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Michigan CDL Air Brakes study materials
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Michigan CDL Air Brakes practice questions

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Question 1Air Brake System Components and Operation

On a vehicle with air brakes, what is the function of the compressor governor?

  1. It converts hydraulic pressure into air pressure at each wheel.
  2. It controls when the compressor cuts in and out to maintain system air pressure. Correct answer.
  3. It releases air from the spring brake chambers during parking.
  4. It measures the amount of air flowing to the brake chambers.

Explanation

The governor senses reservoir pressure and signals the compressor to load (build air) at the cut-in pressure and to unload (stop building) at the cut-out pressure. This keeps system pressure within a safe operating range. It does not meter air to the chambers or control parking brakes.

Question 2Air Brake System Components and Operation

A vehicle's air pressure gauge shows the compressor has cut out at 125 psi. Which statement best describes what the governor has done?

  1. It has closed the supply line to the primary reservoir permanently.
  2. It has engaged the spring brakes to prevent overpressure.
  3. It has unloaded the compressor so it stops building air until pressure drops to cut-in. Correct answer.
  4. It has opened the safety valve to vent excess pressure to the atmosphere.

Explanation

At cut-out pressure the governor unloads the compressor, letting it idle without pumping air until reservoir pressure falls to the cut-in setting. The safety valve is a separate backup device that only opens near its higher setting. Spring brakes are not engaged by high pressure.

Question 3Air Brake System Components and Operation

What is the purpose of the air compressor's safety valve (pop-off valve)?

  1. It drains moisture from the reservoir each time the compressor cycles.
  2. It vents air if reservoir pressure rises above a safe maximum. Correct answer.
  3. It signals the driver when the compressor belt is slipping.
  4. It supplies air directly to the brake chambers during a hard stop.

Explanation

The safety valve is a spring-loaded relief device on the reservoir that opens if the governor fails and pressure climbs above the safe limit. It protects the system from over-pressurization. It does not drain moisture or feed the brake chambers.

Question 4Air Brake System Components and Operation

Why does an air brake system include an alcohol evaporator or air dryer?

  1. To increase the compressor's cut-out pressure setting.
  2. To boost the pressure delivered to the trailer glad hands.
  3. To lubricate the brake shoes and drums.
  4. To reduce moisture and prevent ice formation in the air lines during cold weather. Correct answer.

Explanation

Moisture in compressed air can condense and freeze in valves and lines, causing brake failure in cold weather. An alcohol evaporator or air dryer removes or lowers the freezing point of that moisture. It has no role in setting pressure or lubricating brake friction parts.

Question 5Air Brake System Components and Operation

During a pre-trip inspection, the driver drains the air reservoirs and finds a large amount of water. What is the correct interpretation?

  1. The air dryer or evaporator may not be working and the system needs service. Correct answer.
  2. The safety valve has been opening and should be capped off.
  3. This is normal and no action is needed as long as pressure builds.
  4. The compressor is overloading the system and the governor must be replaced.

Explanation

Excessive water in the reservoirs indicates the moisture-removal device is not functioning properly. Water can freeze or corrode valves and cause brake problems, so the system should be checked. A small amount of condensation is normal, but a large amount is not.

Question 6Air Brake System Components and Operation

What does the supply pressure gauge on a vehicle with air brakes show?

  1. The air pressure available in the system reservoirs. Correct answer.
  2. The amount of vacuum in the booster.
  3. The pressure in the trailer's electrical circuit.
  4. The pressure being applied to the brake chambers at that moment.

Explanation

The supply gauge shows stored air pressure in the reservoirs, telling the driver how much air is available to operate the brakes. Application pressure at the chambers is a different reading. Air brakes do not use vacuum boosters.

Question 7Air Brake System Components and Operation

What is the purpose of the application pressure gauge?

