to track the trailer. Turning toward the driver's side keeps the trailer visible in the left mirror. Speed reduces your ability to correct before the trailer swings off line.

Question 10Combination Vehicle Basics and Risks

When driving a combination vehicle on a downgrade, why should you select a lower gear before starting down?

  1. To keep the trailer brakes from engaging automatically
  2. To reduce the trailer's off-tracking on the curve at the bottom
  3. To keep the engine at high idle so the air compressor runs faster
  4. To use engine braking and avoid relying on the service brakes, which can fade Correct answer.

Explanation

A lower gear lets the engine and driveline absorb speed so the service brakes are used only for trimming. Continuous service brake use overheats the drums or rotors and causes brake fade. Once fade begins on a long grade, there may be no way to recover speed control.

Question 11Combination Vehicle Basics and Risks

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

  1. Lower the landing gear, disconnect the air and electrical lines, release the fifth wheel, then pull out slowly Correct answer.
  2. Release the fifth wheel, pull forward, then lower the landing gear and disconnect the lines
  3. Release the fifth wheel, then lower the landing gear, then disconnect the air lines
  4. Disconnect the air lines, pull the tractor forward, then lower the landing gear

Explanation

The landing gear must be down and the trailer supported before the kingpin is released, or the trailer will drop. Air and electrical lines are disconnected before pulling out so they are not stretched or torn. Only then is the fifth wheel released and the tractor pulled clear slowly.

Question 12Combination Vehicle Basics and Risks

You are inspecting a combination vehicle and notice the trailer emergency (supply) line and service line gladhands are cross-connected. What is the result?

  1. The trailer brakes will release normally but the lights will not work
  2. The fifth wheel jaws will not lock around the kingpin
  3. The trailer brakes will not release properly and braking performance will be affected Correct answer.
  4. The tractor will not start because the neutral safety switch is bypassed

Explanation

The supply line charges the trailer tanks and holds off the emergency brakes, while the service line carries the application signal. Crossed lines mean the trailer may stay braked or apply incorrectly. Color coding and careful coupling prevent this.

Question 13Combination Vehicle Basics and Risks

A combination vehicle is more likely than a single-unit truck to jackknife during hard braking because:

  1. The combination has a shorter wheelbase than a single truck
  2. The trailer has no spring brakes
  3. The tractor's front brakes are disconnected when a trailer is attached
  4. The trailer pushes the tractor and the units can pivot at the fifth wheel Correct answer.

Explanation

The fifth wheel is a pivot point, and under hard braking the trailer's momentum can push the tractor's drive axles sideways. If the drive tires lose traction, the tractor swings around the trailer. Smooth braking and adequate following distance reduce the risk.

Question 14Combination Vehicle Basics and Risks

How does the center of gravity of a loaded combination vehicle affect its stability in a curve?

  1. A lower center of gravity increases the trailer's off-tracking
  2. A higher center of gravity increases the tendency to roll over at a given speed Correct answer.
  3. A higher center of gravity improves traction and reduces rollover risk
  4. Center of gravity has no effect on a combination because of the fifth wheel

Explanation

The higher the center of gravity, the greater the lateral force needed to tip the vehicle, so a tall or top-heavy load rolls at a lower speed. Loads should be kept as low as practical and secured. Speed must be reduced for high-center-of-gravity loads before entering a curve.

Question 15Combination Vehicle Basics and Risks

You are driving a loaded tractor-trailer and the trailer begins to sway side to side at highway speed. What is the recommended response?

  1. Ease off the accelerator and steer smoothly without abrupt corrections Correct answer.
  2. Apply the trailer hand valve fully and hold it
  3. Accelerate to pull the trailer straight
  4. Brake hard and steer sharply toward the shoulder

Explanation

Trailer sway is corrected by reducing speed gradually and keeping the steering steady so the oscillations damp out. Hard braking or sharp steering inputs can amplify the sway and lead to a jackknife or rollover. The driver should slow smoothly and stop in a safe place to check the load and tires.

Question 16Combination Vehicle Basics and Risks

Which statement best describes the effect of a long combination on lane position when passing through an intersection?

