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Maryland CDL General Knowledge study materials
Maryland CDL General Knowledge study resources

Maryland CDL General Knowledge practice questions

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Question 1General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

During a pre-trip inspection, what should you check on the slack adjusters of a vehicle with air brakes?

  1. That they are painted with the date of the last adjustment
  2. That they move no more than about one inch when pulled by hand Correct answer.
  3. That they rotate freely with the pushrod disconnected
  4. That they are fully extended before the trip begins

Explanation

With the brakes released and system aired up, a properly adjusted slack adjuster should not move more than about one inch when you pull it by hand. Greater movement means the brakes may be out of adjustment and the vehicle should be placed out of service until corrected.

Question 2General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

When you must back a tractor-trailer and cannot avoid it, the safest starting position is to back toward which side?

  1. The right, or driver's, side
  2. Whichever side has more room
  3. The side away from the sun
  4. The left side Correct answer.

Explanation

Backing toward the left, or driver's, side gives you a better view of the trailer and lets you look out the driver's side window during the maneuver. Backing toward the right reduces visibility and should be avoided whenever possible.

Question 3General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

You are driving a loaded combination vehicle at highway speed and need to slow for a curve. What is the correct technique?

  1. Stay in the highest gear and let engine drag slow you
  2. Shift to a lower gear before entering the curve, then brake gently through it Correct answer.
  3. Coast through the curve in neutral to save the brakes
  4. Brake hard while in the curve, then downshift as you exit

Explanation

Slow down and downshift before the curve so the vehicle is at a safe speed and in the right gear on entry. Braking hard inside a curve can cause a skid or rollover, so any braking there should be gentle and gradual.

Question 4General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

How does the safe following distance for a 70-foot combination vehicle at 55 mph compare with that of a passenger car?

  1. It is half the car gap because truck brakes are stronger
  2. It must be at least one second for each 10 feet of vehicle length at speeds above 40 mph Correct answer.
  3. It is the same one-second gap used by cars
  4. It is measured only in the rain and snow

Explanation

For speeds above 40 mph, the rule is at least one second of following distance for each 10 feet of vehicle length, which gives a 70-foot rig roughly seven seconds. Add one extra second for speeds over 40 mph in many state guides, and more in poor conditions.

Question 5General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

As you approach a railroad crossing in a commercial vehicle, which action is required before crossing?

  1. Stop and look both ways, and listen with the window open if you are carrying passengers or hazmat Correct answer.
  2. Sound the horn continuously from a quarter mile away
  3. Shift to a higher gear to cross more quickly
  4. Slow to 25 mph and proceed without stopping if no train is visible

Explanation

Certain vehicles, including those carrying passengers or hazardous materials, must stop at railroad crossings and check in both directions before proceeding. Opening the window and listening helps detect an approaching train that may be hidden from view.

Question 6General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

When driving at night, at what point should you switch from high beams to low beams for oncoming traffic?

  1. At about 100 feet, just before passing
  2. Only after the other driver dims first
  3. Within about 500 feet of an oncoming vehicle Correct answer.
  4. Never, because trucks need maximum visibility

Explanation

Dim your high beams within about 500 feet of an oncoming vehicle so you do not blind its driver. When following another vehicle, dim within about 300 feet, and always dim for pedestrians and in fog or heavy rain.

Question 7General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

What is the correct way to drive down a long, steep mountain grade in a heavy vehicle?

  1. Select a low gear before the descent and use engine braking, braking only as needed Correct answer.
  2. Ride the brakes lightly the whole way down to hold a steady speed
  3. Descend in neutral to let gravity do the work
  4. Use the highest gear and brake hard at each curve

Explanation

Choose a gear low enough that engine compression holds your speed, and apply the brakes only to control speed. Continuous light braking overheats the drums and can lead to brake fade or total loss of braking.

Question 8General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

You have been driving for hours and notice you are yawning, drifting in the lane, and unable to keep your eyes focused. What should you do?