  1. It shows how much air pressure the driver is applying to the brakes. Correct answer.
  2. It shows the pressure in the parking brake circuit only.
  3. It shows the compressor's cut-in and cut-out settings.
  4. It shows the pressure in the trailer supply line.

Explanation

An application pressure gauge tells the driver how much braking force is being applied at the chambers, which helps judge braking on grades or slippery surfaces. It is separate from the supply gauge that shows available reservoir pressure. It does not display governor settings.

Question 8Air Brake System Components and Operation

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

  1. It releases air from the trailer emergency line.
  2. It controls the amount of air pressure sent to the brake chambers when the pedal is pressed. Correct answer.
  3. It automatically sets the spring brakes when the engine is shut off.
  4. It regulates compressor output to the wet tank.

Explanation

The foot valve is the driver's service brake control; pressing the treadle meters air from the reservoirs to the brake chambers in proportion to pedal travel. It does not control the compressor or the parking brakes. Spring brake application is handled by the parking brake valve.

Question 9Air Brake System Components and Operation

In a dual air brake system, why are the primary and secondary circuits kept separate?

  1. So the trailer can be supplied with air through both circuits.
  2. So a failure in one circuit still leaves a portion of the braking system able to stop the vehicle. Correct answer.
  3. So the parking brakes can be released without air pressure.
  4. So the compressor can build pressure in both circuits at the same time.

Explanation

Dual circuits provide redundancy: if one circuit loses pressure, the other can still apply some brakes so the vehicle can be brought to a stop. This is a safety requirement, not a means of speeding compressor output. The circuits are deliberately isolated from each other during a failure.

Question 10Air Brake System Components and Operation

The primary circuit of a dual air brake system loses pressure while the vehicle is moving. What should the driver expect?

  1. All braking will be lost immediately with no warning.
  2. The low pressure warning will activate and the secondary circuit will still provide braking. Correct answer.
  3. The compressor will overheat and shut down the engine.
  4. The spring brakes will release automatically to allow continued driving.

Explanation

A dual system is designed so that loss of one circuit triggers the low pressure warning and leaves the other circuit operational. The driver should still have some braking ability and must stop safely to investigate. Total loss of braking does not occur instantly from a single circuit failure.

Question 11Air Brake System Components and Operation

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

  1. It reduces reservoir pressure before it reaches the compressor.
  2. It controls the compressor's unloading cycle.
  3. It speeds up the application and release of air to the brake chambers by using reservoir air close to the chambers. Correct answer.
  4. It measures the air pressure applied to the trailer.

Explanation

A relay valve uses a small control signal from the foot valve to admit reservoir air directly to nearby brake chambers. This shortens the distance air must travel, so brakes apply and release faster. It does not regulate compressor output or measure pressure.

Question 12Air Brake System Components and Operation

What is the purpose of the quick release valve?

  1. It vents the safety valve during overpressure.
  2. It releases the trailer from the tractor when pressure is low.
  3. It exhausts air quickly from the brake chambers so the brakes release faster. Correct answer.
  4. It quickly fills the reservoirs after the compressor starts.

Explanation

The quick release valve is mounted near the brake chambers and lets air escape rapidly when the driver releases the pedal. Faster release reduces brake drag and helps prevent overheating. It does not fill reservoirs or uncouple the trailer.

Question 13Air Brake System Components and Operation

Why must the air compressor be able to build pressure from zero to the governor cut-out within a specified time?

  1. So the safety valve can be tested without the engine running.
  2. So the system can recover quickly enough to keep brakes working after air use. Correct answer.
  3. So the spring brakes can be released without a parking brake valve.
  4. So the trailer supply line can be connected without glad hands.

Explanation

Federal standards require the compressor to build pressure within a specified time so the system can replenish air used during braking. If build-up is too slow, the vehicle may not maintain enough pressure for safe operation. This is a performance check on the compressor and governor, not on other components.

Question 14Air Brake System Components and Operation

During a leak test on a dual air brake system, the driver notes pressure drop with the engine off and the brakes released. What does this indicate?

