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AK CDL Combination study materials
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AK CDL Combination practice questions

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Question 1Combination Vehicle Basics and Trailer Hookup

Before backing a tractor-trailer, what is the most reliable way to know what is behind the trailer?

  1. Rely on the trailer's rear camera
  2. Watch the mirrors only
  3. Sound the horn and back slowly
  4. Get out and look, or use a spotter Correct answer.

Explanation

The safest practice before backing is to get out and look (GOAL) or use a spotter, because mirrors leave large blind spots directly behind and along the sides of a trailer. Cameras and horn use do not replace a direct visual check.

Question 2Combination Vehicle Basics and Trailer Hookup

When coupling a tractor to a trailer, the trailer should be positioned so that the kingpin is:

  1. Centered in the fifth wheel opening Correct answer.
  2. Slightly high so the trailer drops onto the fifth wheel
  3. Offset to the driver side of the slot
  4. Raised above the fifth wheel plate

Explanation

The kingpin should be centered in the fifth wheel throat so the jaws close fully around it. A misaligned or high trailer can cause the tractor to strike the trailer or the jaws to fail to lock.

Question 3Combination Vehicle Basics and Trailer Hookup

After coupling, you should tug-test the connection. What is the purpose of the tug test?

  1. To confirm the fifth wheel jaws have locked around the kingpin Correct answer.
  2. To verify the landing gear is fully raised
  3. To check that the trailer brakes release smoothly
  4. To seat the kingpin deeper into the fifth wheel

Explanation

A tug test-pulling forward gently with the trailer brakes set-confirms the fifth wheel jaws are locked and will not let the trailer separate. If the tractor moves and the trailer does not, the coupling is holding.

Question 4Combination Vehicle Basics and Trailer Hookup

What color are the clearance lights at the front and sides of a combination vehicle typically required to be?

  1. Blue
  2. Red
  3. Amber Correct answer.
  4. White

Explanation

Front and side clearance, identification, and marker lights are amber, while rear clearance and tail lights are red. This color convention helps other drivers judge vehicle size and direction.

Question 5Combination Vehicle Basics and Trailer Hookup

When should you use the trailer hand valve (trolley valve) while driving?

  1. To test the trailer brakes before driving Correct answer.
  2. For normal stopping on the highway
  3. To assist with steering in curves
  4. To hold the vehicle on a hill

Explanation

The hand valve is intended for testing trailer brakes, not for routine stopping, because it does not apply the tractor brakes and can cause skids or jackknife if used while moving. Normal braking uses the foot brake, which applies both tractor and trailer.

Question 6Combination Vehicle Basics and Trailer Hookup

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

  1. Lower landing gear, release fifth wheel, disconnect lines, pull out
  2. Release fifth wheel, lower landing gear, disconnect lines, pull out
  3. Lower landing gear, disconnect lines, release fifth wheel, pull out Correct answer.
  4. Disconnect lines, release fifth wheel, lower landing gear, pull out

Explanation

The safe sequence is to set the trailer brakes, lower the landing gear to take the weight, disconnect the air and electrical lines, then release the fifth wheel and pull out slowly. Doing the steps out of order can drop the trailer or damage lines.

Question 7Combination Vehicle Basics and Trailer Hookup

Why must you chock the trailer wheels or set the trailer spring brakes before uncoupling?

  1. To align the kingpin for the next hookup
  2. To keep the air lines pressurized
  3. To make the landing gear easier to crank
  4. To keep the trailer from rolling when the tractor is removed Correct answer.

Explanation

Once the tractor is removed, nothing else holds the trailer, so the trailer brakes must be set or the wheels chocked to prevent rolling. This is a basic safety step in every uncoupling procedure.

Question 8Combination Vehicle Basics and Trailer Hookup

A combination vehicle generally requires a longer stopping distance than a single vehicle because:

  1. The trailer brakes engage before the tractor brakes
  2. The tires have less contact with the road
  3. The steering axle carries less weight
  4. The combination weighs more and has more momentum Correct answer.

Explanation

A loaded tractor-trailer has much greater mass, so it takes more distance and brake force to stop. Drivers must follow farther behind and plan stops earlier.

Question 9Combination Vehicle Basics and Trailer Hookup

What does the trailer air supply (red) control valve do?

