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OK CDL Air Brakes study materials
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OK CDL Air Brakes practice questions

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Question 1Air Brake System Components and Fundamentals

On a vehicle with a dual air brake system, what is the purpose of the two separate reservoirs?

  1. They store air for the trailer only while the tractor uses a separate compressor
  2. They hold air at two different pressures for loaded and unloaded conditions
  3. They provide a backup supply so one circuit can still stop the vehicle if the other fails Correct answer.
  4. They supply air to the front and rear axles respectively

Explanation

A dual air brake system splits the vehicle into two independent circuits, each with its own reservoir and check valve. If one circuit loses pressure, the other still provides enough air to stop the vehicle, which is the core safety reason for the design.

Question 2Air Brake System Components and Fundamentals

What does the air compressor governor do in an air brake system?

  1. It sets the maximum application pressure at the brake chambers
  2. It drains moisture from the supply reservoir automatically
  3. It limits the compressor's output by unloading it at a set cut-out pressure and reloading it at cut-in Correct answer.
  4. It reduces the pressure delivered to the trailer glad hands

Explanation

The governor senses supply reservoir pressure and controls whether the compressor pumps or idles. It unloads the compressor at the cut-out pressure and lets it resume at the cut-in pressure, keeping system pressure within the designed range.

Question 3Air Brake System Components and Fundamentals

During a pre-trip inspection you build full pressure and then shut the engine off with the brakes released. Which result indicates an acceptable air leakage rate for a single vehicle?

  1. Pressure holds exactly steady with no drop at all
  2. Pressure drops no more than 20 psi in one minute
  3. Pressure drops no more than 3 psi in one minute Correct answer.
  4. Pressure drops no more than 10 psi in five minutes

Explanation

For a single vehicle, the standard leakage test allows a loss of no more than 3 psi in one minute with the engine off and brakes released. A larger drop points to a leak that must be found and repaired before driving.

Question 4Air Brake System Components and Fundamentals

What is the function of the safety valve installed on the air brake system?

  1. It releases air automatically if system pressure climbs above a safe limit Correct answer.
  2. It bleeds air from the service line during a normal brake application
  3. It keeps the trailer from rolling away when the supply line is disconnected
  4. It prevents the spring brakes from applying when pressure is low

Explanation

The safety valve is a pressure-relief device that opens if reservoir pressure rises above its setting, protecting components from over-pressurization. It typically discharges near 150 psi, above normal governor cut-out.

Question 5Air Brake System Components and Fundamentals

What is the role of the alcohol evaporator or air dryer in an air brake system?

  1. It boosts pressure to the trailer during heavy applications
  2. It increases the compressor's pumping capacity
  3. It lubricates the brake chambers with a fine oil mist
  4. It removes moisture and contaminants that could freeze or damage valves Correct answer.

Explanation

Moisture in compressed air can freeze in lines and corrode or jam valves, so an evaporator or dryer conditions the air before it reaches the reservoirs. Removing water and contaminants keeps the system functioning in cold weather and extends component life.

Question 6Air Brake System Components and Fundamentals

In a dual air brake system, the two circuits are commonly described by which primary functions?

  1. Primary and secondary Correct answer.
  2. Service and parking
  3. Front and rear
  4. Supply and control

Explanation

The two circuits of a dual system are called the primary and secondary systems, and each is capable of providing braking if the other fails. They are not simply front/rear or service/parking splits.

Question 7Air Brake System Components and Fundamentals

What component senses pressure in the supply reservoir and sends a signal to the compressor to stop pumping at cut-out pressure?

  1. The inversion valve
  2. The relay valve
  3. The quick release valve
  4. The governor Correct answer.

Explanation

The governor monitors supply reservoir pressure and unloads the compressor when cut-out pressure is reached. It reloads the compressor when pressure falls to the cut-in setting.

Question 8Air Brake System Components and Fundamentals

Which component converts air pressure into mechanical force that applies the foundation brakes?