  1. The governor is set too high and should be adjusted.
  2. The compressor is building air too quickly.
  3. The safety valve is functioning normally.
  4. A leak exists in the system that should be found and repaired before driving. Correct answer.

Explanation

With the engine off and brakes released, pressure should drop only within a small allowable rate. A larger drop means air is escaping somewhere in the system and must be repaired. This test isolates leakage from compressor or governor performance.

Question 15Air Brake System Components and Operation

What is the function of the spring brake (emergency/parking brake) in an air brake system?

  1. It regulates the compressor's cut-in pressure.
  2. It provides extra braking force during normal service stops.
  3. It drains moisture from the reservoirs.
  4. It applies the brakes automatically when air pressure is lost and holds the vehicle when parked. Correct answer.

Explanation

Spring brakes use heavy springs to apply the brakes when air pressure is released or lost, providing emergency and parking braking. Air pressure must be applied to compress the springs and release the brakes. They are not part of normal service braking force.

Question 16Air Brake System Components and Operation

A driver hears a low air pressure warning buzzer while driving and the pressure gauge is falling. What is the correct immediate action?

  1. Continue driving to the nearest terminal because the warning is only advisory.
  2. Release the parking brake repeatedly to restore pressure.
  3. Increase engine speed and keep driving to build pressure.
  4. Stop as soon as it is safe and investigate before continuing. Correct answer.

Explanation

A low pressure warning means the system may soon be unable to apply the service brakes, and spring brakes will apply when pressure drops further. The driver should stop safely and find the cause. Continuing to drive risks an uncontrolled spring brake application.

Question 17Air Brake System Components and Operation

At what point in a dual air brake system should the low pressure warning device activate?

  1. When the compressor first starts building pressure.
  2. Only after the spring brakes have fully applied.
  3. When pressure reaches the governor cut-out setting.
  4. Before pressure drops below the level needed to safely operate the brakes. Correct answer.

Explanation

The warning device must activate before air pressure falls below the minimum needed for safe braking, giving the driver time to stop. It should come on well before the spring brakes apply. It is not tied to cut-out pressure or compressor start-up.

Question 18Air Brake System Components and Operation

A trailer supply line separates and begins venting air. Which component helps preserve the tractor's air supply by isolating the trailer connection?

  1. Brake chamber pushrod
  2. Tractor protection valve Correct answer.
  3. Compressor governor
  4. Reservoir drain valve

Explanation

The tractor protection valve can close off the tractor's trailer air connections when trailer pressure is lost. This helps limit loss from the tractor's air supply.

Question 19Air Brake System Components and Operation

What happens when the trailer emergency (supply) line is disconnected from the tractor?

  1. The trailer service brakes release and the trailer rolls freely.
  2. The tractor compressor stops building pressure.
  3. The trailer air reservoirs fill faster.
  4. The trailer spring brakes apply because supply pressure is lost. Correct answer.

Explanation

The emergency line supplies air that holds off the trailer spring brakes. When it is disconnected, that air escapes and the trailer spring brakes apply, which is a fail-safe design. The trailer does not roll freely, and the tractor compressor is unaffected.

Question 20Air Brake System Components and Operation

What is the function of the slack adjuster?

  1. It measures the air pressure in the brake chambers.
  2. It converts the pushrod stroke into rotation of the brake camshaft to apply the brakes. Correct answer.
  3. It controls the flow of air to the trailer.
  4. It adjusts the compressor's cut-in pressure.

Explanation

The slack adjuster is the lever that links the brake chamber pushrod to the camshaft, turning pushrod travel into brake application. It also takes up slack as linings wear. It does not control air pressure or compressor settings.

Question 21Air Brake System Components and Operation

A driver measures a brake chamber pushrod stroke that exceeds the adjustment limit. What does this mean?