  1. Releases the tractor parking brake
  2. Dumps air from the tractor protection valve
  3. Charges the trailer air tanks and releases the trailer spring brakes Correct answer.
  4. Applies only the trailer service brakes

Explanation

The red trailer air supply knob charges the trailer air system and, when pushed in, releases the trailer spring brakes so the trailer can roll. The yellow parking brake knob controls the tractor parking brakes.

Question 10Combination Vehicle Basics and Trailer Hookup

If the trailer breaks away from the tractor, what is supposed to happen?

  1. The trailer air tanks vent to atmosphere
  2. The trailer service brakes release fully
  3. The tractor accelerates to catch up
  4. The trailer spring brakes apply automatically Correct answer.

Explanation

The breakaway valve senses the loss of air supply and applies the trailer spring brakes, stopping the trailer. This emergency feature is why the air lines and their connections must be inspected regularly.

Question 11Combination Vehicle Basics and Trailer Hookup

When should you check that the trailer spring brakes apply and release properly?

  1. During the pre-trip inspection before driving Correct answer.
  2. Only when the trailer is empty
  3. Whenever the trailer is loaded to maximum weight
  4. Only after the trailer has been in service for a month

Explanation

Spring brake operation is part of the pre-trip inspection, so any defect is found before the vehicle is on the road. Waiting until later or only checking under specific loads leaves unsafe conditions undetected.

Question 12Combination Vehicle Basics and Trailer Hookup

What is the proper way to connect the air lines between tractor and trailer?

  1. Connect both lines at the same time
  2. Connect the service (blue) line first, then the emergency (red) line
  3. Connect either line first; order does not matter
  4. Connect the emergency (red) line first, then the service (blue) line Correct answer.

Explanation

The emergency (red) line is connected first so the trailer air system charges and the spring brakes release, then the service (blue) line is connected. This order reduces the risk of the trailer rolling unexpectedly.

Question 13Combination Vehicle Basics and Trailer Hookup

You notice the trailer is not tracking in line with the tractor and the trailer appears to swing wide in turns. What should you do?

  1. Continue to the next stop and check later
  2. Increase speed to straighten the trailer
  3. Stop and inspect the coupling and hitching Correct answer.
  4. Adjust the mirrors and keep driving

Explanation

Improper tracking can indicate a coupling problem, a bent frame, or a tire issue, and should be checked immediately. Continuing to drive risks a separation or loss of control.

Question 14Combination Vehicle Basics and Trailer Hookup

What is off-tracking in a combination vehicle?

  1. The trailer brakes apply later than the tractor brakes
  2. The tractor drifts to one side on a straight road
  3. The trailer wheels follow a path inside the tractor's path in a turn Correct answer.
  4. The trailer lights flash out of sequence

Explanation

Off-tracking means the rear wheels of the trailer cut inside the path of the front wheels, so the trailer sweeps closer to the inside of a turn. Drivers must swing wider and watch the trailer's rear wheels in mirrors.

Question 15Combination Vehicle Basics and Trailer Hookup

When making a right turn in a combination vehicle, you should:

  1. Turn sharply to keep the trailer in line
  2. Turn from the left lane to widen the turn
  3. Stay in the right lane and swing the tractor slightly left before turning Correct answer.
  4. Accelerate through the turn to reduce off-tracking

Explanation

Swinging the tractor slightly left before a right turn keeps the trailer's rear wheels from cutting the corner and striking the curb or objects. Turning from the wrong lane or too sharply increases the risk of a collision.

Question 16Combination Vehicle Basics and Trailer Hookup

What is the purpose of the tractor protection valve?

  1. To release the trailer spring brakes manually
  2. To close off air to the trailer if pressure drops Correct answer.
  3. To regulate trailer light voltage
  4. To increase air pressure to the trailer

Explanation

The tractor protection valve protects the tractor air supply by closing off air to the trailer when pressure falls too low, helping prevent a complete loss of air. It also helps apply the trailer brakes in an emergency.

Question 17Combination Vehicle Basics and Trailer Hookup

During a pre-trip inspection of a combination vehicle, you should check the fifth wheel for:

  1. Excess grease on the kingpin only
  2. Tire pressure at the fifth wheel
  3. Trailer light operation
  4. Loose mounting bolts and worn jaws Correct answer.