  1. The trailer will track outside the tractor's path and clear the intersection more quickly
  2. The trailer's wheels always follow the exact path of the tractor's steer tires
  3. The rear of the combination may still be in the intersection after the tractor has cleared it Correct answer.
  4. The combination occupies the same footprint as a single-unit truck

Explanation

Because of length and off-tracking, the rear of a long combination can still be crossing the intersection when the driver has already passed. Drivers must not accelerate away until the trailer has cleared. Failing to allow for this blocks cross traffic and causes side impacts.

Question 17Combination Vehicle Basics and Risks

A driver is coupling to a trailer and must verify the trailer height matches the fifth wheel. What is the risk of backing under a trailer that is set too high?

  1. The kingpin may ride over the fifth wheel and the trailer may not couple, or the landing gear may lift off the ground Correct answer.
  2. The trailer brakes will not release because the supply line is blocked
  3. The trailer lights will short out through the kingpin
  4. The tractor's rear axle will unload and lose traction

Explanation

If the trailer nose is too high, the kingpin can pass over the fifth wheel plate instead of locking into the jaws, leaving the trailer uncoupled. Backing hard can also lift the landing gear off the ground or damage the trailer. The trailer should be set so the fifth wheel plate contacts the trailer at the proper height.

Question 18Combination Vehicle Basics and Risks

Why can a combination vehicle with a lightly loaded trailer be more prone to wheel lockup and skidding during hard braking than the same combination fully loaded?

  1. The tractor's compressor runs slower with a light trailer
  2. Light loads reduce tire-to-road friction, so the trailer wheels lock more easily Correct answer.
  3. A light trailer has a shorter stopping distance than a heavy one
  4. The trailer brakes are automatically disconnected when the trailer is empty

Explanation

With less weight on the trailer tires, there is less normal force and therefore less available friction, so the brakes can lock the wheels with less pedal effort. Locked wheels slide and extend stopping distance while reducing steering control. Drivers should brake smoothly and adjust for load condition.

Question 19Combination Vehicle Basics and Risks

On a combination vehicle, what does the tractor protection valve do?

  1. It regulates the electrical voltage sent to the trailer lights
  2. It limits tractor air pressure to the trailer and closes the trailer supply line if pressure drops too low Correct answer.
  3. It locks the trailer brakes whenever the parking brake is released
  4. It prevents the fifth wheel jaws from opening while the engine is running

Explanation

The tractor protection valve protects the tractor's air supply by limiting pressure sent to the trailer and by closing the trailer supply line when pressure falls below a safe level. This causes the trailer emergency brakes to set rather than draining the tractor system. It is a key safety device in the air brake system.

Question 20Combination Vehicle Basics and Risks

A driver must cross railroad tracks in a long combination vehicle whose trailer has low ground clearance. What is the safest procedure?

  1. Back up if a train is seen, then cross when clear
  2. Shift to a lower gear and creep across without stopping, so the whole combination clears the tracks Correct answer.
  3. Accelerate hard so the trailer bounces over the rails
  4. Stop on the tracks to check both directions before proceeding

Explanation

Never stop on the tracks; a long combination takes time to clear and a train cannot stop quickly. Use a low gear and cross without shifting so the engine does not stall and the entire unit clears the rails. If traffic will not allow a full crossing, wait before starting.

Question 21Trailer Air Brake Systems and Safety Devices

On a combination vehicle with air brakes, what is the purpose of the trailer supply (emergency) line?

  1. It carries the electrical signal that operates the trailer stop lamps during braking.
  2. It supplies air only to the trailer service chambers when the foot valve is released.
  3. It vents air pressure to the atmosphere to speed up trailer brake release.
  4. It charges the trailer air tanks and automatically applies the trailer spring brakes if the line loses pressure. Correct answer.

Explanation

The trailer supply line is the emergency line: it charges the trailer reservoirs and, when pressure drops because of a breakaway or coupling failure, the trailer spring brakes apply automatically. The service line is the separate circuit that delivers application pressure when the driver brakes.

Question 22Trailer Air Brake Systems and Safety Devices

Which color is the trailer service line gladhand on a standard combination vehicle?

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

Explanation

By industry convention the service line gladhand is blue and the emergency (supply) line gladhand is red. Matching colors correctly keeps the two circuits from being cross-coupled.

Question 23Trailer Air Brake Systems and Safety Devices

You are coupling a tractor to a trailer and hear a steady leak of escaping air at the gladhands after connecting both lines. What should you do first?

