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Mississippi CDL Combination study materials
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Mississippi CDL Combination practice questions

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Question 1Driving Combination Vehicles Safely

In a combination vehicle, what is the primary purpose of the trailer hand valve (trolley valve)?

  1. To operate the tractor's front brakes when the foot pedal fails
  2. To set the trailer spring brakes when parking on a grade
  3. To release air pressure from the trailer supply line during coupling
  4. To apply the trailer service brakes independently for a test or to help control a skid Correct answer.

Explanation

The hand valve controls the trailer service brakes only, which is why drivers use it to check that trailer brakes work or to help straighten a skid. It is not a parking brake and does not affect the tractor's brakes.

Question 2Driving Combination Vehicles Safely

When coupling a tractor to a semitrailer, at what point should you back under the trailer?

  1. After locking the fifth wheel with the release arm
  2. After raising the landing gear fully
  3. After the trailer has been blocked and the kingpin is aligned with the fifth wheel Correct answer.
  4. After connecting the air lines to the trailer

Explanation

The trailer must be blocked so it cannot roll and the kingpin must line up with the fifth wheel opening before backing under. Backing under an unblocked or misaligned trailer can cause the trailer to move or the tractor to strike it.

Question 3Driving Combination Vehicles Safely

After coupling, you tug-test the tractor against the trailer with the trailer spring brakes set. What does a successful tug test confirm?

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

Explanation

Pulling gently against a braked trailer with the tractor shows whether the fifth wheel has actually latched onto the kingpin. If the jaws are not closed, the trailer will not hold and the tractor will pull out from under it.

Question 4Driving Combination Vehicles Safely

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

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

Explanation

You must set the trailer brakes, lower the landing gear to firm ground, and disconnect the air lines before releasing the fifth wheel. Releasing the fifth wheel first can drop the trailer onto the tractor frame.

Question 5Driving Combination Vehicles Safely

Which condition indicates a trailer has a serious air leak in its brake system?

  1. The hand valve applies the trailer brakes firmly
  2. The trailer air gauge drops quickly and the low-pressure warning sounds Correct answer.
  3. The trailer brakes release slowly when the supply knob is pushed in
  4. The tractor air compressor cycles on and off normally

Explanation

A rapid pressure drop on the gauge with the warning buzzer or light indicates a leak the system cannot keep up with. This must be repaired before the vehicle is driven.

Question 6Driving Combination Vehicles Safely

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

  1. It closes the tractor air supply if the trailer breaks away or loses air Correct answer.
  2. It locks the fifth wheel during hard braking
  3. It balances air pressure between the steer and drive axles
  4. It limits tractor speed when the trailer is loaded

Explanation

The tractor protection valve closes off the tractor's air supply when pressure drops too low or the trailer separates. This preserves enough air to stop the tractor if the trailer is lost.

Question 7Driving Combination Vehicles Safely

Why does a combination vehicle need more stopping distance than a single-unit truck of similar weight?

  1. The tractor's steer axle carries less weight when towing
  2. The trailer brakes are only applied by the hand valve
  3. The combination has more total weight and the trailer can push the tractor forward Correct answer.
  4. The trailer tires have less traction than tractor tires

Explanation

A combination carries more total weight and the trailer's momentum can push the tractor during braking, lengthening stopping distance. Drivers must allow more following distance.

Question 8Driving Combination Vehicles Safely

What is off-tracking in a combination vehicle?

  1. The tendency of the trailer to swing wide when backing
  2. The tendency of the trailer to sway at highway speed
  3. The rear wheels of the trailer following a shorter path than the tractor's front wheels in a turn Correct answer.
  4. The loss of air pressure when the trailer is disconnected

Explanation

Off-tracking means the rear trailer wheels cut inside the path of the tractor's front wheels in a turn. Drivers must swing wider to keep the trailer from striking curbs or other vehicles.