Explanation

The fifth wheel must be secure, with tight mounting bolts and jaws that are not worn or damaged, because these parts carry the trailer and lock the kingpin. Grease condition alone does not confirm the coupling is safe.

Question 18Combination Vehicle Basics and Trailer Hookup

What should you do if the trailer air pressure gauge does not build pressure when you push in the red knob?

  1. Shut down and find the air leak or fault Correct answer.
  2. Release the yellow knob and continue
  3. Increase engine RPM and ignore the gauge
  4. Drive and let it build up on the road

Explanation

If the trailer system will not build pressure, there is a leak or a fault that must be found before driving, since the trailer brakes may not release or may not apply properly. Driving with an unknown air problem is unsafe.

Question 19Combination Vehicle Basics and Trailer Hookup

Which statement about the landing gear is correct?

  1. It should be left down to stabilize the trailer in motion
  2. It should be raised fully before the tractor pulls away Correct answer.
  3. It should be greased but never used
  4. It should be lowered while the trailer is moving

Explanation

Landing gear must be fully raised before moving, because leaving it down can damage the gear, the road, or the trailer and can cause a crash. It is used only to support an uncoupled trailer.

Question 20Combination Vehicle Basics and Trailer Hookup

What is the safest way to check that the trailer brakes are working after coupling?

  1. Release all brakes and coast downhill
  2. Pull forward slowly with the trailer brakes applied and feel resistance Correct answer.
  3. Rely on the dash warning light only
  4. Apply the hand valve at highway speed

Explanation

Pulling gently against applied trailer brakes confirms they hold, and the tractor should feel resistance. This is a controlled check done at low speed before driving.

Question 21Combination Vehicle Basics and Trailer Hookup

Why is a pre-trip walkaround especially important for combination vehicles?

  1. It allows the driver to skip the tug test
  2. It is required only for single-unit trucks
  3. It confirms the coupling, lines, and lights are secure before driving Correct answer.
  4. It replaces the need for mirror checks while driving

Explanation

A combination vehicle has more connection points-fifth wheel, air lines, electrical cord, safety latches-so a walkaround confirms everything is secure before driving. It does not replace in-motion checks or the tug test.

Question 22Combination Vehicle Basics and Trailer Hookup

When the trailer is heavily loaded, how does this affect the tractor's steering axle?

  1. It has no effect on axle loading
  2. It increases weight on the steering axle
  3. It reduces weight on the steering axle Correct answer.
  4. It locks the steering axle

Explanation

A heavy load placed behind the tractor can lift weight off the steering axle, making steering feel light and reducing traction. Load placement and weight distribution must be managed to keep enough weight on the steering axle.

Question 23Combination Vehicle Basics and Trailer Hookup

What is the proper action if you must back a trailer and cannot see behind it?

  1. Back slowly using mirrors only
  2. Ask another driver to watch from inside the cab
  3. Honk and back quickly
  4. Use a spotter or get out and look Correct answer.

Explanation

When you cannot see behind the trailer, use a spotter or get out and look before moving. Backing blind, even slowly, risks striking people or objects.

Question 24Combination Vehicle Basics and Trailer Hookup

What is the correct way to release the tractor parking brake knob (yellow) during a pre-trip check?

  1. Leave it out and start driving
  2. Push it in only after system pressure is up Correct answer.
  3. Push it in before building air pressure
  4. Pull it out while the engine is off

Explanation

The yellow knob should be pushed in only after the air system has built enough pressure to release the parking brakes safely. Releasing it without adequate pressure can leave the vehicle unable to move or hold.

Question 25Combination Vehicle Basics and Trailer Hookup

What should you check about the safety latches on the air line couplers?

  1. That they are removed before driving
  2. That they are greased daily
  3. That they are closed and locked over the couplers Correct answer.
  4. That they are left open for ventilation

Explanation

Safety latches must be closed and locked so the couplers cannot separate while driving. An open or missing latch can let an air line pop off, causing a loss of trailer brakes.

Question 26Combination Vehicle Basics and Trailer Hookup

When coupling a tractor to a trailer, what is the correct height relationship between the fifth wheel and the trailer kingpin plate before you back under the trailer?