  1. Stop at a safe place and rest or sleep before continuing Correct answer.
  2. Open the window, turn up the radio, and keep driving
  3. Drink coffee and continue for several more hours
  4. Speed up to reach your destination sooner

Explanation

Yawning, lane drift, and difficulty focusing are warning signs of fatigue that will not be fixed by fresh air, music, or caffeine. The only reliable remedy is to stop in a safe location and rest or sleep before resuming the trip.

Question 9General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

What does a blood alcohol concentration of 0.04 mean for a commercial driver operating a CMV?

  1. The driver may drive if no erratic behavior is observed
  2. The driver may drive after a 30-minute waiting period
  3. The driver is considered under the influence and is prohibited from driving Correct answer.
  4. The driver is legal because the limit for CDL holders is 0.08

Explanation

Commercial drivers are held to a stricter standard: a BAC of 0.04 or higher while operating a CMV is a violation, and any measurable alcohol can justify being placed out of service. The 0.08 limit applies to non-commercial drivers, not CDL holders on the job.

Question 10General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

Your vehicle begins to skid on a wet road. What is the recommended recovery technique?

  1. Turn the wheel sharply back and forth to regain grip
  2. Brake hard and steer opposite the skid
  3. Stop braking, steer in the direction you want the vehicle to go, and avoid overcorrecting Correct answer.
  4. Accelerate to pull the vehicle straight

Explanation

Release the brakes so the wheels can roll and steer where you want the front of the vehicle to go, making smooth corrections. Hard braking or overcorrecting can turn a simple skid into a jackknife or rollover.

Question 11General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

What is the purpose of the retarder on a heavy vehicle, and when should it be turned off?

  1. It slows the vehicle on downgrades and should be shut off on slippery surfaces Correct answer.
  2. It is used only for parking on hills
  3. It cools the brakes during long descents
  4. It replaces the service brakes entirely and stays on at all times

Explanation

A retarder, such as an engine or driveline retarder, helps control speed on long grades without heating the service brakes. On wet, icy, or slippery pavement it can cause the drive wheels to lose traction, so it should be switched off there.

Question 12General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

You are driving a 12-foot-6-inch vehicle and see a low clearance sign reading 12 feet 4 inches. What should you do?

  1. Deflate the tires slightly to gain clearance
  2. Take another route, because your vehicle is taller than the posted clearance Correct answer.
  3. Proceed slowly, since the sign usually allows a small margin
  4. Assume the sign is wrong and continue

Explanation

If your vehicle's height exceeds the posted clearance, you cannot safely pass under the structure and must find another route. Signs are posted at the actual clearance, so there is no guaranteed margin for error.

Question 13General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

When may you use the lights on your vehicle to warn other drivers that you are about to back up?

  1. Backup lights are enough, so no walk-around is needed
  2. Flashing high beams signals others to clear the way
  3. Honking twice replaces the need to check
  4. You should not rely on backup lights; get out and check behind the vehicle first Correct answer.

Explanation

Backup lights and alarms do not guarantee the area behind the vehicle is clear, so you should walk around and check before backing. If you cannot see behind the trailer, use a helper who stays in view in your mirror.

Question 14General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

What is the correct response if your front wheels leave the paved surface onto the shoulder?

  1. Hold the wheel firmly, ease off the accelerator, and steer gently back onto the pavement Correct answer.
  2. Brake hard and jerk the wheel back onto the road
  3. Accelerate and steer sharply to climb back on
  4. Apply the parking brake to slow quickly

Explanation

Grip the wheel, slow gradually by easing off the accelerator, and steer back onto the pavement with small, smooth inputs once you have slowed. Hard braking or sharp steering on a soft shoulder can pull the vehicle into a rollover or across the road.

Question 15General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

How should you position your vehicle in the lane when strong crosswinds are pushing against a tall trailer?