  1. The low pressure warning switch
  2. The brake chamber Correct answer.
  3. The treadle valve
  4. The slack adjuster

Explanation

The brake chamber contains a diaphragm and pushrod that turn air pressure into mechanical force, which the slack adjuster then transfers to the brake mechanism. Without the chamber, air pressure could not move the foundation brake parts.

Question 9Air Brake System Components and Fundamentals

A driver notices the low air pressure warning does not activate until pressure has dropped below 60 psi. What is the correct assessment?

  1. This is a defect because the warning must activate before pressure falls below 60 psi Correct answer.
  2. This is normal because warnings only apply to the trailer circuit
  3. This is acceptable because the spring brakes will hold the vehicle
  4. This is acceptable if the vehicle has a working air dryer

Explanation

The low pressure warning device must activate before system pressure drops below 60 psi, giving the driver time to stop safely. A warning that waits until below 60 psi does not provide the required margin and must be repaired.

Question 10Air Brake System Components and Fundamentals

What is the purpose of the tractor protection valve?

  1. It controls the trailer spring brakes during normal parking
  2. It prevents air loss from the tractor if the trailer breaks away or the trailer line fails Correct answer.
  3. It regulates the compressor cut-in pressure
  4. It sets the pressure at which the safety valve opens

Explanation

The tractor protection valve closes off the tractor's air supply to the trailer lines when pressure drops or the trailer separates, protecting the tractor's ability to brake. It also helps keep the tractor from losing all its air through a broken trailer line.

Question 11Air Brake System Components and Fundamentals

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

  1. It reduces the pressure going to the trailer supply line
  2. It automatically adjusts slack in the brake linkage
  3. It uses a small control signal to quickly deliver or exhaust a large volume of air at the brake chambers Correct answer.
  4. It warns the driver when pressure is low

Explanation

A relay valve takes a low-volume control signal and uses reservoir air to apply or release the brakes quickly at a remote location, such as the rear axle or trailer. This shortens application and release time compared with routing all air through the foot valve.

Question 12Air Brake System Components and Fundamentals

A driver performs the applied leakage test and finds pressure dropping 5 psi in one minute with the brakes fully applied. What does this indicate?

  1. The leakage exceeds the allowable limit and the vehicle should be repaired before use Correct answer.
  2. The drop is caused by the air dryer purging and can be ignored
  3. The test was invalid because the engine was off
  4. The leakage is within limits for a single vehicle

Explanation

With brakes applied, the allowable leakage for a single vehicle is no more than 3 psi in one minute, so a 5 psi drop exceeds the limit. Excess leakage under application suggests a problem in the service side of the system that must be corrected.

Question 13Air Brake System Components and Fundamentals

What is the purpose of the spring brakes in an air brake system?

  1. They provide extra braking force during hard service applications
  2. They release the brakes when the park control is pushed in
  3. They apply automatically when air pressure drops below a safe level Correct answer.
  4. They hold the trailer only while it is coupled to the tractor

Explanation

Spring brakes use heavy springs to apply the brakes when air pressure is lost, so they function as an automatic emergency and parking brake. Air pressure compresses the springs to release the brakes during normal operation.

Question 14Air Brake System Components and Fundamentals

When the park brake control is pulled out, what happens in the spring brake chambers?

  1. Air is vented through the safety valve to hold pressure steady
  2. Air is routed to the service chambers to add force
  3. Air is supplied to compress the springs and release the brakes
  4. Air is exhausted so the springs extend and apply the brakes Correct answer.

Explanation

Pulling the park control exhausts air from the spring brake chambers, allowing the springs to extend and apply the brakes. Pushing the control back in re-supplies air to compress the springs and release the brakes.

Question 15Air Brake System Components and Fundamentals

What is the function of the quick release valve?

  1. It vents the trailer line when glad hands are disconnected
  2. It quickly applies the spring brakes in an emergency
  3. It releases air from the supply reservoir during compressor unload
  4. It speeds up exhausting air from brake chambers when the brake is released Correct answer.