  1. Shut down, chock the wheels, and check that both gladhands are fully seated and the couplers are locked. Correct answer.
  2. Disconnect the service line and continue with only the emergency line connected.
  3. Close the trailer shut-off valves and drive to the nearest shop.
  4. Increase engine rpm to build pressure faster than the leak.

Explanation

A continuous leak at the coupling point usually means a gladhand is not fully seated, has a damaged seal, or the coupler is not locked. Secure the vehicle, set the parking brake, and chock the wheels before reaching under or between the units to inspect and reseat the connection.

Question 24Trailer Air Brake Systems and Safety Devices

During a pre-trip inspection of a combination vehicle, you release the trailer spring brakes and hear a loud popping or hissing sound from under the trailer. What does this most likely indicate?

  1. An air leak or a spring brake chamber problem that must be repaired before driving. Correct answer.
  2. Normal operation of the trailer spring brake chambers.
  3. The trailer spring brakes are releasing as designed.
  4. The trailer supply line is properly charged.

Explanation

Spring brakes release with a controlled release of air, but a loud pop or sustained hiss points to a leaking chamber, diaphragm, or line. A combination vehicle with a defective spring brake circuit can lose emergency braking, so the defect must be corrected before the vehicle is moved.

Question 25Trailer Air Brake Systems and Safety Devices

What is the function of the tractor protection valve on an air-braked combination vehicle?

  1. It balances air pressure between the left and right trailer service chambers.
  2. It releases air from the trailer reservoirs when the engine is shut off.
  3. It automatically applies the tractor parking brakes whenever the foot valve is depressed.
  4. It limits tractor air pressure to the trailer to a safe maximum and closes the trailer supply if pressure drops too low. Correct answer.

Explanation

The tractor protection valve guards the tractor air supply. It limits pressure going to the trailer and, at low system pressure, closes off the trailer supply so the tractor retains enough air for its own brakes while the trailer spring brakes apply.

Question 26Trailer Air Brake Systems and Safety Devices

With a fully charged system, you make a full brake application and hold it. What does a steady drop in the tractor air gauge indicate?

  1. The trailer spring brakes have applied.
  2. A leak in the air brake system that must be located and repaired. Correct answer.
  3. The low air pressure warning has been tested.
  4. The compressor is cycling normally.

Explanation

With the brakes applied and the system otherwise stable, the gauge should hold pressure. A continuous drop shows air escaping somewhere in the system, such as a line, fitting, or chamber, and the leak must be found and fixed before the vehicle is operated.

Question 27Trailer Air Brake Systems and Safety Devices

What is the purpose of the trailer spring brakes in a breakaway situation?

  1. They apply only when the driver pulls the parking control valve.
  2. They apply automatically when the trailer supply line is severed. Correct answer.
  3. They release air to let the trailer roll clear of the tractor.
  4. They hold the trailer only while the engine is running.

Explanation

Spring brakes are held off by air pressure. If the trailer separates from the tractor and the supply line is torn apart, pressure is lost and the springs apply the brakes automatically, stopping the trailer.

Question 28Trailer Air Brake Systems and Safety Devices

After connecting the trailer, you perform the trailer supply valve check. You push in the red trailer air supply knob and the trailer brakes release. Then you pull the knob out. What should happen?

  1. The trailer brakes should release and stay released.
  2. The trailer spring brakes should apply and the trailer should not roll. Correct answer.
  3. The tractor brakes should apply but the trailer brakes should stay released.
  4. The low air pressure warning should sound and the engine should shut down.

Explanation

Pulling out the trailer air supply knob shuts off air to the trailer supply line, which causes the trailer spring brakes to apply. With the knob out, the trailer should be held and should not roll, confirming the emergency circuit works.

Question 29Trailer Air Brake Systems and Safety Devices

The low air pressure warning device on a combination vehicle must activate before pressure falls below what level?

  1. 90 psi
  2. 60 psi Correct answer.
  3. 45 psi
  4. 110 psi

Explanation

Federal standards require the low air pressure warning to activate before the system pressure drops below 60 psi. The warning alerts the driver while enough pressure remains to stop safely.

Question 30Trailer Air Brake Systems and Safety Devices

Your combination vehicle has a front brake limiting (blended) valve, and you are driving on a slippery road. What does the valve do?