  1. Keep both hands on the wheel and reduce speed, staying centered with small corrections Correct answer.
  2. Move to the shoulder and stop in the travel lane
  3. Steer into the wind and hold the wheel turned
  4. Speed up to push through the gusts

Explanation

In strong crosswinds, slow down and make small, steady steering corrections to stay centered, keeping both hands on the wheel. Speeding up or holding the wheel turned can cause overcorrection when the gust ends.

Question 16General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

Before shifting a manual transmission, what must you do to change gears smoothly in a heavy vehicle?

  1. Skip gears to save time and fuel
  2. Double-clutch by releasing the accelerator, pressing the clutch, shifting to neutral, and matching engine speed Correct answer.
  3. Hold the clutch partly engaged while accelerating
  4. Shift without using the clutch at all

Explanation

Double-clutching lets the engine and transmission speeds match so gears engage without grinding. You release the accelerator, press the clutch, shift to neutral, release the clutch, then press it again and select the next gear.

Question 17General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

What is the safest way to handle a curve when driving a tanker with a high center of gravity?

  1. Enter the curve at the posted limit and brake in the middle
  2. Accelerate through the curve to stabilize the load
  3. Shift gears while in the curve
  4. Slow to a safe speed before the curve and avoid braking hard inside it Correct answer.

Explanation

High-center-of-gravity vehicles such as tankers are prone to rollover, so speed must be reduced before the curve and steering kept smooth. Braking hard or shifting mid-curve upsets the vehicle and can cause a rollover.

Question 18General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

What should you do if your brakes fail while descending a grade?

  1. Downshift, use the emergency or parking brake gradually, and look for an escape ramp Correct answer.
  2. Pump the accelerator to build engine braking
  3. Steer into the opposing lane to warn traffic
  4. Turn off the engine and coast to a stop

Explanation

Downshift to use engine braking, apply the emergency or parking brake gradually to avoid locking the wheels, and steer toward an escape ramp or other safe runoff area. Shutting off the engine can lock the steering and remove power assist.

Question 19General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

How far ahead should you look while driving a commercial vehicle at highway speed?

  1. About 30 seconds ahead at all times
  2. About 2 seconds ahead
  3. Only as far as the vehicle in front
  4. About 12 to 15 seconds ahead of the vehicle Correct answer.

Explanation

Aim your view about 12 to 15 seconds ahead, which at highway speed is roughly a quarter mile, so you can spot hazards and plan ahead. At lower speeds, look closer, but always scan beyond the vehicle directly in front.

Question 20General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

When is it legal and safe to use the horn on a commercial vehicle?

  1. Only as a warning of danger, not to express irritation Correct answer.
  2. Whenever traffic is moving slowly
  3. Continuously while passing through intersections
  4. To greet other drivers you recognize

Explanation

The horn is a safety device meant to warn others of danger, such as a vehicle drifting into your lane. Using it to express frustration or greet others can startle drivers and is often restricted by local rules.

Question 21General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

You are entering a curve too fast and feel the trailer start to roll. What is the correct response?

  1. Steer sharply to the outside of the curve
  2. Ease off the accelerator smoothly and avoid hard braking or sharp steering Correct answer.
  3. Brake hard immediately to shed speed
  4. Accelerate to shift weight forward

Explanation

If a rollover begins, hard braking or sharp steering can trigger it, so ease off the accelerator smoothly while keeping the wheel steady. The goal is to reduce speed gradually without upsetting the load further.

Question 22General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

What is required before driving a vehicle with air brakes after the system has been drained of pressure?

  1. Pump the brake pedal repeatedly while driving
  2. Build pressure to the governed level and check for leaks and proper brake response Correct answer.
  3. Release the parking brake and go without checking
  4. Drive immediately, since the compressor will catch up

Explanation

After air pressure is lost, run the engine until the compressor builds pressure to the normal governed range, then check the system for leaks and confirm the brakes respond properly. Driving with low pressure can leave you without service brakes.

