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Minnesota CDL Air Brakes Test study materials
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Minnesota CDL Air Brakes Test practice questions

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Question 1Air Brake System Components and Fundamentals

In a typical air brake system, what is the primary function of the compressor?

  1. It releases air pressure from the spring brakes
  2. It builds and maintains air pressure in the storage tanks Correct answer.
  3. It filters moisture and oil out of the air supply
  4. It converts air pressure into mechanical force at the wheel

Explanation

The compressor is engine-driven and pumps air into the system to build and hold the required pressure in the storage tanks. It does not apply brakes, release parking brakes, or filter air; those are handled by other components. Without adequate compressor output, pressure will not recover after repeated brake use.

Question 2Air Brake System Components and Fundamentals

What is the purpose of the air dryer in an air brake system?

  1. It stores a reserve of air for emergency braking
  2. It removes moisture and contaminants from the compressed air Correct answer.
  3. It regulates the pressure delivered to the brake chambers
  4. It cools the compressor outlet to increase airflow

Explanation

An air dryer is installed between the compressor and the storage tanks to remove water vapor and contaminants from the compressed air. Removing moisture helps prevent corrosion and freeze-ups in valves and lines. Pressure regulation and air storage are functions of the governor and tanks, not the dryer.

Question 3Air Brake System Components and Fundamentals

The governor on an air brake system controls which of the following?

  1. The rate at which the spring brakes apply
  2. The compressor's cut-in and cut-out pressure points Correct answer.
  3. The pressure shown on the dash gauge only
  4. The amount of air sent to the rear brake chambers

Explanation

The governor senses system pressure and engages or disengages the compressor at preset cut-in and cut-out pressures. This keeps tank pressure within the normal operating range. It does not meter air to individual brake chambers or affect spring brake application timing.

Question 4Air Brake System Components and Fundamentals

What is the function of the alcohol evaporator or injector sometimes found on air brake systems?

  1. It lubricates the compressor's internal moving parts
  2. It introduces alcohol into the air system to prevent moisture from freezing Correct answer.
  3. It increases the compressor's cut-out pressure
  4. It seals small leaks in the air lines

Explanation

An alcohol evaporator adds alcohol vapor to the compressed air so that any moisture in the system is less likely to freeze in cold weather. This protects valves and lines from ice blockage. It does not lubricate the compressor, change governor settings, or seal leaks.

Question 5Air Brake System Components and Fundamentals

A driver notices that air pressure builds very slowly after starting the engine. Which component is the most likely cause?

  1. A misadjusted slack adjuster
  2. A worn or malfunctioning compressor Correct answer.
  3. A leaking service brake chamber diaphragm
  4. A stuck relay valve in the open position

Explanation

Slow pressure buildup points to a problem on the supply side, most often a weak or worn compressor or a slipping compressor drive belt. Service brake and relay valve problems typically cause pressure loss or brake drag rather than slow buildup. Slack adjuster adjustment affects brake stroke, not air production.

Question 6Air Brake System Components and Fundamentals

What does the safety valve on an air tank do?

  1. It drains accumulated moisture when the engine is shut off
  2. It releases air if tank pressure exceeds a preset maximum Correct answer.
  3. It supplies air to the trailer emergency line
  4. It prevents air from flowing back into the compressor

Explanation

The safety valve is a pressure-relief device that opens if tank pressure rises above its preset limit, protecting the system from over-pressurization. It is not a drain valve and does not control airflow direction. It also has no role in supplying the trailer emergency line.

Question 7Air Brake System Components and Fundamentals

What is the purpose of the drain cock located at the bottom of an air storage tank?

  1. To supply air to the low-pressure warning buzzer
  2. To release excess pressure during compressor cycling
  3. To equalize pressure between the wet and dry tanks
  4. To allow accumulated water and oil to be drained from the tank Correct answer.

Explanation

Moisture and oil collect at the bottom of air tanks, and the drain cock lets the driver remove them manually. Draining tanks regularly helps prevent corrosion and freeze-ups. The drain cock does not regulate pressure, feed the warning buzzer, or equalize tanks.