Explanation

A quick release valve is located near the brake chambers and exhausts air locally when the control pressure is released, shortening release time. Faster release reduces brake drag and improves response.

Question 16Air Brake System Components and Fundamentals

A vehicle's compressor cut-out pressure is 120 psi and cut-in is 100 psi. What does this mean for system operation?

  1. The low pressure warning activates at 120 psi and resets at 100 psi
  2. The compressor pumps until 100 psi, then idles until pressure falls to 120 psi
  3. The safety valve opens at 100 psi and closes at 120 psi
  4. The compressor pumps until 120 psi, then idles until pressure falls to 100 psi Correct answer.

Explanation

Cut-out is the upper pressure at which the governor unloads the compressor, and cut-in is the lower pressure at which it reloads. So the compressor pumps to 120 psi, idles, and restarts when pressure drops to 100 psi.

Question 17Air Brake System Components and Fundamentals

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

  1. It carries air only when the park brake is set
  2. It carries the control signal for service brake applications
  3. It charges the trailer reservoirs and signals a breakaway to apply trailer spring brakes Correct answer.
  4. It exhausts air from the trailer chambers during release

Explanation

The supply line charges the trailer's air reservoirs and, if it loses pressure or separates, causes the trailer spring brakes to apply. The service line carries the control signal for normal brake applications.

Question 18Air Brake System Components and Fundamentals

A driver finds that the spring brakes will not release after the system reaches normal pressure. Which condition is most likely responsible?

  1. The safety valve is stuck open
  2. The governor cut-out pressure is too high
  3. The park control valve is not supplying air to the spring chambers Correct answer.
  4. The air dryer purge valve is cycling too often

Explanation

Spring brakes release only when air is supplied to the spring chambers through the park control valve. If the valve does not deliver air, the springs stay applied even at normal reservoir pressure.

Question 19Air Brake System Components and Fundamentals

What is the purpose of the check valve in an air brake reservoir circuit?

  1. It limits the maximum pressure in the service line
  2. It releases air to the atmosphere when pressure is too high
  3. It allows air to flow into the reservoir but prevents it from flowing back toward the compressor Correct answer.
  4. It drains water from the reservoir automatically

Explanation

Check valves permit air to enter a reservoir while blocking backflow, which preserves stored air if the supply side loses pressure. This protection is part of why each circuit can keep braking if the other fails.

Question 20Air Brake System Components and Fundamentals

What is the function of the inversion valve in a dual air brake system?

  1. It inverts the compressor output to reduce pressure at the governor
  2. It allows a spring brake to be modulated or released during a partial pressure loss Correct answer.
  3. It converts service air pressure into an electrical warning signal
  4. It reverses the flow direction in the supply line to charge the trailer

Explanation

An inversion valve responds to a loss of pressure in one circuit by allowing the spring brakes to be applied or modulated for emergency stopping. It helps provide controlled braking rather than an abrupt full spring application.

Question 21Air Brake System Components and Fundamentals

During a test, the low pressure warning activates at 70 psi and the spring brakes apply automatically at about 45 psi. How should this be evaluated?

  1. Both behaviors are acceptable because the warning comes before spring application
  2. The warning is late because it must activate before 60 psi Correct answer.
  3. The spring brake application is too early and indicates a defect
  4. The warning should activate only after the spring brakes apply

Explanation

The warning device must activate before pressure drops below 60 psi, so activation at 70 psi is actually within the requirement, but the question asks for evaluation of the warning point; a warning at 70 psi is early enough. The spring brakes applying around 45 psi is normal emergency behavior.

Question 22Air Brake System Components and Fundamentals

How does a relay valve improve brake response on a long-wheelbase vehicle?

  1. It eliminates the need for slack adjusters on the rear axle
  2. It raises system pressure above the governor cut-out to speed application
  3. It uses stored reservoir air near the chambers so air does not have to travel from the foot valve Correct answer.
  4. It keeps the spring brakes partially applied for faster stops

Explanation

A relay valve responds to a small control signal from the foot valve and delivers air from a nearby reservoir directly to the chambers. This shortens the distance air must travel, reducing lag on long vehicles.