Question 23General Knowledge (vehicle inspection, basic control, shifting, backing, speed management, space management, hazard perception, emergency procedures, railroad crossings, night driving, mountain driving, fatigue, alcohol and drugs)

What is the effect of using a higher gear than necessary while climbing a steep grade with a heavy load?

  1. The transmission locks out lower gears automatically
  2. The vehicle climbs faster with better fuel economy
  3. The engine cools better in a higher gear
  4. The engine may lug and lose power, so you should downshift before the grade Correct answer.

Explanation

Climbing in too high a gear makes the engine lug, lose power, and possibly stall, so shift to a lower gear before the grade begins. Waiting until the engine bogs down leaves you struggling for power on the slope.

Question 24Air Brakes

On a vehicle with air brakes, what is the purpose of the supply reservoir (often called the wet tank)?

  1. It receives air directly from the compressor and traps moisture and oil before air moves to the other tanks Correct answer.
  2. It regulates the compressor so it never exceeds 100 psi
  3. It stores air only for the parking brake system
  4. It holds hydraulic fluid that assists the air compressor

Explanation

The compressor feeds the supply reservoir first, and because compressed air cools there, water and oil drop out of the airstream. Air then passes through a one-way check valve to the primary and secondary reservoirs. This arrangement keeps contaminants out of the working circuits.

Question 25Air Brakes

The governor on an air brake system performs which function?

  1. It balances air pressure between the left and right brake chambers
  2. It drains moisture from the supply reservoir automatically
  3. It controls when the compressor builds air and when it stops Correct answer.
  4. It limits how far the brake pedal can be pushed down

Explanation

The governor senses reservoir pressure and signals the compressor to load or unload. It typically cuts the compressor in at about 100 psi and cuts it out around 125 psi. This cycling keeps system pressure within the safe operating range.

Question 26Air Brakes

During the static leak test on a straight truck with a fully charged system and the engine off, what is the maximum allowable pressure loss with the brakes released?

  1. 4 psi in one minute
  2. 5 psi in two minutes
  3. 2 psi in one minute Correct answer.
  4. 3 psi in one minute

Explanation

With a fully charged system, engine off, and brakes released, pressure should not drop more than 2 psi in one minute for a single vehicle. A loss faster than that indicates a leak that must be found and repaired before driving. Combination vehicles are allowed 3 psi in one minute.

Question 27Air Brakes

After you press and hold the brake pedal firmly during the applied leak test, pressure should not drop more than:

  1. 3 psi in one minute for a combination vehicle Correct answer.
  2. 10 psi in three minutes
  3. 1 psi in one minute for a straight truck
  4. 6 psi in one minute for any vehicle

Explanation

With the brakes applied and held, the allowable loss is 3 psi in one minute for a combination vehicle and 4 psi in one minute for a single vehicle. A steady pedal with only slight drop is normal. A rapid drop means a leak in the applied circuit.

Question 28Air Brakes

What does the low air pressure warning device on an air brake vehicle need to do?

  1. Sound only when the parking brake is released
  2. Give a visible and audible warning before pressure drops below 60 psi Correct answer.
  3. Flash a light when pressure exceeds 125 psi
  4. Warn the driver only after the spring brakes have fully applied

Explanation

The warning must activate before air pressure falls below 60 psi so the driver has time to stop safely. It is both a light and a buzzer on most vehicles. Waiting until the spring brakes apply leaves no margin for a controlled stop.

Question 29Air Brakes

You are inspecting a tractor-trailer and find that the spring brakes have not applied even though system pressure has dropped well below the warning level. What is the most likely explanation?

  1. The low pressure warning buzzer is disconnected
  2. The compressor is cycling normally
  3. The governor cut-out pressure is set too high
  4. The emergency relay valve or spring brake circuit is faulty Correct answer.