  1. It reduces air pressure to the front brakes to help prevent a front wheel skid. Correct answer.
  2. It increases air pressure to the front brakes to shorten stopping distance.
  3. It applies the trailer spring brakes whenever the front wheels lock.
  4. It disables the trailer service brakes so only the tractor brakes work.

Explanation

A front brake limiting valve reduces air pressure delivered to the steering axle brakes in the slippery setting, lowering the chance of front wheel lockup and loss of steering control. In the normal setting full pressure is available.

Question 31Trailer Air Brake Systems and Safety Devices

You are inspecting a trailer equipped with spring brakes. Which condition indicates the spring brakes may be mechanically caged or otherwise disabled?

  1. The pushrod extends when the parking control is released.
  2. The slack adjuster moves when the brake is applied.
  3. The trailer rolls freely with the supply line disconnected and the system bled down. Correct answer.
  4. The chamber shows a normal service stroke on application.

Explanation

With the supply line disconnected and air bled off, spring brakes should apply and hold the trailer. If the trailer rolls freely in that condition, the springs are not doing their job, possibly because a chamber is caged or the brake is otherwise disabled.

Question 32Trailer Air Brake Systems and Safety Devices

During a pre-trip inspection you apply the trailer hand valve with the trailer supply charged. What should you observe?

  1. The trailer spring brakes apply permanently until the valve is reset.
  2. Only the tractor drive axle brakes apply.
  3. The trailer brakes apply and remain applied after the valve is released.
  4. The trailer brakes apply, and they release when the hand valve is returned. Correct answer.

Explanation

The trailer hand valve delivers service air to the trailer brakes, so they should apply while the valve is held and release when it is returned. It is a service brake control, not a parking brake, and should not be used to hold a parked vehicle.

Question 33Trailer Air Brake Systems and Safety Devices

Why should the trailer hand valve never be used to hold a parked combination vehicle?

  1. It applies the tractor parking brakes but not the trailer brakes.
  2. It applies only the trailer brakes and can release if air pressure bleeds off, allowing the vehicle to roll. Correct answer.
  3. It overcharges the trailer reservoirs and damages the compressor.
  4. It automatically releases the trailer spring brakes when pressure drops.

Explanation

The hand valve is a service brake control that depends on stored air pressure. If pressure leaks away, the trailer brakes release and the vehicle can roll. Parking requires the spring parking brakes and, where appropriate, wheel chocks.

Question 34Trailer Air Brake Systems and Safety Devices

What does the trailer air supply knob (red octagon) control on a combination vehicle?

  1. Air flow to the trailer supply line, which charges the trailer and releases its spring brakes. Correct answer.
  2. The trailer service brake application pressure.
  3. The trailer ABS warning lamp circuit.
  4. The tractor parking brakes only.

Explanation

The red trailer air supply knob controls air to the trailer emergency/supply line. Pushing it in charges the trailer and releases the spring brakes; pulling it out shuts off supply and applies them.

Question 35Trailer Air Brake Systems and Safety Devices

You are driving a combination vehicle and the low air pressure warning activates. What is the correct response?

  1. Continue to the next exit while downshifting to save air.
  2. Move to a safe stop as soon as possible, keeping the brakes applied after stopping. Correct answer.
  3. Pump the brake pedal repeatedly to build pressure.
  4. Release the trailer supply knob while moving to conserve tractor air.

Explanation

When the warning sounds, pressure is approaching the point where spring brakes will apply automatically. The safe action is to stop as soon as possible in a safe location and keep the brakes applied, because the spring brakes may set once pressure drops further.

Question 36Trailer Air Brake Systems and Safety Devices

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

  1. It limits the maximum pressure in the trailer reservoirs.
  2. It converts electrical signals from the tractor into air pressure.
  3. It releases the trailer spring brakes when the supply knob is pulled out.
  4. It senses control pressure from the tractor and delivers air from the trailer reservoir to the trailer brake chambers. Correct answer.

Explanation

A relay valve uses a small control signal from the tractor to open a larger supply of air from the trailer reservoir directly to the brake chambers. This shortens response time and reduces pressure lag on a long combination vehicle.