Question 23Air Brake System Components and Fundamentals

A driver reports that the trailer brakes apply when the trailer supply line is disconnected, but the tractor brakes remain fully functional. What explains this behavior?

  1. The governor has unloaded the compressor permanently
  2. The tractor protection valve has isolated the trailer and the trailer spring brakes applied on pressure loss Correct answer.
  3. The safety valve has vented the tractor reservoirs
  4. The relay valve on the tractor has failed closed

Explanation

When the supply line loses pressure, the tractor protection valve closes to protect tractor air, and the trailer spring brakes apply because their release air is gone. This is the designed breakaway response.

Question 24Air Brake System Components and Fundamentals

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

  1. It adds alcohol to the air to lower the freezing point of moisture in the lines Correct answer.
  2. It filters out carbon particles from the compressor
  3. It increases the compressor's cut-out pressure
  4. It lubricates the brake chamber diaphragm

Explanation

An alcohol evaporator introduces alcohol vapor into the air system to lower the freezing point of any moisture present, helping prevent ice blockages in cold weather. It is a moisture-control measure, not a pressure or lubrication device.

Question 25Air Brake System Components and Fundamentals

Which statement best describes the relationship between the service brake system and the parking/emergency brake system on an air-braked vehicle?

  1. The parking brake uses hydraulic pressure while the service brake uses air
  2. They share the same brake chambers and apply force through the same pushrods
  3. The service brake can only operate when the parking brake is applied
  4. They are independent systems, with spring brakes providing parking and emergency functions Correct answer.

Explanation

The service brake system uses air pressure to apply the foundation brakes during normal driving, while the spring brake system provides parking and emergency braking. They are distinct functions, though both act on the same foundation brake mechanism.

Question 26Air Brake System Components and Fundamentals

On a typical air brake system, which gauge reading tells the driver the pressure available in the circuit that applies the rear service brakes?

  1. The low air pressure warning buzzer
  2. The application pressure gauge
  3. The primary (rear) system gauge Correct answer.
  4. The secondary (front) system gauge

Explanation

Dual systems use separate reservoirs and gauges so the driver can monitor each circuit independently. The primary gauge reflects the circuit that supplies the rear service brakes, while the secondary gauge reflects the front circuit. A fault in one circuit can then be identified before total loss of braking.

Question 27Air Brake System Components and Fundamentals

What is the function of the safety valve installed on an air brake reservoir?

  1. It limits the rate at which the compressor can build pressure
  2. It drains accumulated moisture and oil from the bottom of the tank
  3. It releases air to the atmosphere if reservoir pressure exceeds a set limit Correct answer.
  4. It prevents air from flowing back toward the compressor

Explanation

The safety valve is a pressure-relief device that protects the tank and downstream components from over-pressurization if the governor fails. It is set above normal cut-out pressure so it stays closed during routine operation. It does not manage moisture, backflow, or compressor speed.

Question 28Air Brake System Components and Fundamentals

Which component in the air brake system removes moisture and oil vapor from the compressed air before it reaches the reservoirs?

  1. The air dryer Correct answer.
  2. The alcohol evaporator
  3. The pressure regulator
  4. The governor

Explanation

An air dryer uses a desiccant bed to absorb water vapor and traps oil carryover so that clean, dry air reaches the reservoirs. Moisture in the system can freeze in cold weather and corrode valves. The dryer is distinct from devices that control compressor cycling or add alcohol.

Question 29Air Brake System Components and Fundamentals

What does the governor do when reservoir pressure reaches the cut-out setting?

  1. It sends supply pressure to the trailer emergency line
  2. It unloads the compressor so it stops pumping air into the system Correct answer.
  3. It opens the safety valve to vent excess pressure
  4. It closes the check valve to isolate the wet tank

Explanation

The governor senses reservoir pressure and, at cut-out, signals the compressor to unload so it circulates air without building pressure. When pressure falls to cut-in, the governor reloads the compressor. This cycling keeps system pressure within the designed operating range.