Explanation

Spring brakes apply automatically when air pressure drops because springs push the chambers on. If they fail to apply at low pressure, the emergency/spring brake control circuit is not releasing air as it should. This is a serious defect that puts the vehicle out of service.

Question 30Air Brakes

Before driving a vehicle with air brakes, the air pressure should build from 85 to 100 psi within what time frame?

  1. 1 minute for a single vehicle and 2 minutes for a combination
  2. 10 seconds for any vehicle
  3. 3 minutes for a single vehicle and 4 minutes for a combination
  4. 45 seconds for both single and combination vehicles Correct answer.

Explanation

The build-up test requires pressure to rise from 85 to 100 psi within 45 seconds on both single and combination vehicles. Faster build-up is acceptable; slower build-up indicates a compressor or governor problem. This test is done with the engine at operating rpm.

Question 31Air Brakes

How do you perform the test that checks whether the spring brakes will apply automatically when air is lost?

  1. Drain all air from the system and watch for the spring brakes to apply Correct answer.
  2. Hold the brake pedal down for one minute and watch for pressure loss
  3. Release the parking brake and time how long the compressor takes to build pressure
  4. Pump the pedal five times with the engine running

Explanation

With the wheels chocked and the parking brake released, you drain the reservoirs and confirm the spring brakes apply as pressure falls. The parking brake control should then be pulled and the vehicle should not move. This verifies the emergency braking function.

Question 32Air Brakes

What is the function of the slack adjusters in an air brake system?

  1. They regulate compressor output pressure
  2. They store reserve air for emergency braking
  3. They drain condensation from the reservoirs
  4. They keep the brake shoes close to the drums as linings wear Correct answer.

Explanation

Slack adjusters take up the gap between the brake shoes and the drums as the linings wear. Proper adjustment keeps pushrod stroke within limits so the brakes apply fully. Excessive pushrod travel means the brakes are out of adjustment.

Question 33Air Brakes

On a vehicle with automatic slack adjusters, when is it acceptable to manually adjust them?

  1. Whenever the pushrod stroke appears long, as a routine practice
  2. Never, under any circumstances
  3. Only during a brake repair or when the manufacturer's procedure calls for it Correct answer.
  4. Every time you do the pre-trip inspection

Explanation

Automatic slack adjusters are meant to adjust themselves during normal brake use. Manual adjustment is limited to brake repairs or when the manufacturer's procedure requires it. Routinely adjusting them can mask a defect and is not a substitute for repair.

Question 34Air Brakes

On a vehicle with air brakes, what does the alcohol evaporator do?

  1. It adds alcohol to the air system to prevent moisture from freezing in the lines Correct answer.
  2. It lubricates the compressor's internal bearings
  3. It cleans the brake shoes of oil and grease
  4. It reduces the pressure delivered to the front brakes

Explanation

An alcohol evaporator introduces alcohol vapor into the air system so condensed moisture does not freeze in valves and lines during cold weather. It supplements, but does not replace, regular draining of the reservoirs. It is not a cleaning or lubrication device.

Question 35Air Brakes

Why should you drain air from the reservoirs at the end of each day of driving?

  1. To remove water and oil that accumulate in the tanks Correct answer.
  2. To release the spring brakes for parking
  3. To reset the governor cut-in pressure
  4. To test the low pressure warning light

Explanation

Compressed air carries moisture and oil that condense in the reservoirs, and draining removes them before they reach valves and cause freeze-ups or corrosion. Most tanks have manual drain cocks at the bottom. Automatic drain valves do this on some vehicles.

Question 36Air Brakes

You are driving a loaded tractor-trailer down a long mountain grade. What is the correct technique for using the brakes?

  1. Use a low gear and apply the brakes firmly enough to feel a noticeable slowing, then release and repeat Correct answer.
  2. Shift to neutral and use the service brakes only
  3. Rely on the engine brake alone and never touch the service brakes
  4. Hold light steady pressure on the pedal all the way down

Explanation

Brakes should be used in short, firm applications that noticeably reduce speed, then released to cool. A low gear and engine braking help control speed. Light continuous pressure overheats the drums and can lead to brake fade.