Question 8Air Brake System Components and Fundamentals

Which component applies the brakes when the driver presses the brake pedal?

  1. The safety valve
  2. The foot valve (treadle valve) Correct answer.
  3. The governor
  4. The alcohol evaporator

Explanation

The foot valve, also called the treadle valve, meters air from the supply tanks to the brake chambers when the pedal is pressed. It is the driver's primary control over service braking. The governor, safety valve, and evaporator serve supply-side functions and do not apply the brakes.

Question 9Air Brake System Components and Fundamentals

What is the function of a relay valve in an air brake system?

  1. It automatically applies the spring brakes when pressure is lost
  2. It reduces tank pressure to a safe operating level
  3. It filters oil from the compressed air before it reaches the foot valve
  4. It speeds up brake application and release at the wheels by using air from a nearby tank Correct answer.

Explanation

A relay valve uses a small control signal from the foot valve to send a large volume of air from a nearby reservoir directly to the brake chambers. This shortens the time needed to apply and release brakes, especially on long vehicles. It does not regulate tank pressure, apply spring brakes, or filter air.

Question 10Air Brake System Components and Fundamentals

Why is a spring brake chamber designed so that the brakes apply when air pressure is lost?

  1. To prevent the service brakes from overheating
  2. To provide a fail-safe parking and emergency brake Correct answer.
  3. To reduce the amount of air the compressor must produce
  4. To allow the driver to adjust slack automatically

Explanation

Spring brakes use a powerful spring to apply the brakes when air pressure is released, so a loss of pressure results in braking rather than a runaway vehicle. This provides fail-safe emergency and parking braking. They do not reduce compressor demand, adjust slack, or prevent overheating.

Question 11Air Brake System Components and Fundamentals

In a dual air brake system, what is the purpose of having two separate air circuits?

  1. To eliminate the need for a parking brake
  2. To double the compressor's output pressure
  3. To allow the trailer to be operated without a tractor
  4. To provide a backup circuit so partial braking remains if one circuit fails Correct answer.

Explanation

A dual circuit system separates the front and rear (or primary and secondary) brake circuits so that a failure in one circuit still leaves braking capability in the other. This improves safety over a single-circuit system. It does not eliminate the parking brake or increase compressor output.

Question 12Air Brake System Components and Fundamentals

What does the low air pressure warning device indicate to the driver?

  1. That system pressure has dropped to an unsafe level and braking may be affected Correct answer.
  2. That the air dryer needs replacement
  3. That the trailer has been disconnected
  4. That the compressor has reached its cut-out pressure

Explanation

The low pressure warning, typically a buzzer and light, activates when system pressure falls below a safe operating level, warning the driver that braking performance may be compromised. The driver should stop safely and investigate. It does not signal normal cut-out, dryer condition, or trailer connection.

Question 13Air Brake System Components and Fundamentals

A driver hears the low air pressure warning activate while driving and the gauge shows pressure dropping steadily. What is the safest immediate action?

  1. Shift to a lower gear and use engine braking while moving to a safe stop Correct answer.
  2. Continue driving to the nearest terminal at normal speed
  3. Turn off the warning device and monitor the gauge
  4. Pump the brake pedal repeatedly to rebuild pressure

Explanation

A steady pressure drop with the warning active means braking capability is being lost, so the driver should reduce speed using engine braking and gearing and stop in a safe location as soon as possible. Pumping the pedal uses more air and worsens the problem. Continuing at speed or disabling the warning is unsafe.

Question 14Air Brake System Components and Fundamentals

What is the function of the tractor protection valve?

  1. It drains moisture from the trailer tanks
  2. It closes the tractor air lines to the trailer when pressure drops, helping protect the tractor's air supply Correct answer.
  3. It applies the tractor's spring brakes during normal driving
  4. It regulates the compressor's cut-in pressure

Explanation

The tractor protection valve senses tractor air pressure and closes off the trailer supply lines if pressure drops too low or the trailer breaks away. This preserves the tractor's remaining air for its own brakes. It does not regulate compressor cycling, apply spring brakes during normal driving, or drain trailer tanks.