Question 9Driving Combination Vehicles Safely

When making a right turn in a combination vehicle, what should you do to account for off-tracking?

  1. Swing into the left lane only after checking that it is clear, then complete the turn Correct answer.
  2. Stay in the right lane and turn tightly to keep traffic behind you
  3. Turn from the left lane and swing wide to the left before turning right
  4. Turn sharply and accelerate through the turn

Explanation

To keep the trailer's rear wheels from cutting the corner, the driver may swing wide to the left, but only after checking that the left lane is clear. Turning tightly risks the trailer striking the curb or a vehicle.

Question 10Driving Combination Vehicles Safely

When backing a tractor-semitrailer, which technique helps keep the trailer tracking straight?

  1. Turn the steering wheel sharply in the direction you want the trailer to go
  2. Use the mirrors and make small steering corrections, backing slowly Correct answer.
  3. Rely on the backup alarm to warn others
  4. Back quickly to reduce the time in reverse

Explanation

Backing a combination vehicle requires slow speed and small, controlled steering inputs while watching the mirrors. Sharp corrections can cause the trailer to jackknife.

Question 11Driving Combination Vehicles Safely

What causes a tractor-trailer to jackknife during braking?

  1. The hand valve is left open during normal driving
  2. The drive wheels of the tractor lock and lose traction while the trailer pushes forward Correct answer.
  3. The trailer is loaded heavier than the tractor
  4. The trailer brakes lock while the tractor wheels keep rolling

Explanation

A jackknife happens when the tractor's drive wheels lock and slide while the trailer keeps pushing, causing the combination to fold. Proper braking and speed control reduce this risk.

Question 12Driving Combination Vehicles Safely

What is the safest response if the trailer begins to skid while you are braking?

  1. Hold the brakes firmly and steer opposite the skid
  2. Apply the hand valve fully to straighten the trailer
  3. Accelerate hard to pull the trailer straight
  4. Release the brakes and steer in the direction you want to go Correct answer.

Explanation

Releasing the brakes allows the wheels to roll again and regain traction, and steering in the intended direction helps straighten the vehicle. Holding the brakes or steering against the skid can worsen it.

Question 13Driving Combination Vehicles Safely

What should you do before driving a combination vehicle with a new or changed load?

  1. Rely on the shipper's assurance that the load is legal
  2. Adjust the mirrors and drive immediately
  3. Check that the load is secured and the trailer is properly coupled Correct answer.
  4. Increase speed gradually to test the brakes

Explanation

The driver is responsible for ensuring the load is secured and the coupling is correct before moving. A walk-around inspection and coupling check prevent load shifts and separations.

Question 14Driving Combination Vehicles Safely

Why is it important to check the trailer's sliding tandem position before driving?

  1. It affects weight distribution on the axles and handling Correct answer.
  2. It controls the trailer's spring brakes
  3. It changes the color of the trailer lights
  4. It determines the trailer's air pressure

Explanation

Moving the tandem axles changes how weight is distributed among the tractor and trailer axles, which affects legal axle loads and stability. A poor position can overload axles or make handling unsafe.

Question 15Driving Combination Vehicles Safely

What is a key difference in handling between a combination vehicle and a single-unit truck in a curve?

  1. A single-unit truck has a higher rollover risk
  2. There is no difference in rollover risk
  3. A combination vehicle tends to roll over at lower speeds in a curve Correct answer.
  4. A combination vehicle can take curves faster because of the trailer's weight

Explanation

Combination vehicles have a higher center of gravity and longer wheelbase, making them more prone to rollover in curves. Drivers must slow down more than they would in a single-unit truck.

Question 16Driving Combination Vehicles Safely

How does the length of a combination vehicle affect passing and lane changes?

  1. It makes the vehicle easier to see, so less caution is needed
  2. It allows quicker lane changes because of the trailer's momentum
  3. It requires more time and space to complete a pass or lane change Correct answer.
  4. It has no effect on passing maneuvers

Explanation

A longer combination needs more time and distance to pass or change lanes, and the driver must ensure the trailer has cleared before returning to the lane. Cutting back too soon can strike another vehicle.