Question 37Air Brakes

What causes brake fade on an air brake vehicle?

  1. Air pressure builds too quickly in the reservoirs
  2. The slack adjusters are set too tight
  3. The low pressure warning buzzer stays on
  4. Excessive heat from overuse of the brakes reduces friction Correct answer.

Explanation

Overheated drums and linings lose friction, so the brakes become less effective even though the pedal feels normal. Fade is best prevented by controlling speed with gearing and using short, firm brake applications. Once fade occurs, stopping distance increases sharply.

Question 38Air Brakes

On a combination vehicle, why is the trailer required to have its own air reservoir and spring brakes?

  1. To increase the total air pressure available to the tractor
  2. To let the trailer's brakes be applied with hydraulic pressure
  3. To allow the trailer to brake and park even if it separates from the tractor Correct answer.
  4. To reduce the weight of the trailer's brake chambers

Explanation

Trailer reservoirs and spring brakes let the trailer stop and hold itself if the air lines break or the trailer separates. The emergency relay valve applies the trailer brakes when supply pressure is lost. This is a key safety feature of combination air brake systems.

Question 39Air Brakes

What is the purpose of the tractor protection valve?

  1. It releases the trailer spring brakes during parking
  2. It prevents the compressor from building more than 100 psi
  3. It closes the air supply to the trailer if the tractor's air pressure drops to a low level Correct answer.
  4. It regulates the pressure in the front steering axle brakes

Explanation

The tractor protection valve cuts off air to the trailer when tractor pressure falls too low, which helps keep enough air for the tractor's own brakes. It also closes the lines if the trailer breaks away. This protects the tractor's ability to stop.

Question 40Air Brakes

How can you check that the trailer air supply and service lines are properly connected and sealed?

  1. Charge the system, shut the engine off, and confirm pressure loss stays within the allowed limits Correct answer.
  2. Disconnect the lines and see if air escapes
  3. Watch for pressure to drop on the trailer gauge when you release the parking brake
  4. Listen for a steady hiss at the glad hands

Explanation

After charging the system and shutting off the engine, watch the gauges to confirm the loss stays within the allowed limits. A leaking glad hand or line shows up as a faster drop. Applying the brakes and repeating the check tests the service line under pressure.

Question 41Air Brakes

What is the difference between the primary and secondary air reservoirs on a dual circuit system?

  1. The primary serves the rear brakes and the secondary serves the front brakes, so one circuit can still stop the vehicle Correct answer.
  2. The primary is used only when parked and the secondary only when moving
  3. The primary holds air for the horn and the secondary holds air for the brakes
  4. The primary is pressurized by the compressor and the secondary is pressurized by the engine

Explanation

Dual circuits split the brakes so a failure in one circuit still leaves braking on the other. Typically one circuit serves the rear brakes and the other the front. This redundancy is why the system is called dual circuit.

Question 42Air Brakes

A driver notices the air pressure gauge shows pressure rising above 130 psi and the safety valve has not released. What should the driver do?

  1. Stop and have the system checked, since the governor or safety valve is not working Correct answer.
  2. Open a reservoir drain cock and keep driving
  3. Continue driving because high pressure is harmless
  4. Shift to a higher gear to lower compressor speed

Explanation

The governor should unload the compressor at about 125 psi, and a safety valve provides backup protection. Pressure climbing above that range means one of those devices has failed. Continuing to drive risks bursting lines or components, so the vehicle should be taken out of service for repair.

Question 43Air Brakes

When should you use the parking brake control (yellow diamond knob) on an air brake vehicle?