Question 15Air Brake System Components and Fundamentals

What is the purpose of the trailer air supply valve (also called the red trailer valve)?

  1. It controls service brake application on the trailer only
  2. It measures the trailer's air pressure for the dash gauge
  3. It supplies or cuts off air to the trailer and can set the trailer spring brakes Correct answer.
  4. It adjusts the trailer's slack adjusters automatically

Explanation

The trailer air supply valve controls air to the trailer's supply line; pulling it out cuts off supply and causes the trailer spring brakes to apply. It is used for parking and for emergencies. It does not meter service braking, adjust slack, or serve as the pressure gauge sender.

Question 16Air Brake System Components and Fundamentals

What does a quick-release valve do in an air brake system?

  1. It vents excess pressure from the compressor inlet
  2. It rapidly refills the wet tank after each brake application
  3. It quickly applies the spring brakes in an emergency
  4. It speeds up the release of air from brake chambers when the brake is let off Correct answer.

Explanation

A quick-release valve exhausts air from the brake chambers close to the wheels when the brake pedal is released, shortening release time, especially on long wheelbases. It does not apply spring brakes, refill the wet tank, or vent the compressor inlet.

Question 17Air Brake System Components and Fundamentals

What is the function of a pressure protection valve in the air brake system?

  1. It controls the amount of air delivered to each brake chamber
  2. It relieves pressure from the tanks when the engine is shut down
  3. It converts air pressure into a mechanical pushrod stroke
  4. It prevents air from flowing to non-safety accessories until braking pressure is adequate Correct answer.

Explanation

A pressure protection valve holds back air to non-essential accessories, such as air horns or seats, until system pressure reaches a safe level. This ensures the brakes get priority for available air. It does not relieve tank pressure, meter brake chamber air, or produce pushrod stroke.

Question 18Air Brake System Components and Fundamentals

A driver applies the brakes and notices that the pushrod stroke on one chamber is longer than the others and the brake feels weak. Which component is most likely worn?

  1. The governor
  2. The alcohol evaporator
  3. The slack adjuster or the brake chamber Correct answer.
  4. The safety valve

Explanation

Excessive pushrod travel on one wheel usually indicates a worn slack adjuster or a brake chamber problem such as a leaking diaphragm or a broken return spring. This reduces braking force at that wheel. The governor, safety valve, and evaporator do not affect individual pushrod stroke.

Question 19Air Brake System Components and Fundamentals

What is meant by the term 'cut-out pressure' in an air brake system?

  1. The pressure at which the low air warning activates
  2. The pressure at which the compressor stops pumping air into the tanks Correct answer.
  3. The pressure at which the safety valve opens
  4. The pressure at which the spring brakes release

Explanation

Cut-out pressure is the upper set point at which the governor disengages the compressor so tank pressure stops rising. Cut-in pressure is the lower point at which the compressor restarts. The warning device and safety valve operate at different, lower or higher thresholds.

Question 20Air Brake System Components and Fundamentals

Why should air tanks be drained of moisture regularly, particularly in cold weather?

  1. To keep the slack adjusters properly lubricated
  2. To increase the compressor's output volume
  3. To reduce the cut-out pressure setting automatically
  4. To prevent water from freezing and blocking valves and lines Correct answer.

Explanation

Water that accumulates in the tanks can travel through the system and freeze in cold weather, blocking valves and lines and disabling the brakes. Regular draining removes this moisture before it causes problems. Draining does not change compressor output, governor settings, or slack adjuster lubrication.

Question 21Air Brake System Components and Fundamentals

During a pre-trip inspection, the driver builds normal pressure and then shuts off the engine. Air pressure drops noticeably within a minute with no brake applied. What does this most likely indicate?

  1. That the alcohol evaporator is working properly
  2. That the governor cut-out pressure is set too high
  3. An air leak somewhere in the system that should be repaired Correct answer.
  4. Normal operation because the compressor is off

Explanation

With the engine off and brakes released, pressure should hold fairly steady; a noticeable drop indicates a leak in the system. Leaks must be found and repaired before driving. Normal systems do not lose significant pressure simply because the compressor is off.