Question 17Driving Combination Vehicles Safely

What should you check when inspecting the fifth wheel before a trip?

  1. That the fifth wheel is tilted downward
  2. That the release arm is left in the open position for easy coupling
  3. That the jaws are closed and the release arm is in the locked position Correct answer.
  4. That the fifth wheel is greased only on the top surface

Explanation

The fifth wheel jaws must be fully closed around the kingpin and the release arm must be locked. An open release arm or unlatched jaws can allow the trailer to separate.

Question 18Driving Combination Vehicles Safely

What is the purpose of the trailer air supply knob (red knob) on the dashboard?

  1. To apply the trailer brakes only when parking
  2. To adjust the trailer's suspension height
  3. To supply or cut off air to the trailer's brake system Correct answer.
  4. To control the trailer's electrical lights

Explanation

The red trailer air supply knob controls air flow to the trailer. Pulling it out cuts off supply and sets the trailer spring brakes; pushing it in charges the trailer system.

Question 19Driving Combination Vehicles Safely

When should you use the trailer hand valve while driving?

  1. To hold the vehicle on a hill
  2. Only for testing trailer brakes or to help control a skid Correct answer.
  3. Whenever you need to stop quickly
  4. Regularly to save the tractor brakes

Explanation

The hand valve is intended for testing trailer brakes and for helping straighten a skid. Using it as a service brake can cause the trailer to swing or jackknife.

Question 20Driving Combination Vehicles Safely

What is the recommended following distance for a combination vehicle at highway speed?

  1. Four seconds total regardless of speed
  2. One second total
  3. Two seconds total
  4. One second for each 10 feet of vehicle length Correct answer.

Explanation

A common rule is at least one second of following distance for each 10 feet of vehicle length at speeds under 40 mph, with an extra second added at higher speeds. This allows for the longer stopping distance of a combination.

Question 21Driving Combination Vehicles Safely

Why should you avoid using the trailer hand valve to hold a combination vehicle on a hill?

  1. It prevents the trailer lights from working
  2. It can cause the trailer brakes to overheat and fail Correct answer.
  3. It drains the tractor's coolant
  4. It disables the tractor's steering

Explanation

Holding the vehicle with the hand valve keeps the trailer brakes applied without the tractor brakes, which can overheat and fade the trailer brakes. The service brake or parking brake should be used instead.

Question 22Driving Combination Vehicles Safely

What should you do if the trailer begins to swing out while you are driving on a slippery road?

  1. Brake hard and hold the wheel straight
  2. Steer in the direction the trailer is swinging and ease off the accelerator Correct answer.
  3. Accelerate to pull it straight
  4. Apply the hand valve fully

Explanation

Steering in the direction of the trailer's swing and easing off the accelerator helps bring the trailer back in line. Hard braking or acceleration can worsen the sway.

Question 23Driving Combination Vehicles Safely

What is the correct way to test the trailer brakes with the hand valve before a trip?

  1. With the vehicle stopped, apply the hand valve and try to pull forward slowly Correct answer.
  2. Apply the hand valve while the trailer air supply knob is pulled out
  3. Test the hand valve only after the trailer is loaded
  4. Drive at highway speed and apply the hand valve fully

Explanation

With the vehicle stopped and the trailer brakes set by the hand valve, a gentle pull forward should show whether the trailer brakes hold. This confirms the trailer brakes are working before driving.

Question 24Driving Combination Vehicles Safely

What is the purpose of the trailer's spring brakes?

  1. To provide normal service braking
  2. To increase trailer speed on downgrades
  3. To control the trailer's lights
  4. To apply automatically when air pressure is lost or the supply line is disconnected Correct answer.