  1. The brakes may be out of adjustment and must be adjusted before driving. Correct answer.
  2. The compressor is building too much pressure.
  3. The trailer supply line is leaking.
  4. The safety valve needs to be replaced.

Explanation

Excessive pushrod stroke means the slack adjuster has not taken up enough wear, so the brakes are out of adjustment and may not apply fully. The vehicle must be taken out of service until the brakes are adjusted. This is a mechanical adjustment issue, not a pressure or leak problem.

Question 22Air Brake System Components and Operation

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

  1. It adds alcohol vapor to the air to lower the freezing point of moisture in the system. Correct answer.
  2. It lubricates the brake camshaft.
  3. It increases the compressor's output pressure.
  4. It removes oil from the compressor.

Explanation

The alcohol evaporator introduces alcohol vapor into the air system so any moisture present is less likely to freeze in cold weather. This helps prevent frozen valves and lines. It does not change compressor output or lubricate mechanical parts.

Question 23Air Brake System Components and Operation

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

  1. It converts air pressure into mechanical force that applies the brakes. Correct answer.
  2. It stores compressed air for the system.
  3. It regulates the compressor's cut-out pressure.
  4. It controls the flow of air to the trailer.

Explanation

The brake chamber receives air pressure and uses a diaphragm and pushrod to convert that pressure into mechanical force. That force is transferred through the slack adjuster to apply the brakes. The chamber does not store air or regulate compressor output.

Question 24Air Brake System Components and Operation

Why does a dual air brake system use a pressure differential valve?

  1. It shuts off the compressor when one circuit fails.
  2. It alerts the driver when pressure in one circuit differs significantly from the other. Correct answer.
  3. It balances the air pressure between the two circuits so they stay equal.
  4. It drains the reservoirs automatically.

Explanation

The pressure differential valve detects a significant pressure difference between the primary and secondary circuits and triggers the warning light. This alerts the driver to a circuit failure. It does not equalize the circuits or control the compressor.

Question 25Air Brake System Components and Operation

What does the brake pedal feel like when the air system is functioning normally?

  1. The pedal should feel spongy and pulse rapidly.
  2. The pedal should sink to the floor with little resistance.
  3. The pedal should have a firm feel that resists pressure as the brakes apply. Correct answer.
  4. The pedal should be completely loose with no return spring action.

Explanation

In a properly operating air brake system, the foot valve meters air and the pedal feels firm as pressure builds in the chambers. A pedal that sinks or feels spongy can indicate a leak or maladjustment. The pedal should also return normally when released.

Question 26Air Brake System Components and Operation

The air compressor on a heavy truck is typically driven by which of the following?

  1. An electric motor wired to the charging system
  2. A dedicated air turbine fed from the wet tank
  3. A belt or gear drive off the engine Correct answer.
  4. A hydraulic motor powered by the power steering pump

Explanation

The compressor is engine-driven, usually through a belt or gear train, so it only builds air while the engine is running. Because it depends on engine speed, compressor output rises and falls with engine rpm.

Question 27Air Brake System Components and Operation

During a stationary pressure-build check, the compressor unloads at its upper control pressure and resumes loading after pressure falls. What controls this cycling?

  1. The brake drum responding to wheel speed
  2. The drain valve measuring brake temperature
  3. The parking-brake knob measuring tire pressure
  4. The governor responding to reservoir pressure Correct answer.

Explanation

The governor responds to air-system pressure and controls when the compressor loads or unloads. The described cycle regulates the air supply rather than measuring wheel or tire conditions.

Question 28Air Brake System Components and Operation

Air brake reservoirs are drained at the end of the day mainly to remove which of the following?

  1. Nitrogen that builds up from tire inflation
  2. Moisture and water that can freeze or corrode the system Correct answer.
  3. Air that has become too warm to operate the brakes
  4. Excess compressor oil that lubricates the valves

Explanation

Compressed air carries water vapor that condenses in the tanks, and draining removes that liquid before it can freeze in lines or corrode valves. Most tanks have manual drain cocks for this purpose, and automatic drain valves also exist.