Question 22Air Brake System Components and Fundamentals

What is the function of the brake chamber diaphragm?

  1. It regulates the pressure delivered from the foot valve
  2. It stores a reserve of air for emergency braking
  3. It seals the chamber so air pressure can move the pushrod Correct answer.
  4. It mechanically adjusts the slack in the brake linkage

Explanation

The diaphragm is a flexible seal inside the brake chamber that separates the pressurized air side from the pushrod side. When air pressure enters, it flexes and pushes the pushrod to apply the brake. It does not store air, regulate supply pressure, or adjust slack.

Question 23Air Brake System Components and Fundamentals

A driver wants to test the service brakes for air loss. After building full pressure, which procedure best checks for excessive leakage?

  1. Drain all air from the tanks and restart the engine
  2. Fan the brake pedal and watch how fast pressure drops
  3. Release the parking brake and accelerate to highway speed
  4. Fully apply the brakes and hold them while watching the gauge for a steady drop Correct answer.

Explanation

A service brake leak test is done by building full pressure, fully applying and holding the brakes, and watching the gauge for an excessive steady drop. This reveals leaks in the applied brake system. Fanning the pedal tests recovery, not static leakage, and draining all air is unsafe.

Question 24Air Brake System Components and Fundamentals

Which statement best describes how the air compressor is typically driven on a heavy truck?

  1. It is driven by exhaust gas pressure
  2. It is driven by an electric motor powered by the battery
  3. It is driven by the engine through a belt or gear drive Correct answer.
  4. It is driven by the vehicle's wheel rotation

Explanation

The air compressor is mounted on the engine and driven by a belt, gear, or direct drive from the engine. This lets it produce compressed air whenever the engine runs. It is not electrically driven, exhaust-driven, or powered by wheel rotation.

Question 25Air Brake System Components and Fundamentals

What does the supply line (emergency line) on a trailer air brake system do?

  1. It supplies air to charge the trailer tanks and releases the trailer spring brakes Correct answer.
  2. It carries the air that applies the trailer service brakes during normal braking
  3. It carries electrical signals for the trailer lights
  4. It drains moisture from the trailer tanks automatically

Explanation

The supply, or emergency, line charges the trailer's air tanks and provides the pressure that keeps the trailer spring brakes released. The service line carries the signal that applies service brakes. The supply line is not electrical and does not drain tanks.

Question 26Air Brake System Components and Fundamentals

Which component stores compressed air so that the brakes can be applied several times after the compressor stops running?

  1. The governor
  2. The air storage tank Correct answer.
  3. The brake shoe
  4. The slack adjuster

Explanation

Air storage tanks hold the compressed air that the compressor produces, and that reserve lets the driver make repeated brake applications even when the engine and compressor are no longer supplying air. Without stored volume, any single pedal application would use up the available pressure almost immediately.

Question 27Air Brake System Components and Fundamentals

What is the function of the governor in a heavy-truck air brake system?

  1. It automatically drains moisture from the supply tank each cycle
  2. It reduces the pressure sent to the trailer service line during a brake application
  3. It limits how far each brake chamber pushrod can travel
  4. It regulates compressor loading and unloading to hold system pressure within a set range Correct answer.

Explanation

The governor senses system pressure and signals the compressor to load, building pressure, or unload, stopping the build. This cycling keeps reservoir pressure between the cut-in and cut-out values instead of rising without limit or falling too low.

Question 28Air Brake System Components and Fundamentals

Which component converts air pressure into the mechanical force that moves the slack adjuster and applies the brake?

  1. The low pressure indicator
  2. The safety valve
  3. The alcohol evaporator
  4. The brake chamber Correct answer.

Explanation

The brake chamber receives air pressure on its diaphragm, which pushes a plate and pushrod outward. That pushrod motion is transferred through the slack adjuster to the foundation brake, creating the applying force.

Question 29Air Brake System Components and Fundamentals

In an air brake system, what does the foot valve (treadle valve) do?