Explanation

Spring brakes are held off by air pressure and apply automatically when pressure drops or the supply line is disconnected. This provides a fail-safe parking and emergency brake.

Question 25Driving Combination Vehicles Safely

Why is it important to check that the trailer's landing gear is fully raised before driving?

  1. It keeps the trailer lights working
  2. It prevents the landing gear from striking the road or railroad tracks Correct answer.
  3. It increases the trailer's air pressure
  4. It improves fuel economy

Explanation

Landing gear left partly down can strike the road surface, railroad tracks, or other objects, causing damage or a crash. It must be fully raised and secured before moving.

Question 26Driving Combination Vehicles Safely

In a combination vehicle, what does the yellow parking brake knob control when it is pulled out?

  1. It applies only the tractor steer-axle brakes
  2. It applies the spring brakes on the tractor and trailer together Correct answer.
  3. It releases only the trailer spring brakes while the tractor stays braked
  4. It modulates trailer service brakes proportionally to pedal pressure

Explanation

The yellow knob is the parking brake control. Pulling it out exhausts air from the spring brake chambers on both the tractor and the trailer, so all spring brakes apply. Pushing it in charges those chambers and releases the parking brakes.

Question 27Driving Combination Vehicles Safely

You are coupling a tractor to a loaded semitrailer. After backing under the trailer, at what point should you tug-test the coupling?

  1. After raising the landing gear and connecting the air lines
  2. Only after you have driven a few hundred feet at low speed
  3. Before raising the landing gear, with the trailer brakes released
  4. After locking the fifth wheel but before connecting any air lines, with the trailer spring brakes set Correct answer.

Explanation

The tug test confirms the kingpin is locked in the fifth wheel. With the trailer spring brakes set, pull gently forward against them; if the trailer holds and the tractor does not pull out from under it, the coupling is secure. Do this before raising the landing gear so the trailer cannot drop.

Question 28Driving Combination Vehicles Safely

What is the purpose of the supply line (emergency line) in a tractor-trailer air brake system?

  1. It vents trailer brake chambers directly to atmosphere during a stop
  2. It carries the air pressure that releases the trailer spring brakes and signals a breakaway Correct answer.
  3. It delivers modulated service brake pressure to the trailer relay valve
  4. It supplies air only to the trailer's suspension and tires

Explanation

The supply line charges the trailer reservoirs and holds off the trailer spring brakes. If it loses pressure or separates, the tractor protection valve closes and the trailer spring brakes apply, which is why it is also called the emergency line.

Question 29Driving Combination Vehicles Safely

You are driving a loaded tractor-semitrailer and the trailer begins to pass you on the left during a hard stop. What is the best immediate action?

  1. Accelerate briefly to pull the trailer back in line
  2. Apply the trailer hand valve fully to straighten the rig
  3. Release the brakes completely and steer hard right
  4. Keep steady pressure on the brakes and steer in the direction the trailer is moving Correct answer.

Explanation

A trailer swinging out means its wheels have lost lateral grip and the rig is folding. Releasing the brakes restores wheel roll and lets the trailer realign, and steering in the direction of the swing keeps the tractor under the trailer. Stabbing the hand valve or accelerating worsens the fold.

Question 30Driving Combination Vehicles Safely

Before descending a long steep grade with a heavy combination vehicle, which practice best protects the brakes?

  1. Shift to a low gear before the descent and use light, steady braking as needed Correct answer.
  2. Coast in neutral to save fuel and brake only at the bottom
  3. Stay in high gear and brake firmly whenever speed climbs above the limit
  4. Rely on the trailer hand valve to control speed down the grade

Explanation

Selecting a low gear before the grade lets engine drag hold speed and keeps the service brakes cool for when they are needed. Light, steady applications are far safer than heavy repeated braking, which overheats drums and fades the brakes.