  1. The fifth wheel should be positioned so it contacts the trailer plate and lifts it slightly Correct answer.
  2. The fifth wheel should be well above the trailer plate so it drops onto the kingpin
  3. The fifth wheel should be slightly lower so the trailer rides up onto it
  4. The fifth wheel should be far below the trailer plate to avoid striking it

Explanation

Backing under with the fifth wheel just touching and lifting the trailer plate slightly ensures the kingpin enters the jaws and the locking mechanism engages fully. If the fifth wheel is too low the tractor can strike the trailer, and if too high the kingpin can pass over the fifth wheel.

Question 27Combination Vehicle Basics and Trailer Hookup

After backing under a trailer and hearing the fifth wheel lock, what should you do before raising the landing gear?

  1. Release the trailer spring brakes and roll the trailer backward
  2. Lower the landing gear first, then check the coupling
  3. Pull the tractor forward slowly to confirm the kingpin is locked
  4. Do a visual check and a tug test to confirm the coupling is secure Correct answer.

Explanation

A visual inspection of the fifth wheel jaws plus a gentle tug test with the trailer brakes set confirms the kingpin is captured before you commit to raising the gear. Pulling forward hard or moving the trailer before confirming the lock risks a dropped trailer.

Question 28Combination Vehicle Basics and Trailer Hookup

How should you position the trailer apron (kingpin plate) relative to the fifth wheel when preparing to couple?

  1. Centered over the fifth wheel with the tractor and trailer in line Correct answer.
  2. Offset to one side so the jaws can grab the kingpin
  3. Raised well above the fifth wheel
  4. Angled so the trailer can slide into the jaws

Explanation

The tractor and trailer should be lined up and the apron centered over the fifth wheel so the kingpin enters the jaws squarely. Misalignment can damage the jaws or prevent the lock from engaging.

Question 29Combination Vehicle Basics and Trailer Hookup

When coupling the air lines, which line should you connect first and why?

  1. Either line, because the order does not matter
  2. The blue service line, because it controls the brakes
  3. The red emergency line, because it supplies air to charge the trailer system Correct answer.
  4. The electrical cord, because the trailer needs power to release its brakes

Explanation

The red emergency (supply) line charges the trailer air system and must be connected first so the trailer has air to work with. The blue service line is connected after the supply line is in place.

Question 30Combination Vehicle Basics and Trailer Hookup

What is the primary reason for locking the trailer spring brakes before you begin to couple?

  1. To keep the trailer from rolling while you back under it Correct answer.
  2. To help the kingpin align with the jaws
  3. To charge the trailer air tanks faster
  4. To test the tractor protection valve

Explanation

With the trailer spring brakes set, the trailer stays parked while you back the tractor under it. This prevents the trailer from creeping or rolling during coupling.

Question 31Combination Vehicle Basics and Trailer Hookup

What is the purpose of the trailer hand valve (trolley valve) on the steering column or dash?

  1. To apply only the trailer brakes for testing or controlling a skid Correct answer.
  2. To supply air to the trailer tanks
  3. To release the tractor parking brakes
  4. To set the trailer spring brakes for parking

Explanation

The hand valve applies the trailer service brakes only, and it is useful for checking that the trailer brakes work. It should not be used for parking because it can leak off over time.

Question 32Combination Vehicle Basics and Trailer Hookup

What does the yellow (tractor parking brake) knob do when pulled out?

  1. Applies the tractor parking brakes and can also set the trailer spring brakes Correct answer.
  2. Applies the trailer spring brakes only
  3. Releases all air from the trailer tanks
  4. Charges the trailer air system

Explanation

The yellow knob applies the tractor parking brakes and, depending on the system, also sets the trailer spring brakes. It is the parking control for the combination.

Question 33Combination Vehicle Basics and Trailer Hookup

When should you open the trailer air tank drain valves during a pre-trip inspection?

  1. Periodically to drain moisture and check for contamination Correct answer.
  2. Only after the brakes have been applied for an hour
  3. Only when the trailer is fully loaded
  4. Never, because the tanks are sealed

Explanation

Draining the air tanks removes condensed moisture that can corrode the system and freeze in cold weather. It also lets you check that the air is clean and free of oil or excess water.

Question 34Combination Vehicle Basics and Trailer Hookup

If the trailer is equipped with a converter dolly for double trailers, what does the dolly do?

  1. It converts a single trailer into a set of doubles by providing a second coupling point Correct answer.
  2. It supplies electrical power to the second trailer
  3. It raises the trailer for clearance
  4. It increases the trailer's braking capacity

Explanation

A converter dolly has its own axle and fifth wheel, allowing a second trailer to be coupled behind the first. It provides the extra articulation and coupling point needed for doubles.