  1. It sets the compressor cut-out pressure each time the pedal is released
  2. It holds the trailer supply line charged while the parking brakes are set
  3. It automatically adjusts slack in the brake linkage as linings wear
  4. It meters air pressure to the brake chambers in proportion to how far the pedal is pushed Correct answer.

Explanation

The foot valve is the driver's service brake control. It meters air from the reservoirs toward the brake chambers, so pushing farther increases the delivered pressure and the braking force.

Question 30Air Brake System Components and Fundamentals

A driver notices that the air pressure gauge climbs to about 120 psi, then holds steady and the compressor can be heard unloading. A short time later pressure falls to roughly 100 psi and the compressor loads again. What does this behavior indicate?

  1. The compressor is worn and cannot build pressure above 100 psi
  2. The safety valve is venting excess pressure from the wet tank
  3. The foot valve is leaking air into the service line
  4. The governor is cycling the compressor between its cut-out and cut-in pressures Correct answer.

Explanation

Normal operation has the compressor load at the cut-in pressure and unload at the cut-out pressure, which is exactly the rise-and-hold then fall-and-reload pattern described. A steady hold at the upper value with a later reload near the lower value is the governor doing its job.

Question 31Air Brake System Components and Fundamentals

Which component protects the service brake chambers from being fed by the parking brake circuit, and also keeps the parking spring from releasing until enough pressure is available?

  1. The spring brake control valve Correct answer.
  2. The relay valve
  3. The alcohol evaporator
  4. The quick-release valve

Explanation

The spring brake control valve (parking brake valve) controls air to the spring brake chambers. It releases the parking spring only when system pressure is sufficient, and it prevents service and parking functions from being applied against each other.

Question 32Air Brake System Components and Fundamentals

What is the purpose of the alcohol evaporator (alcohol injector) installed on some air brake systems?

  1. It adds alcohol vapor to the air system to reduce moisture freezing in valves and lines Correct answer.
  2. It lubricates the compressor by mixing oil with the air supply
  3. It absorbs and removes oil that passes the compressor rings
  4. It raises system pressure when the compressor cannot keep up

Explanation

Alcohol evaporators introduce alcohol vapor into the air stream so that condensed moisture in tanks, valves, and lines is less likely to freeze in cold weather. They supplement, but do not replace, regular tank draining.

Question 33Air Brake System Components and Fundamentals

A driver wants to check that the compressor is building pressure properly during a pre-trip inspection. Which observation confirms the compressor is working?

  1. With the parking brake released, the trailer wheels roll freely
  2. With the engine running and the system below cut-out, the gauge pressure rises steadily Correct answer.
  3. With the engine off, the gauge pressure holds steady for one minute
  4. With the engine at idle, the low pressure warning buzzer stays silent

Explanation

The compressor's job is to build pressure, so a steady rise on the gauge while the engine runs and the system is below cut-out shows it is loading and pumping. A gauge that will not climb points to a compressor, governor, or leak problem.

Question 34Air Brake System Components and Fundamentals

On a vehicle with a dual air brake system, what is the function of the check valves located between the supply reservoir and the individual circuits?

  1. They limit the maximum pressure that any brake chamber can receive
  2. They automatically drain moisture from each circuit's reservoir
  3. They release the parking spring when pressure drops below a set value
  4. They prevent air from one circuit from flowing back into the other circuit or the supply tank Correct answer.

Explanation

Check valves are one-way devices that keep the circuits isolated. If one circuit loses pressure, the check valves keep that loss from draining the other circuit, which preserves a braking capability.

Question 35Air Brake System Components and Fundamentals

A driver applies the brakes and hears a continuous hiss from one brake chamber area that continues even after the pedal is released. Which condition does this most likely point to?

  1. The safety valve opening to relieve overpressure
  2. The governor unloading the compressor at cut-out pressure
  3. A leaking diaphragm or chamber seal that is losing air from the service side Correct answer.
  4. A normal quick-release valve exhausting after the pedal is released

Explanation

A steady leak that continues after the pedal is released suggests a failed diaphragm or seal in the service side of that chamber, because the air is escaping instead of holding the pushrod out. A normal release exhaust is brief, and governor or safety valve events are heard at the tank or compressor, not at a chamber.