Question 31Driving Combination Vehicles Safely

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

  1. Lower the landing gear, release the fifth wheel, then disconnect the air lines
  2. Lower the landing gear, disconnect the air lines and electrical cord, then release the fifth wheel Correct answer.
  3. Disconnect the air lines, lower the landing gear, then release the fifth wheel
  4. Release the fifth wheel first, then lower the landing gear and disconnect the air lines

Explanation

The landing gear must carry the trailer before the tractor is freed, and the air lines and cord must be separated so they are not torn off when the tractor pulls away. Only then is the fifth wheel released.

Question 32Driving Combination Vehicles Safely

During a pre-trip inspection of a combination vehicle, you find the trailer's pushrod stroke exceeds the adjustment limit. What does this indicate?

  1. The spring brake chamber has failed and must be replaced
  2. The supply line is leaking at the gladhand
  3. The slack adjuster is out of adjustment and the wheel brake may not apply fully Correct answer.
  4. The trailer relay valve is stuck in the released position

Explanation

Excessive pushrod travel means the slack adjuster is out of adjustment, so the brake chamber stroke is used up before the shoe contacts the drum. Braking force at that wheel is reduced and the vehicle should be placed out of service until corrected.

Question 33Driving Combination Vehicles Safely

Why must you check that the trailer's air line gladhands are sealed and locked when coupling?

  1. To ensure the trailer lights operate during the trip
  2. To keep water from entering the trailer's suspension system
  3. To prevent air loss that could drain the trailer reservoirs and apply the spring brakes Correct answer.
  4. To keep the electrical cord from chafing on the trailer frame

Explanation

Gladhands that are not locked and sealed leak air from the supply and control lines. Enough leakage drains the trailer reservoirs, and low pressure causes the spring brakes to apply and can trigger the tractor protection valve, bringing the combination to a stop.

Question 34Driving Combination Vehicles Safely

You are approaching a right turn in a long combination vehicle and there is a vehicle stopped in the lane to your right. What is the safest action?

  1. Turn from the left lane to give the trailer more room
  2. Swing wide to the left and complete the turn without stopping
  3. Stop before the turn and wait until the right side is clear enough to swing wide Correct answer.
  4. Sound the horn and proceed since you have the right of way

Explanation

A long combination needs extra room on the right because the rear wheels cut the corner. If a vehicle is in that space, waiting until it clears prevents crushing it against the curb. Turning from the left lane or swinging blindly creates new conflicts.

Question 35Driving Combination Vehicles Safely

What is the correct procedure for testing that the tractor protection valve is working?

  1. Build full pressure, then hold the foot pedal down for one minute and watch the gauge
  2. Turn the engine off and check that the valve bleeds off within thirty seconds
  3. Build full pressure, then open a trailer supply line gladhand and watch for the valve to close near 45 psi Correct answer.
  4. Release the parking brake and listen for the valve to click open

Explanation

The tractor protection valve is designed to close when supply pressure drops to roughly 45 psi, trapping the remaining tractor air and applying the trailer spring brakes. Fanning down by opening a gladhand is the standard way to confirm it closes.

Question 36Driving Combination Vehicles Safely

When should you activate the trailer hand valve while driving a combination vehicle?

  1. To hold the vehicle stationary at a loading dock
  2. When descending a steep grade to save the tractor brakes
  3. Whenever you need to slow down more quickly than the foot pedal allows
  4. Only to test the trailer brakes before a trip Correct answer.

Explanation

The hand valve is a test device. Using it to stop or hold the vehicle applies trailer brakes alone, which can cause a trailer skid or jackknife, and it does not apply the tractor brakes. It should be used only to check trailer brake operation.

Question 37Driving Combination Vehicles Safely

What is the effect of a sliding trailer tandem moved all the way to the rear on a loaded combination vehicle?