Question 35Combination Vehicle Basics and Trailer Hookup

When pulling double trailers, why must you be especially careful about the rear trailer's tracking in curves?

  1. The rear trailer swings wider than a single trailer
  2. The rear trailer is heavier than the lead trailer
  3. The rear trailer brakes are always weaker
  4. The rear trailer cuts the corner more sharply than a single trailer Correct answer.

Explanation

With multiple articulation points, the rear trailer off-tracks and cuts corners more sharply than a single trailer. Drivers must allow extra room and watch the rear trailer in turns.

Question 36Combination Vehicle Basics and Trailer Hookup

What is the primary purpose of the glad hands on the air lines?

  1. To hold the electrical cord in place
  2. To drain moisture from the lines
  3. To lock the trailer to the fifth wheel
  4. To seal the couplers and prevent air leaks when lines are connected Correct answer.

Explanation

Glad hands are the couplers that connect the tractor and trailer air lines, and their seals prevent air from escaping at the joint. A poor seal causes air loss and weak brakes.

Question 37Combination Vehicle Basics and Trailer Hookup

After coupling and connecting the air lines, what should you do if you hear a steady hiss of air at the glad hands?

  1. Disconnect and reconnect the lines to reseat the seals, then recheck Correct answer.
  2. Apply the trailer hand valve to stop the leak
  3. Ignore it because some leakage is normal
  4. Increase engine RPM to overcome the leak

Explanation

A steady hiss indicates a leaking connection that will drain air and weaken the brakes. Reseating the couplers or replacing worn seals is required before driving.

Question 38Combination Vehicle Basics and Trailer Hookup

When backing a tractor-trailer, which technique helps keep the trailer tracking where you want it?

  1. Back up quickly to reduce the time the trailer drifts
  2. Rely only on the cab's rear window
  3. Turn the wheel fully in one direction and hold it
  4. Steer in small corrections and watch the trailer's rear in the mirrors Correct answer.

Explanation

Small steering corrections and continuous mirror checks let you control trailer direction while backing. Large or fast inputs cause the trailer to swing out of control.

Question 39Combination Vehicle Basics and Trailer Hookup

What should you check about the fifth wheel mounting during a pre-trip inspection?

  1. That the plate is greased only on the top surface
  2. That the jaws are welded shut
  3. That the mounting bolts are tight and the plate is securely attached to the tractor frame Correct answer.
  4. That the plate is painted a bright color

Explanation

Loose or missing mounting bolts can let the fifth wheel shift or separate from the frame, which is a serious safety hazard. The plate must be firmly bolted to the tractor.

Question 40Combination Vehicle Basics and Trailer Hookup

Why is it important to check that the trailer's spring brakes hold when you release the tractor parking brake during a pre-trip test?

  1. To check that the landing gear is raised
  2. To verify the trailer lights are working
  3. To test the electrical cord connection
  4. To confirm the trailer will not roll when parked on a grade Correct answer.

Explanation

If the trailer spring brakes fail to hold, the trailer could roll away when parked or during coupling. Testing them confirms the parking brake system is functioning.

Question 41Pre-Trip Inspection of Combination Vehicles

During a pre-trip inspection of a combination vehicle, what should you check on the fifth wheel coupling after the trailer is connected?

  1. That the trailer apron is touching the ground
  2. That the release lever is pulled out so the jaws stay open
  3. That the jaws have closed around the kingpin and the release lever is in the locked position Correct answer.
  4. That the kingpin is resting on top of the fifth wheel plate

Explanation

A secure coupling requires the fifth wheel jaws to fully close around the kingpin, with the locking mechanism engaged and the release lever secured. A pull test with the trailer brakes locked confirms the coupling will hold.

Question 42Pre-Trip Inspection of Combination Vehicles

How much slack should there normally be in the air lines between the tractor and trailer?

  1. No slack at all, with the lines pulled tight
  2. About six inches of slack regardless of trailer length
  3. Enough slack for the lines to drag on the ground
  4. Enough to allow the combination to turn and flex without pulling apart Correct answer.

Explanation

Air lines need enough slack to accommodate turning and articulation between tractor and trailer, but not so much that they sag, drag, or snag. Lines that are too tight can separate or chafe.