Question 29Air Brake System Components and Operation

In a dual air brake system, which circuit typically supplies the rear (drive) axle brakes on a heavy truck?

  1. The trailer supply circuit
  2. The secondary circuit
  3. The parking brake circuit
  4. The primary circuit Correct answer.

Explanation

By convention the primary circuit feeds the rear axle service brakes and the secondary circuit feeds the front axle, each with its own reservoir. Either circuit alone must be able to stop the vehicle.

Question 30Air Brake System Components and Operation

What does the safety valve on an air reservoir do?

  1. Drains condensation automatically each time the engine starts
  2. Releases air to the atmosphere if reservoir pressure exceeds a set limit Correct answer.
  3. Prevents the parking brake from releasing below 60 psi
  4. Shuts off the compressor when the engine overheats

Explanation

The safety valve is a pressure-relief device that opens if the governor fails and pressure climbs above a preset value, protecting the tank from overpressure. It is a backup, not the normal means of controlling pressure.

Question 31Air Brake System Components and Operation

A driver finds the air compressor will not build pressure above about 85 psi and the gauge holds steady there. Which component is the most likely cause?

  1. The quick release valves are leaking
  2. The brake chambers have worn diaphragms
  3. The governor cut-out setting is faulty or misadjusted Correct answer.
  4. The slack adjusters are out of adjustment

Explanation

If pressure stabilizes below the normal cut-out, the governor may be unloading the compressor too early. Slack adjusters, quick release valves, and diaphragms would cause stroke, leak, or balance problems rather than a low, stable ceiling.

Question 32Air Brake System Components and Operation

How does an air dryer protect the air brake system?

  1. It adds alcohol vapor to the air to raise the dew point
  2. It removes moisture from the compressed air before it reaches the reservoirs Correct answer.
  3. It filters out brake dust from the wheel ends
  4. It cools the compressor oil so the compressor runs longer

Explanation

The air dryer uses a desiccant to absorb water vapor from the compressed air, and it periodically purges collected moisture through a purge valve. Dry air reduces freeze-ups and corrosion in valves and lines.

Question 33Air Brake System Components and Operation

When the parking brake control (yellow knob) is pulled out, what happens inside the spring brake chambers?

  1. Air pressure is applied to the spring side to set the brakes
  2. Air is released from the spring side and the spring applies the brakes Correct answer.
  3. Air pressure is routed to the service chamber to hold the brakes
  4. Air is released from the service side while the spring stays compressed

Explanation

The spring brake is held off by air pressure; pulling the knob exhausts that air so the spring mechanically applies the brakes. This is why a loss of air pressure causes the brakes to set.

Question 34Air Brake System Components and Operation

A driver observes that the low air pressure warning device activates, but the spring brakes do not apply until pressure falls much lower. What does this indicate?

  1. The warning device is defective because it must trigger at the same pressure the spring brakes apply
  2. The spring brakes are defective because they must apply before the warning sounds
  3. The warning device and the spring brake protection valve are set to different pressures, which is normal design Correct answer.
  4. The compressor is overcharging the system

Explanation

The warning is required to activate by roughly 60 psi, while the spring brakes normally begin to apply at a lower pressure through the protection valve. The gap is intentional so the driver gets warning before brake application.

Question 35Air Brake System Components and Operation

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

  1. It provides a means of applying the spring brakes if the service circuit loses pressure Correct answer.
  2. It reverses the direction of air flow in the supply line
  3. It converts hydraulic pressure to air pressure at the wheel
  4. It inverts the signal from the treadle valve to the front axle

Explanation

The inversion valve allows a pressure signal to apply the spring brakes as a backup when a service circuit fails, so the vehicle can still be stopped. It works with the spring brake chambers and the parking control.