Question 36Air Brake System Components and Fundamentals

Which statement best describes the relationship between the service brake and the parking (spring) brake in a typical spring brake chamber?

  1. They use separate air sections and a strong spring so the parking brake applies when air is released Correct answer.
  2. The service brake and parking brake cannot be used on the same wheel end
  3. They share one air section so the parking brake cannot be applied unless the service brake is applied
  4. The parking brake is applied by air pressure and released by the spring

Explanation

A spring brake chamber has a service section and a spring section, separated by a divider. The spring applies the parking brake when its air is released, and air pressure compresses the spring to release the parking brake.

Question 37Air Brake System Components and Fundamentals

A driver is checking slack adjuster travel and finds that one pushrod moves noticeably farther than the others on the same axle. What is the most appropriate interpretation?

  1. That brake may be out of adjustment or have a worn chamber and should be inspected and corrected before driving Correct answer.
  2. The shorter-travel brakes are the ones that need adjustment
  3. The longer travel proves the brake is working better than the others
  4. The longer travel is normal and requires no attention

Explanation

Excessive or uneven pushrod stroke on one wheel usually means that brake is out of adjustment, has a worn or leaking chamber, or has a mechanical problem. Brakes on the same axle should have similar stroke, so the outlier should be inspected and adjusted or repaired before operation.

Question 38Air Brake System Components and Fundamentals

What is the function of the compressor unloader mechanism?

  1. It opens the safety valve when reservoir pressure exceeds the limit
  2. It releases air from the brake chambers after each service application
  3. It drains condensate from the wet tank each time the compressor cycles
  4. It stops the compressor from pumping when the governor signals that cut-out pressure has been reached Correct answer.

Explanation

The unloader holds the compressor intake valves open, or otherwise bypasses pumping, so the compressor stops building pressure once cut-out is reached. It resumes pumping when the governor signals cut-in pressure.

Question 39Air Brake System Components and Fundamentals

A driver performs a static leak test: with the engine off, full pressure built, and the brakes released, the gauge drops more than the allowed amount in one minute. What does this result most directly indicate?

  1. The compressor is overcharging the reservoirs
  2. There is a leak somewhere in the charged air system that must be located and repaired Correct answer.
  3. The slack adjusters are over-adjusted
  4. The governor cut-out setting is too high

Explanation

With the engine off and brakes released, the only way gauge pressure can fall is for stored air to escape, so an excessive drop points to a leak in the charged system. The test isolates leaks from compressor output because nothing is supplying air during the check.

Question 40Air Brake System Components and Fundamentals

Why does a spring brake chamber have a mechanical release (caging) bolt, and when is it used?

  1. To manually compress and hold the parking spring so the vehicle can be moved when air pressure is unavailable Correct answer.
  2. To lock the service brake in the applied position for a leak test
  3. To bleed moisture out of the spring section of the chamber
  4. To adjust the service brake pushrod stroke during routine maintenance

Explanation

If system air is lost, the parking spring stays applied and the vehicle cannot roll. The caging bolt lets a technician mechanically compress and hold the spring so the brake releases for towing or moving, and it must be removed before the vehicle returns to service.

Question 41Dual Air Brake System and Safety Features

In a dual air brake system, what is the primary purpose of having two separate air circuits?

  1. To increase the total reservoir capacity so the compressor runs less often
  2. To allow one circuit to supply air pressure if the other circuit loses pressure Correct answer.
  3. To let the driver choose which set of brakes to apply during normal driving
  4. To reduce the number of gauges the driver must monitor

Explanation

The dual system splits braking into two independent circuits so that a failure in one circuit still leaves a protected circuit able to apply some braking force. This redundancy is the core safety reason the system exists.