Question 43Pre-Trip Inspection of Combination Vehicles

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

  1. Missing seals with the couplers still connected
  2. Cracked or worn seals and properly engaged couplers Correct answer.
  3. Loose couplers that swivel freely
  4. Seals coated in oil to improve the seal

Explanation

Glad hand seals should be present, uncracked, and in good condition, and the couplers should be fully engaged and locked. Worn or missing seals cause air leaks that can reduce braking performance.

Question 44Pre-Trip Inspection of Combination Vehicles

What is the purpose of the trailer spring brakes during a pre-trip inspection?

  1. They release the trailer brakes when the supply line is charged
  2. They hold the trailer level while it is parked on a slope
  3. They provide the main service braking for the trailer during normal driving
  4. They apply automatically when air pressure is lost, so they can be used to test the coupling Correct answer.

Explanation

Spring brakes are held applied by mechanical spring force and release only when air pressure builds, so they apply automatically on an air loss. Locking them with the trailer parking valve lets you pull-test the coupling.

Question 45Pre-Trip Inspection of Combination Vehicles

Before backing under a trailer, what should you verify about the trailer height relative to the fifth wheel?

  1. The trailer apron is well above the fifth wheel so it drops onto the plate
  2. The trailer apron is slightly lower than the fifth wheel so the plate lifts it Correct answer.
  3. The trailer landing gear is fully retracted
  4. The trailer kingpin is removed

Explanation

The trailer should be positioned so the fifth wheel will contact the apron and lift the trailer slightly as you back under, allowing the kingpin to enter the jaws. If the trailer is too high, the kingpin can strike and damage the fifth wheel.

Question 46Pre-Trip Inspection of Combination Vehicles

What should you check on the landing gear during a pre-trip inspection?

  1. That the gear is partly lowered to support the trailer
  2. That the gear is fully raised, the crank is secured, and there are no cracks or bends Correct answer.
  3. That the crank handle is removed and stored in the cab
  4. That the gear feet are buried in the ground

Explanation

Landing gear must be fully raised so it clears the road, with the crank secured and the legs, braces, and feet free of damage. Damaged or partly lowered gear can drag or collapse.

Question 47Pre-Trip Inspection of Combination Vehicles

How should you check the trailer lights during a pre-trip inspection?

  1. Turn on the lights and walk around to confirm clearance, brake, turn, and marker lights work Correct answer.
  2. Rely on the trailer ABS light to confirm all lights work
  3. Have someone watch while you sound the horn
  4. Check only the brake lights from the cab

Explanation

All required trailer lights, including clearance, marker, brake, turn, and tail lights, must be checked for proper operation, typically by turning them on and walking around the vehicle. The ABS light does not confirm lamp function.

Question 48Pre-Trip Inspection of Combination Vehicles

During a pre-trip inspection, what should you check on the tractor protection valve?

  1. That it is capped off with tape
  2. That it is in place and not damaged, and that it holds air when the trailer supply is closed Correct answer.
  3. That it is removed so air can flow freely
  4. That it leaks air continuously to signal it is working

Explanation

The tractor protection valve keeps air from escaping from the tractor if the trailer breaks away or the lines separate. It must be intact and able to hold air when the trailer supply valve is closed.

Question 49Pre-Trip Inspection of Combination Vehicles

After coupling to a trailer, you perform a pull test. What does a proper pull test confirm?

  1. That the fifth wheel jaws are locked and will not release under load Correct answer.
  2. That the landing gear is fully raised
  3. That the air lines are the correct length
  4. That the trailer brakes are adjusted correctly

Explanation

In a pull test you set the trailer brakes, release the tractor brakes, and gently pull forward; if the coupling is secure the tractor will not move away from the trailer. This verifies the jaws are locked around the kingpin.

Question 50Pre-Trip Inspection of Combination Vehicles

You are inspecting a combination vehicle and find a small air leak at a glad hand. What is the correct action?

  1. Tighten the glad hand by hand and continue
  2. Ignore it if the leak is small
  3. Repair or replace the seal or coupler before operating the vehicle Correct answer.
  4. Increase engine RPM to compensate for the leak

Explanation

Any air leak in the coupling system can reduce braking capability and must be corrected before the vehicle is operated. Small leaks grow and can cause spring brakes to apply unexpectedly.