Question 30Air Brake System Components and Fundamentals

A driver observes that the air pressure gauge climbs very slowly after a long idle period, and the compressor never seems to unload. Which condition best explains this?

  1. A drain cock left open on the wet tank
  2. A leaking or slipping compressor drive belt or an internal compressor fault Correct answer.
  3. A governor stuck in the unloaded position
  4. A safety valve set too low

Explanation

If the compressor runs continuously without reaching cut-out, it is producing air too slowly or losing it as fast as it is made. A slipping or broken drive belt or worn compressor internals fits this pattern. A governor stuck unloaded would prevent pressure from building at all rather than build slowly.

Question 31Air Brake System Components and Fundamentals

How does a pressure protection valve affect the operation of an air brake system?

  1. It prevents the spring brakes from releasing below a set pressure
  2. It balances pressure between the primary and secondary circuits
  3. It blocks air flow to non-critical accessories until brake system pressure reaches a safe level Correct answer.
  4. It limits maximum pressure delivered to the service brake chambers

Explanation

A pressure protection valve isolates accessories such as air seats, horns, or suspension so that a leak in those circuits cannot drain the brake reservoirs. It opens only after brake system pressure reaches a preset minimum. This preserves the air needed for braking.

Question 32Air Brake System Components and Fundamentals

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

  1. It introduces alcohol vapor into the air system to prevent moisture from freezing in valves and lines Correct answer.
  2. It removes carbon deposits from the compressor discharge
  3. It absorbs water in the reservoirs and must be drained periodically
  4. It lubricates the compressor pistons during operation

Explanation

Alcohol evaporators are used on systems without an air dryer to keep condensed moisture from freezing in valves and lines during cold weather. The alcohol vapor mixes with the air and lowers the freezing point of the water present. It does not dry, clean, or lubricate the system.

Question 33Air Brake System Components and Fundamentals

A driver applies the brakes and hears a steady hiss that continues as long as the pedal is held, with pressure slowly falling. Which component is the most likely source?

  1. A leaking service brake chamber or diaphragm Correct answer.
  2. A normally functioning relay valve exhausting air
  3. A governor in the cut-in phase
  4. A quick release valve during normal release

Explanation

A continuous hiss while the brakes are applied indicates air escaping under pressure, commonly from a ruptured diaphragm or a cracked chamber. Normal exhaust occurs only briefly during application and release. A steady leak will also show up as pressure loss on the gauge.

Question 34Air Brake System Components and Fundamentals

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

  1. It meters air pressure to the service brake chambers in proportion to how far the pedal is pressed Correct answer.
  2. It divides supply air between the primary and secondary reservoirs
  3. It releases air from the trailer supply line when the tractor is parked
  4. It holds the spring brakes applied until system pressure is restored

Explanation

The treadle (foot) valve is the driver's primary control for service braking. Its pedal travel and force determine how much delivery pressure reaches the service chambers, giving graduated braking. It does not store air or manage spring brake release.

Question 35Air Brake System Components and Fundamentals

Why are the front and rear service brake circuits supplied through separate reservoirs with check valves?

  1. So the parking brakes release independently of system pressure
  2. So a failure in one circuit still leaves the other circuit with a protected air supply Correct answer.
  3. So the compressor can recharge both circuits at different pressures
  4. So the front brakes always apply before the rear brakes

Explanation

Dual-circuit design isolates the two halves of the system so that a leak or rupture in one circuit cannot drain the other. Check valves prevent air from flowing back out of a good reservoir into a failed circuit. This redundancy lets the vehicle retain some braking capability after a single failure.

Question 36Air Brake System Components and Fundamentals

During a static leak check, the driver notes that pressure drops noticeably faster with the brakes released than with them applied. What does this suggest?

  1. The compressor is overcharging the system
  2. The service brake chambers have ruptured diaphragms
  3. The low pressure warning switch is faulty
  4. A leak exists in the supply side of the system, upstream of the service valves Correct answer.