  1. Whenever you park, after the spring brakes have applied Correct answer.
  2. Whenever you want to test the service brakes
  3. Only while driving on wet roads
  4. Only when the low pressure warning sounds

Explanation

The parking brake control releases air so the spring brakes apply and hold the vehicle when parked. It should be pulled after stopping and before leaving the vehicle. It is not a substitute for the service brakes while driving.

Question 44Air Brakes

What does the brake pedal (treadle valve) do in an air brake system?

  1. It sends air pressure to the brake chambers in proportion to how far it is pushed Correct answer.
  2. It mechanically pulls the brake shoes against the drums
  3. It signals the governor to start the compressor
  4. It opens the drain valves to release moisture

Explanation

The treadle valve meters air from the reservoirs to the brake chambers based on pedal travel. More pedal means more air pressure and stronger braking. Light pedal travel gives light braking, which helps the driver control application.

Question 45Air Brakes

You must make an emergency stop on a vehicle with air brakes, and the service brakes seem weak. What is the correct action?

  1. Shift to neutral and coast to a stop
  2. Apply the parking brake control to bring on the spring brakes Correct answer.
  3. Turn off the engine to increase braking power
  4. Pump the brake pedal rapidly several times

Explanation

If service brakes are weak, the parking brake control applies the spring brakes for emergency stopping. Pumping the pedal uses up reserve air and can make things worse. Shutting off the engine does not increase braking and may disable power steering.

Question 46Air Brakes

What is the correct way to test the low air pressure warning device during a pre-trip inspection?

  1. Disconnect the warning buzzer and listen for silence
  2. Release the parking brake and check that the light stays off
  3. Start the engine and watch the gauge rise to cut-out
  4. With the engine off, pump the brake pedal to reduce pressure and watch for the warning before 60 psi Correct answer.

Explanation

With the engine off, fanning the brake pedal lowers system pressure so you can confirm the warning activates before 60 psi. The light and buzzer should come on together. Pressure should then be allowed to rebuild before driving.

Question 47Combination Vehicles

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

  1. Use the mirrors and back slowly without leaving the cab
  2. Rely on the backup alarm to warn anyone behind
  3. Get out and check behind the trailer, or use a helper who stays visible Correct answer.
  4. Sound the horn twice and back immediately

Explanation

A driver cannot see directly behind a trailer from the cab, and mirrors leave large blind spots near the rear corners. The safe practice is to walk back and look before backing, or to use a helper who stays in view in the mirror and signals you.

Question 48Combination Vehicles

When you are coupling a tractor to a trailer, how should the trailer be positioned relative to the tractor?

  1. Trailer raised so the kingpin is above the fifth wheel plate
  2. Trailer tilted down so the kingpin noses into the plate
  3. Trailer angled sharply to one side of the tractor
  4. Trailer level so the kingpin is slightly below the fifth wheel Correct answer.

Explanation

The trailer should be positioned so the kingpin is slightly lower than the fifth wheel, letting the plate ride up under the trailer as you back in. If the trailer is too high, the tractor can strike the trailer and the kingpin may pass over the plate.

Question 49Combination Vehicles

What is the purpose of the trailer air supply control (the red button) in a tractor-trailer?

  1. It dumps air from the trailer tanks to test leaks
  2. It charges the trailer air tanks and releases the trailer spring brakes Correct answer.
  3. It releases the tractor parking brake only
  4. It controls the trailer service brakes during normal stops

Explanation

The red trailer air supply knob sends air to the trailer and releases its spring parking brakes when pushed in. Pulling it out shuts off supply air and applies the trailer spring brakes.

Question 50Combination Vehicles

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

  1. Testing the trailer brakes while moving Correct answer.
  2. Releasing the trailer spring brakes
  3. Making normal stops in traffic
  4. Parking the combination on a grade

Explanation

The hand valve applies only the trailer brakes, so it is useful for checking that the trailer brakes work when you pull the knob while rolling slowly. It is not a parking brake, and normal stops are made with the foot brake.