  1. It shifts weight to the tractor drive axles and reduces trailer axle loading
  2. It increases the load on the trailer axles and lightens the steering axle Correct answer.
  3. It reduces off-tracking but overloads the trailer tires
  4. It has no effect on axle weights but improves turning radius

Explanation

Moving the tandem rearward places more of the trailer's weight behind the trailer axles and takes weight off the tractor, including the steering axle. That can overload trailer tires and make steering feel light, so axle weights must be checked after any adjustment.

Question 38Driving Combination Vehicles Safely

What should you do with the trailer spring brakes before backing a tractor-semitrailer under a trailer to couple?

  1. Leave them set so the trailer stays put during coupling Correct answer.
  2. Release them so the trailer can roll slightly if bumped
  3. Release them and chock only the rear trailer wheels
  4. Apply them only after the fifth wheel has locked

Explanation

The trailer spring brakes should stay applied so the trailer cannot roll when the tractor contacts it. They also make the tug test meaningful, because the tractor can pull against a braked trailer to prove the kingpin is locked.

Question 39Driving Combination Vehicles Safely

You are driving a combination vehicle at highway speed when the trailer starts to fishtail on a wet road. What is the best response?

  1. Release the accelerator and steer gently to keep the tractor aligned with the trailer Correct answer.
  2. Apply the trailer hand valve to settle the trailer
  3. Accelerate slightly and steer in the direction the trailer is moving
  4. Brake hard and hold the steering wheel straight

Explanation

Easing off the throttle reduces speed without locking wheels, and gentle steering keeps the tractor pointed where the trailer is heading so the rig can straighten. Hard braking or trailer-only braking removes grip and turns a fishtail into a jackknife.

Question 40Driving Combination Vehicles Safely

Which combination of conditions most increases the risk of a rollover in a combination vehicle?

  1. A low center of gravity, a light load, and a downhill grade
  2. A high center of gravity, a full load, and a sweeping curve taken at speed Correct answer.
  3. A short wheelbase, a level road, and moderate acceleration
  4. An empty trailer, a straight road, and light braking

Explanation

Rollover risk rises with a high center of gravity and with speed carried into a curve, because centrifugal force grows quickly with speed. A loaded trailer raises the center of gravity, and the trailer can roll before the tractor shows much lean.

Question 41Air Brakes and Braking Systems

On a combination vehicle with air brakes, what does the red octagonal trailer air supply valve (trailer protection valve) do when pulled out?

  1. It isolates the trailer relay valve from the tractor protection valve
  2. It closes the trailer supply line and applies the trailer spring brakes Correct answer.
  3. It charges the trailer tanks while releasing the tractor parking brakes
  4. It releases air from the trailer service line to modulate braking

Explanation

Pulling the red trailer air supply knob closes the supply line to the trailer, which drops the trailer reservoir pressure and causes the trailer spring brakes to set. It also works with the tractor protection valve to keep the tractor from losing air through a broken trailer line. Pushing the knob in restores supply so the trailer brakes can release.

Question 42Air Brakes and Braking Systems

In a dual air brake system, roughly how does the pressure available for braking compare between the two circuits?

  1. Both circuits share a single reservoir and drop together
  2. The front circuit always has priority and drains the rear
  3. The rear circuit has no reservoir and relies on the front circuit
  4. Each circuit has its own reservoir and is protected by a one-way check valve Correct answer.

Explanation

A dual air system splits into two separate circuits, each with its own reservoir, so a failure in one does not empty the other. One-way check valves prevent air from flowing back out of a good circuit when the other fails. This is why a single circuit failure still leaves partial braking.

Question 43Air Brakes and Braking Systems

After you release the parking brake control in a combination vehicle with air brakes, what should you check before driving?

  1. That the tractor and trailer parking brakes have fully released and the vehicle rolls freely Correct answer.
  2. That the low air pressure warning has sounded and reset
  3. That the trailer supply valve is still pulled out
  4. That the compressor is unloaded and the governor has cut out

Explanation

A pre-trip check confirms that spring brakes release fully on both tractor and trailer, since a dragging brake can overheat or prevent movement. If the vehicle will not roll freely, a parking brake may be stuck applied. The trailer supply valve must be pushed in, not pulled out, for normal driving.