Question 36Air Brake System Components and Operation

Why do some systems use a relay valve near the rear axle rather than routing the treadle signal directly to each brake chamber?

  1. To reduce the number of brake chambers required at the rear
  2. To keep the rear brakes from ever applying with the front
  3. To filter moisture from the air before it reaches the chambers
  4. To shorten the distance air must travel and speed up brake application and release Correct answer.

Explanation

A relay valve uses a small control signal to open a large local supply of air, reducing lag caused by long lines. This makes rear brake response faster and more balanced.

Question 37Air Brake System Components and Operation

What does the tractor protection valve do when the trailer air supply pressure drops too low?

  1. It closes off and prevents the tractor from losing its air to the trailer Correct answer.
  2. It applies only the trailer service brakes
  3. It increases supply pressure to keep the trailer brakes released
  4. It vents the trailer supply line to the atmosphere slowly

Explanation

The tractor protection valve protects the tractor's air supply by closing the trailer lines when pressure falls below a safe level. This also causes the trailer spring brakes to set, preventing a runaway trailer.

Question 38Air Brake System Components and Operation

A driver performs a static leak test and hears a continuous hiss near a brake chamber with the brakes released. What is the most appropriate first step?

  1. Continue driving and recheck at the next stop
  2. Locate the leaking component and repair it before operating the vehicle Correct answer.
  3. Pump the brake pedal to reseat the diaphragm
  4. Increase engine speed to build pressure faster

Explanation

A continuous leak with brakes released indicates a defective diaphragm, fitting, or hose that must be identified and repaired. Operating with an air leak can lead to a pressure loss and emergency brake application.

Question 39Air Brake System Components and Operation

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

  1. It cleans the desiccant in the air dryer
  2. It removes alcohol contamination from the air supply
  3. It adds alcohol vapor to the air to lower the dew point and reduce freeze-ups Correct answer.
  4. It lubricates the compressor with an alcohol-based oil

Explanation

An alcohol evaporator introduces alcohol vapor into the air system so moisture is less likely to freeze in lines and valves during cold weather. It is a freeze-prevention aid, not a cleaner or lubricant.

Question 40Air Brake System Components and Operation

Which condition would cause the service brakes on one axle to apply more slowly than those on the other axle?

  1. An air dryer that is purging normally
  2. A governor set to cut out at 120 psi
  3. A properly functioning relay valve at that axle
  4. A restricted or kinked line feeding that axle's brake chambers Correct answer.

Explanation

A restriction in the line delays air delivery to the chambers, creating a lag in application on that axle. Balanced braking depends on unrestricted, equal-length air paths.

Question 41Air Brake System Components and Operation

What is the purpose of the brake chamber return spring?

  1. To keep the spring brake compressed during normal operation
  2. To release the service brake and retract the pushrod when air is exhausted Correct answer.
  3. To apply the service brake when air pressure is lost
  4. To hold the slack adjuster against the chamber

Explanation

When the driver releases the pedal, air exhausts from the chamber and the return spring pushes the diaphragm and pushrod back, releasing the brake. It does not apply the brake.

Question 42Air Brake System Components and Operation

A driver notices that the tractor's air pressure drops rapidly only when the trailer is connected. Which area should be inspected first?

  1. The tractor's front brake chambers
  2. The trailer supply and service lines and their couplings Correct answer.
  3. The compressor drive belt
  4. The governor cut-in setting

Explanation

Pressure loss that appears only with the trailer connected points to the trailer lines, gladhands, or trailer air system. Isolating the trailer and testing the tractor alone confirms the location.

Question 43Air Brake System Components and Operation

Why is a pressure differential valve used in a dual air brake system?

  1. To alert the driver when one circuit has lost pressure compared with the other Correct answer.
  2. To prevent the compressor from charging either circuit
  3. To bypass the governor during heavy brake use
  4. To equalize pressure between circuits so both always read the same

Explanation

The pressure differential valve senses an imbalance between the primary and secondary circuits and triggers the warning device. It warns of a failed circuit rather than equalizing the two.