Question 37Trailer Air Brake Systems and Safety Devices

During your walk-around inspection, you notice the trailer air line gladhand seals are cracked and flattened. What is the correct action?

  1. Tape the seals and continue the trip.
  2. Reverse the gladhands so the good seal faces outward.
  3. Coat the seals with oil to improve the seal.
  4. Replace the defective seals before operating the vehicle. Correct answer.

Explanation

Cracked or flattened gladhand seals leak air, which can prevent the trailer from charging fully and can trigger spring brake application. Defective seals must be replaced before the vehicle is operated.

Question 38Trailer Air Brake Systems and Safety Devices

You are backing a trailer and the trailer supply line separates from the tractor. What should happen?

  1. Only the tractor brakes apply while the trailer brakes stay released.
  2. The tractor protection valve closes and the trailer spring brakes apply. Correct answer.
  3. The trailer service brakes release and the trailer rolls free.
  4. The trailer brakes apply only if the driver presses the foot valve.

Explanation

Separation of the supply line causes a rapid pressure loss that the tractor protection valve senses, closing the trailer supply. With supply pressure gone, the trailer spring brakes apply automatically and stop the trailer.

Question 39Trailer Air Brake Systems and Safety Devices

What does the yellow diamond-shaped parking brake control knob operate on a combination vehicle?

  1. The trailer hand valve circuit.
  2. The tractor parking (spring) brakes, and in many tractors it also controls the trailer supply. Correct answer.
  3. The trailer spring brakes only.
  4. The tractor and trailer service brakes together.

Explanation

The yellow knob controls the tractor parking spring brakes. On many tractors it is also the trailer air supply control, so pulling it applies both tractor and trailer parking brakes; the exact arrangement should be confirmed during the pre-trip inspection.

Question 40Trailer Air Brake Systems and Safety Devices

You are inspecting a combination vehicle and find that the trailer service line has been connected to the trailer emergency gladhand and the emergency line to the service gladhand. What is the likely result?

  1. The trailer brakes may not release properly and the system will not function as designed. Correct answer.
  2. The trailer ABS will compensate and the brakes will work normally.
  3. Only the trailer stop lamps will be affected.
  4. The trailer brakes will work normally because both lines carry air.

Explanation

Cross-coupling the service and emergency lines sends air to the wrong circuit, so the trailer may not charge or release correctly and service braking will not respond as designed. Color-coded gladhands exist to prevent exactly this error.

Question 41Trailer Air Brake Systems and Safety Devices

What is the purpose of the alcohol evaporator or air dryer in a combination vehicle air brake system?

  1. To remove moisture and contaminants that could freeze or damage brake components. Correct answer.
  2. To increase reservoir pressure above governor cut-out.
  3. To lubricate the brake chambers automatically.
  4. To supply air directly to the trailer service line.

Explanation

Moisture in an air system can freeze in cold weather and block valves or lines, and contaminants can damage seals. An evaporator or dryer conditions the air so the brake system stays reliable.

Question 42Trailer Air Brake Systems and Safety Devices

You drain the trailer air reservoirs during the pre-trip inspection and observe a large amount of water and oil. What does this indicate?

  1. The trailer spring brakes are overcharged.
  2. The reservoirs were properly drained and need no further action.
  3. The trailer relay valve is functioning normally.
  4. The compressor or air dryer is passing contaminants and the system needs service. Correct answer.

Explanation

Reservoirs normally collect a little moisture, but a large amount of water or oil means the air dryer or compressor is not conditioning the air properly. Contaminants can freeze, corrode, and damage brake valves, so the system should be serviced.

Question 43Trailer Air Brake Systems and Safety Devices

During the pre-trip inspection, how do you check that the trailer spring brakes will hold the trailer?

  1. Open the reservoir drain cocks while the engine is running.
  2. Release the parking control and confirm the trailer rolls freely.
  3. Apply the foot valve and listen for air loss.
  4. Disconnect the supply line, let air bleed off, and confirm the trailer does not roll. Correct answer.

Explanation

With the supply line disconnected and air bled from the trailer, the spring brakes should apply and hold the trailer. If the trailer rolls, the spring brakes are not working and the vehicle must not be driven.

Question 44Trailer Air Brake Systems and Safety Devices

What is the function of the trailer spring brake chamber's caging bolt?