Explanation

Leaks in the charged supply side, such as reservoir fittings or supply lines, lose air continuously whether or not the brakes are applied. A leak in the service side would typically show up or worsen when the brakes are applied and delivery pressure is present. Comparing both conditions helps localize the fault.

Question 37Air Brake System Components and Fundamentals

What does the supply reservoir (wet tank) do in a typical air brake system?

  1. It stores air exclusively for the trailer brake circuit
  2. It receives air directly from the compressor and allows moisture and oil to condense before the air moves on Correct answer.
  3. It supplies air only to the spring brake chambers
  4. It regulates compressor cut-in and cut-out pressure

Explanation

The supply or wet tank is the first tank after the compressor and acts as a collection point where moisture and oil drop out of the air. Air then passes through check valves to the primary and secondary reservoirs. It does not control governor settings or serve only one circuit.

Question 38Air Brake System Components and Fundamentals

A truck's low air pressure warning activates at about 60 psi, but the spring brakes do not apply automatically until pressure falls near 20 psi. How should this be evaluated?

  1. The warning is too late and should activate below 20 psi
  2. The warning is within the required range, but the spring brake application point is too low and the system needs service Correct answer.
  3. The spring brakes should apply at the same moment the warning sounds
  4. Both settings are normal because the warning and the spring brake application are unrelated

Explanation

The low pressure warning must operate before pressure drops below about 60 psi so the driver has time to act. Spring brakes should apply automatically well before pressure falls to a level that leaves no safe braking, typically around 20 to 45 psi depending on the system. A truck holding spring brakes off until near 20 psi has a fault requiring inspection.

Question 39Air Brake System Components and Fundamentals

A driver reports that air pressure builds normally but the spring brakes will not release. Which component is the most likely cause?

  1. The park control (spring brake) valve is not supplying air to release the chambers Correct answer.
  2. The safety valve is opening at the wrong pressure
  3. The low pressure warning switch is stuck closed
  4. The compressor drive belt is slipping

Explanation

Spring brakes release only when air pressure is delivered to the spring brake chambers through the park control valve and related valving. If pressure builds normally but the brakes stay set, the release path is the prime suspect. A slipping belt or faulty warning switch would not prevent release once pressure is present.

Question 40Air Brake System Components and Fundamentals

Why is the inversion valve important in a dual air brake system equipped with spring brakes?

  1. It inverts the signal from the governor to the compressor
  2. It converts supply line pressure into trailer service pressure
  3. It provides a modulated spring brake application if the primary service circuit loses pressure Correct answer.
  4. It reverses the direction of air flow from the compressor during unloading

Explanation

The inversion valve allows the spring brakes to be applied in a controlled, modulated way when service brake pressure is lost, rather than only in a full emergency application. This preserves some graduated braking capability during a primary circuit failure. It does not affect compressor flow or trailer signaling.

Question 41Dual Air Brake System and Safety Devices

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

  1. To allow one circuit to supply air to the other when pressure drops
  2. To provide a backup source of air in case one circuit fails Correct answer.
  3. To balance air pressure between the front and rear axles
  4. To increase the total reservoir capacity for long downgrades

Explanation

A dual air brake system splits the brakes into two independent circuits, each with its own reservoir and lines. If one circuit loses pressure or fails, the other circuit can still apply enough braking force to stop the vehicle. This redundancy is the core safety benefit of the split system.

Question 42Dual Air Brake System and Safety Devices

In a typical dual air brake system, which components are usually on separate circuits?

  1. The front axle brakes and the rear axle brakes Correct answer.
  2. The service brakes and the parking brakes only
  3. The low-pressure warning buzzer and the gauge
  4. The compressor and the governor

Explanation

Most dual systems divide the vehicle so the front axle brakes are on one circuit and the rear axle brakes are on another. This split means a failure in one circuit still leaves braking at the other axle. The parking brake and warning devices are not the primary split points.

Question 43Dual Air Brake System and Safety Devices

What does the safety valve on an air brake reservoir do?