Question 44Air Brake System Components and Operation

A driver applies the brakes and the pedal travels much farther than normal before resistance is felt, with no air loss indicated. What is the most likely explanation?

  1. A governor set too low
  2. Excessive pushrod stroke caused by out-of-adjustment slack adjusters Correct answer.
  3. A plugged air dryer purge valve
  4. A faulty low pressure warning switch

Explanation

Long pedal travel with normal air pressure usually means the foundation brakes must move farther to contact the drum, which happens when slack adjusters are out of adjustment. Pushrod stroke should be measured against the limit.

Question 45Air Brake System Components and Operation

During a compressor build-up test from 85 to 100 psi at normal governed speed, the time exceeds the manufacturer's limit. Which cause is most consistent with this result?

  1. A leaking compressor discharge line or worn compressor Correct answer.
  2. A misadjusted parking brake
  3. A low coolant level in the engine
  4. A slack adjuster adjusted too tight

Explanation

Slow build-up means the compressor is not delivering air efficiently, often from a discharge leak, worn rings, or a slipping drive belt. Brake adjustment and cooling system condition do not govern compressor output.

Question 46Air Brake System Components and Operation

A vehicle with a dual air brake system has a failed primary circuit. Which statement best describes the resulting braking capability?

  1. The secondary circuit still provides service braking, though stopping distance increases Correct answer.
  2. The spring brakes release fully and cannot be applied
  3. The primary circuit automatically recharges from the secondary reservoir
  4. The vehicle has no service brakes and must rely on the parking brake

Explanation

The two circuits are isolated so a failure in one still leaves the other available for service braking, with longer stopping distance and a warning to the driver. The failed circuit is not automatically recharged from the other.

Question 47Air Brake System Components and Operation

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

  1. It quickly applies the spring brakes in an emergency
  2. It releases the parking brake automatically at start-up
  3. It dumps trailer supply air when pressure is low
  4. It speeds up the exhaust of air from brake chambers when the pedal is released Correct answer.

Explanation

The quick release valve is located near the chambers and exhausts air locally when the control pressure drops, so brakes release faster than if air had to travel back to the treadle valve. It affects release, not application.

Question 48Air Brake System Components and Operation

A driver finds that the spring brakes will not release even though the reservoir gauge reads 110 psi. Which component is the most likely cause?

  1. The governor cut-in setting
  2. The air dryer desiccant
  3. The parking brake control valve or its delivery line to the spring chambers Correct answer.
  4. The front service brake chambers

Explanation

Spring brakes release when air reaches the spring side of the chamber through the parking control valve. If system pressure is normal but the brakes stay set, the control valve or its delivery plumbing is the likely fault.

Question 49Air Brake System Components and Operation

In an air brake system, what does the treadle valve do when the driver presses the brake pedal?

  1. It releases air from the spring brake chambers to set the parking brake
  2. It signals the governor to unload the compressor
  3. It drains condensation from the wet tank
  4. It meters air pressure from the reservoirs to the brake chambers in proportion to pedal force Correct answer.

Explanation

The foot valve meters reservoir air to the service chambers, with delivery pressure increasing as pedal force increases. Releasing the pedal exhausts that air so the brakes release.

Question 50Air Brake System Components and Operation

What is the function of the low air pressure warning device in an air brake system?

  1. It warns the driver before air pressure falls to a level where braking is compromised Correct answer.
  2. It automatically applies the parking brake at 45 psi
  3. It prevents the compressor from building more than 100 psi
  4. It shuts down the engine when pressure drops

Explanation

The warning device, usually a buzzer and light, alerts the driver when pressure drops to about 60 psi so the vehicle can be stopped safely before spring brakes apply. It does not itself apply the brakes or stop the engine.