  1. It adjusts the slack adjuster during service brake application.
  2. It provides a manual means to compress and hold the spring so the vehicle can be moved when air pressure is unavailable. Correct answer.
  3. It automatically resets the spring brake after each application.
  4. It vents excess air from the service chamber.

Explanation

The caging bolt is a manual release tool: it compresses the spring brake mechanically so the brake can be released without air pressure, allowing the vehicle to be moved for service. A caged brake provides no emergency braking and must be restored before driving.

Question 45Coupling and Uncoupling Procedures

Before backing a tractor under a semitrailer to couple, what should you check about the trailer?

  1. That the landing gear is fully retracted against the frame
  2. That the trailer is at least one foot higher than the tractor frame
  3. That the trailer is raised so its kingpin is above the fifth wheel plate
  4. That the trailer brakes are released and the wheels are chocked Correct answer.

Explanation

The trailer must be held still by chocks or its parking brakes released only when you are ready to move it; for coupling you need the trailer secured so it cannot roll while you back under. The trailer should be at normal height with the kingpin aligned to enter the fifth wheel throat, not raised above it. Confirm the trailer brakes are set or wheels chocked before you begin backing.

Question 46Coupling and Uncoupling Procedures

When you back the tractor toward the trailer, at what point should you stop and get out to check alignment?

  1. After the trailer has been lifted off its landing gear
  2. Only after the air lines have been connected
  3. Before the tractor touches the trailer, then again as the kingpin nears the fifth wheel Correct answer.
  4. Only after the kingpin has fully entered the fifth wheel

Explanation

You should stop before contact and look at the alignment, then stop again when the kingpin is close to the fifth wheel so you can confirm it will enter the throat squarely. Backing blind until the kingpin is fully engaged risks striking the trailer or missing the fifth wheel. Getting out to check at these points is the standard safe procedure.

Question 47Coupling and Uncoupling Procedures

After the kingpin enters the fifth wheel, how do you confirm the coupling is locked?

  1. Listen for the air dryer to cycle
  2. Check that the trailer has lifted off the landing gear
  3. Pull the tractor forward gently and check that the trailer moves with it Correct answer.
  4. Look for the release handle to be in the forward position only

Explanation

The pull test is the definitive check: with the trailer brakes set, gently pull forward and confirm the tractor does not move away from the trailer, which proves the kingpin is locked. Visual checks of the handle and jaw are useful but not sufficient alone. The trailer lifting off the landing gear does not by itself prove the lock is secure.

Question 48Coupling and Uncoupling Procedures

You have locked the fifth wheel and completed the pull test. What is the correct next step?

  1. Connect the air lines and electrical cord, then raise the landing gear Correct answer.
  2. Lower the landing gear to take the weight off the fifth wheel
  3. Raise the landing gear fully and stow the crank handle
  4. Release the trailer parking brakes and drive away

Explanation

After the coupling is verified, connect the glad hands and the electrical cord so the trailer has air and lights, then crank the landing gear up and secure the handle. Raising the gear before the air lines are connected leaves the trailer without brakes or lights. The landing gear should be raised, not lowered, once the trailer is supported by the tractor.

Question 49Coupling and Uncoupling Procedures

Why must you check that the trailer apron sits flat on the fifth wheel after coupling?

  1. A gap means the landing gear was cranked too high
  2. A gap proves the tractor frame is bent
  3. A gap shows the air lines are crossed
  4. A gap indicates the kingpin is not fully seated and the coupling is unsafe Correct answer.

Explanation

The trailer apron should rest flat against the fifth wheel plate with no visible gap. A gap means the kingpin has not seated fully in the jaws, so the trailer could separate from the tractor. This is a reason to stop and recheck the coupling before moving.

Question 50Coupling and Uncoupling Procedures

What is the first thing you should do when preparing to uncouple a tractor-semitrailer?

  1. Crank the landing gear down until it touches the ground
  2. Disconnect the electrical cord
  3. Park on firm, level ground and set the trailer parking brakes Correct answer.
  4. Pull the fifth wheel release handle

Explanation

Uncoupling starts with a safe position: firm, level ground so the trailer will not roll, and the trailer parking brakes set so it stays put when the tractor is released. Only after that do you lower the landing gear and release the fifth wheel. Releasing the fifth wheel first would let the trailer drop.