Question 44Air Brakes and Braking Systems

During a static leak test on a combination vehicle with the engine off and brakes released, pressure should not drop more than about how much in one minute?

  1. 20 psi
  2. 1 psi
  3. 10 psi
  4. 3 psi Correct answer.

Explanation

With a fully charged system, engine off, and brakes released, leakage should be no more than about 3 psi per minute in a combination vehicle. A larger drop indicates a leak that must be found and repaired. The applied-brake leak test allows a somewhat higher limit.

Question 45Air Brakes and Braking Systems

How is the slack adjuster related to the brake chamber pushrod on an air-braked axle?

  1. It vents moisture from the brake chamber
  2. It stores air for the spring brake during parking
  3. It converts pushrod stroke into rotation of the brake camshaft Correct answer.
  4. It measures reservoir pressure and signals the governor

Explanation

The slack adjuster is the lever arm that connects the pushrod to the brake camshaft, turning linear pushrod travel into camshaft rotation. Its length setting determines how much stroke is needed to apply the brakes. Excess stroke usually means the slack adjuster needs adjustment or a component is worn.

Question 46Air Brakes and Braking Systems

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

  1. 100 psi
  2. 60 psi Correct answer.
  3. 80 psi
  4. 30 psi

Explanation

Federal standards require a visible and audible warning before air pressure falls below 60 psi. At that point the spring brakes may begin to apply automatically. Drivers should stop and find the leak rather than continue.

Question 47Air Brakes and Braking Systems

What is the function of the relay valve on a trailer air brake system?

  1. It applies air from the trailer reservoir to the brake chambers when signaled by the service line Correct answer.
  2. It holds the trailer spring brakes off during normal operation
  3. It filters oil and water from the compressor output
  4. It reduces reservoir pressure to the trailer supply line

Explanation

The relay valve uses a small service line signal to open a large passage from the trailer reservoir to the brake chambers. This shortens the time needed to apply and release trailer brakes on a long vehicle. It does not set or hold off spring brakes.

Question 48Air Brakes and Braking Systems

A driver performs a tug test after coupling a tractor to a trailer. What does a successful tug test confirm?

  1. The trailer air lines are connected in the correct order
  2. The fifth wheel jaws are locked around the kingpin Correct answer.
  3. The trailer spring brakes are fully released
  4. The landing gear is fully retracted

Explanation

With the trailer brakes set, pulling gently against the trailer checks that the fifth wheel jaws have closed on the kingpin. If the jaws are not locked, the trailer will creep or separate from the tractor. The test does not verify air line routing or landing gear position.

Question 49Air Brakes and Braking Systems

Why should you avoid fanning the brake pedal in a combination vehicle with air brakes?

  1. It disables the low pressure warning
  2. It releases the trailer spring brakes unexpectedly
  3. It causes the relay valve to reverse airflow
  4. It can exhaust air faster than the compressor can rebuild it Correct answer.

Explanation

Each brake application uses reservoir air, and repeated fanning can drop pressure below the warning threshold before the compressor catches up. This can lead to automatic spring brake application or loss of braking. Steady, controlled pressure is preferred on long downgrades.

Question 50Air Brakes and Braking Systems

On a long downgrade, which technique best controls speed in a combination vehicle with air brakes?

  1. Shift to neutral and rely on the service brakes
  2. Use the trailer hand valve to brake the trailer only
  3. Hold the brake pedal firmly the entire way down
  4. Select a low gear and use light, steady brake applications with engine braking Correct answer.

Explanation

A low gear lets engine braking carry much of the load, and short steady applications keep the brakes from overheating. Holding the brakes continuously can cause fade or heat cracks. The trailer hand valve is for testing, not for controlling speed on a grade.