Question 42Dual Air Brake System and Safety Features

On a dual air brake system, the two pressure gauges or the two needles on a single gauge show the pressure in:

  1. The compressor and the governor
  2. The service and parking brake chambers
  3. The primary and secondary circuits Correct answer.
  4. The front and rear tires

Explanation

The gauges report pressure in the primary circuit and the secondary circuit separately, so a driver can spot a failure in one circuit. Tire and compressor readings are unrelated to these gauges.

Question 43Dual Air Brake System and Safety Features

Which component in the air supply system prevents the compressor from pumping beyond a set maximum pressure?

  1. The alcohol evaporator
  2. The quick release valve
  3. The governor Correct answer.
  4. The safety valve

Explanation

The governor monitors reservoir pressure and unloads or cuts in the compressor to hold pressure within a set range. The safety valve is a backup that only opens if pressure climbs too high.

Question 44Dual Air Brake System and Safety Features

What is the function of the safety valve in an air brake system?

  1. It balances pressure between the primary and secondary circuits
  2. It drains moisture from the reservoir each time the brakes are applied
  3. It releases air if reservoir pressure exceeds a safe maximum Correct answer.
  4. It signals the driver when pressure drops below the warning level

Explanation

The safety valve is a pressure-relief device that opens if reservoir pressure rises above a safe limit, protecting the system from over-pressurization. It is a backup to the governor, not a normal operating control.

Question 45Dual Air Brake System and Safety Features

The low air pressure warning device on a vehicle with a dual air brake system must activate before pressure drops below what level?

  1. 90 psi
  2. 60 psi Correct answer.
  3. 45 psi
  4. 120 psi

Explanation

The warning buzzer and light must come on before the pressure in the system falls below about 60 psi. This gives the driver time to stop safely before braking ability is lost.

Question 46Dual Air Brake System and Safety Features

What does the spring brake (emergency/parking brake) do automatically if air pressure is lost?

  1. It releases so the wheels can roll freely
  2. It disconnects the driveline from the wheels
  3. It reroutes air to the front circuit only
  4. It applies because the springs push the brake chambers Correct answer.

Explanation

Spring brakes are held off by air pressure; when pressure drops, the springs extend and apply the brakes automatically. This provides emergency and parking braking without driver action.

Question 47Dual Air Brake System and Safety Features

The air compressor on a dual air brake system is normally driven by:

  1. The engine through a belt or gear drive Correct answer.
  2. An electric motor
  3. The vehicle's transmission output shaft
  4. A hydraulic pump

Explanation

The compressor is engine-driven, typically by a belt or gear, so it builds air whenever the engine runs. It is not electrically or hydraulically powered.

Question 48Dual Air Brake System and Safety Features

What is the purpose of the alcohol evaporator in an air brake system?

  1. To lubricate the compressor
  2. To increase reservoir pressure
  3. To filter dirt from the air
  4. To reduce the chance of moisture freezing in the air lines Correct answer.

Explanation

Alcohol vapor added to the air supply lowers the freezing point of moisture so it is less likely to freeze in lines and valves during cold weather. It does not lubricate, boost pressure, or filter dirt.

Question 49Dual Air Brake System and Safety Features

A driver notices that the pressure in one circuit of the dual air brake system has dropped to zero while the other circuit still holds normal pressure. What is the most likely explanation?

  1. The compressor drive belt has broken
  2. The governor has failed and unloaded the compressor
  3. The safety valve has opened in both circuits
  4. One circuit has a leak or failure, but the other circuit is still protected Correct answer.

Explanation

The dual system is designed so that a failure in one circuit does not drain the other, which is why one gauge can read zero while the other stays normal. A compressor or governor problem would affect both circuits together.

Question 50Dual Air Brake System and Safety Features

During a pre-trip test, the driver builds normal pressure, shuts off the engine, and listens. No audible leak is heard, but the gauges drop steadily over several minutes. What should the driver conclude?

  1. The system is fine because a steady drop is normal after shutdown
  2. There is an air leak that must be repaired before driving Correct answer.
  3. The spring brakes are releasing on their own
  4. The governor is over-pressurizing the system

Explanation

With the engine off and brakes released, pressure should hold; a steady drop indicates a leak somewhere in the system. A leak must be found and repaired before the vehicle is driven.