  1. Releases air to the atmosphere if reservoir pressure exceeds a set limit Correct answer.
  2. Drains moisture from the bottom of the tank
  3. Signals the governor to unload the compressor
  4. Balances pressure between the two circuits

Explanation

The safety valve is a pressure-relief device set to open before tank pressure reaches a dangerous level. It protects the reservoir and lines from over-pressurization if the governor or compressor unloading fails. It does not drain water or control the compressor.

Question 44Dual Air Brake System and Safety Devices

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

  1. 60 psi Correct answer.
  2. 80 psi
  3. 100 psi
  4. 45 psi

Explanation

Federal standards require a visible and audible warning to activate before the air pressure falls below 60 psi. The warning alerts the driver while there is still enough pressure to stop safely. Waiting until 45 psi would leave too little margin.

Question 45Dual Air Brake System and Safety Devices

What is the function of the governor in an air brake system?

  1. It limits the maximum application force at the brake chambers
  2. It prevents air from flowing back into the compressor
  3. It regulates the compressor so reservoir pressure stays within a set range Correct answer.
  4. It controls the rate of air release during a parking brake application

Explanation

The governor senses reservoir pressure and controls whether the compressor is loading or unloading. It cuts the compressor in at a low set point and out at a high set point, keeping pressure in the proper range. It does not meter brake chamber force or manage parking brake release.

Question 46Dual Air Brake System and Safety Devices

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

  1. To prevent moisture in the air lines from freezing Correct answer.
  2. To raise the octane of the fuel
  3. To increase reservoir pressure on cold days
  4. To lubricate the compressor pistons

Explanation

An alcohol evaporator introduces alcohol vapor into the air system to lower the freezing point of any moisture present. This helps prevent ice blocking lines and valves in cold weather. It has nothing to do with fuel or compressor lubrication.

Question 47Dual Air Brake System and Safety Devices

During a static leak test on a dual air brake system, a fully charged system with the engine off and brakes released should not lose more than what amount of pressure in one minute?

  1. 3 psi Correct answer.
  2. 10 psi
  3. 2 psi
  4. 5 psi

Explanation

For a combination vehicle the allowable static leak rate with brakes released is generally no more than 3 psi in one minute. A larger drop indicates a leak that must be found and repaired. This test confirms the system holds pressure when it is not being applied.

Question 48Dual Air Brake System and Safety Devices

What does the spring brake chamber do when air pressure is lost in the system?

  1. It disconnects the service brake circuit
  2. It applies the brake automatically Correct answer.
  3. It releases the brake to allow the vehicle to roll
  4. It vents all remaining air to the atmosphere

Explanation

Spring brakes use a large spring to apply the brakes when air pressure is removed. Losing air pressure therefore causes the brakes to apply automatically, which is a fail-safe design. Air pressure must be present to hold the spring off and keep the brakes released.

Question 49Dual Air Brake System and Safety Devices

A driver notices the low air pressure warning buzzer sounds and the gauge reads 55 psi while driving. What is the safest immediate action?

  1. Pump the brake pedal rapidly to build pressure back up
  2. Move the vehicle to a safe stop and investigate the pressure loss Correct answer.
  3. Shift to a lower gear and use engine braking only
  4. Continue driving but reduce speed by half

Explanation

A warning below 60 psi means the system has lost enough pressure that braking may soon be compromised. The driver should bring the vehicle to a safe stop and find the cause before continuing. Pumping the pedal will not restore lost air and may worsen the situation.

Question 50Dual Air Brake System and Safety Devices

Why must the air compressor be able to build pressure from 85 to 100 psi within a specified time during the buildup test?

  1. To ensure the spring brakes release fully
  2. To confirm the compressor and governor can restore working pressure at a safe rate Correct answer.
  3. To check that the low-pressure warning activates
  4. To verify the safety valve opens at the correct pressure

Explanation

The buildup test checks that the compressor can recover pressure quickly enough to keep up with normal brake use. If buildup is too slow, the system may not maintain adequate pressure during repeated applications. The test is about compressor and governor performance, not warning devices.