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KS CDL Air Brakes study materials
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KS CDL Air Brakes practice questions

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Question 1Air brake system components and their functions

On a standard air brake system, what is the primary function of the air compressor?

  1. It converts air pressure into mechanical force at the slack adjuster.
  2. It builds and maintains the supply of compressed air in the reservoirs. Correct answer.
  3. It filters moisture and oil out of the air before it reaches the treadle valve.
  4. It releases air pressure from the brake chambers during a stop.

Explanation

The compressor is the source of compressed air for the whole system; it charges the reservoirs so that stored air is available for braking. Moisture removal is the job of the air dryer or filters, not the compressor itself.

Question 2Air brake system components and their functions

What is the purpose of the safety valve installed on the air reservoir?

  1. It drains accumulated water each time the brakes are applied.
  2. It signals the driver with a buzzer when pressure is low.
  3. It closes off the reservoir so the compressor cannot recharge it.
  4. It opens automatically to release air if reservoir pressure exceeds a safe limit. Correct answer.

Explanation

The safety valve is a pressure-relief device that vents air if pressure rises above its setting, protecting the tank and system from over-pressurization. It is a protective device, not a control or warning device.

Question 3Air brake system components and their functions

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

  1. It eliminates the need for a parking brake.
  2. It allows the compressor to run at a lower pressure.
  3. It doubles the braking force available at the wheels.
  4. If one circuit loses pressure, the other circuit can still provide braking. Correct answer.

Explanation

Dual circuits are a redundancy feature: a failure or leak in one circuit leaves the other circuit able to apply brakes. Total braking force is not increased by having two circuits.

Question 4Air brake system components and their functions

What does the governor do in an air brake system?

  1. It regulates how much air reaches each brake chamber during a stop.
  2. It measures the amount of air lost through leaks.
  3. It limits the maximum speed of the vehicle on downgrades.
  4. It controls when the compressor loads and unloads to maintain proper reservoir pressure. Correct answer.

Explanation

The governor senses reservoir pressure and cycles the compressor between loading and unloading so pressure stays within the designed range. It works on the compressor, not on the individual brake chambers.

Question 5Air brake system components and their functions

What is the function of the slack adjuster?

  1. It stores compressed air for emergency braking.
  2. It converts the pushrod stroke from the brake chamber into rotation of the brake camshaft. Correct answer.
  3. It dries the air before it enters the supply reservoir.
  4. It senses reservoir pressure and signals the governor.

Explanation

The slack adjuster is a lever on the brake camshaft that turns pushrod travel into camshaft rotation, applying the brakes. It also provides a means of adjusting brake clearance.

Question 6Air brake system components and their functions

What does the low air pressure warning device do when reservoir pressure drops below a safe level?

  1. It shuts off the compressor to prevent overpressure.
  2. It opens the safety valve to vent the remaining air.
  3. It gives a visible and audible warning to the driver before braking ability is lost. Correct answer.
  4. It automatically applies the parking brake to stop the vehicle.

Explanation

The warning device alerts the driver with a light and buzzer when pressure falls into an unsafe range, giving time to stop safely. It does not itself apply brakes or vent air.

Question 7Air brake system components and their functions

What is the purpose of the air dryer in a heavy vehicle air brake system?

  1. It increases the pressure of the air leaving the compressor.
  2. It lubricates the brake chambers with oil mist.
  3. It regulates the application rate of the service brakes.
  4. It removes moisture and contaminants from the compressed air before storage. Correct answer.

Explanation

The air dryer uses a desiccant to remove water vapor and contaminants so that clean, dry air reaches the reservoirs. Dry air reduces corrosion and prevents freeze-ups in the system.

Question 8Air brake system components and their functions

What type of brake chamber uses a large spring to apply the brakes when air pressure is lost?

  1. Service brake chamber
  2. Spring brake chamber Correct answer.
  3. Relay valve chamber
  4. Quick release valve chamber

Explanation

Spring brake chambers contain a powerful spring that applies the brakes when air pressure is released, providing emergency and parking braking. Service chambers apply brakes only when air pressure is delivered to them.

Question 9Air brake system components and their functions

When the driver presses the brake pedal (treadle valve), how does air reach the brake chambers on a typical truck?

  1. The foot valve sends a control signal that causes relay valves to deliver air from the reservoirs to the chambers. Correct answer.
  2. The governor opens and lets air flow to the chambers.
  3. The safety valve releases air into the chambers.
  4. The foot valve directly connects the compressor to the brake chambers.

Explanation

The foot valve meters a control signal; relay valves near the axles then admit reservoir air to the chambers in proportion to that signal. This keeps pedal effort low and response quick.

Question 10Air brake system components and their functions

A driver hears the low pressure buzzer and sees the warning light while driving. What is the safest immediate action?

  1. Move to a safe location and stop, because braking ability may soon be lost. Correct answer.
  2. Pump the brake pedal repeatedly to rebuild pressure.
  3. Shift to a higher gear to reduce engine load.
  4. Continue at highway speed to reach the next scheduled stop.

Explanation

A low pressure warning means the system cannot guarantee full braking, so the driver should stop safely as soon as possible. Continuing to drive risks losing service brakes entirely.

Question 11Air brake system components and their functions

During a pre-trip inspection, the driver fully applies the parking brake control (spring brakes). What should happen to the pushrods on the spring brake chambers?

  1. They should rotate the slack adjuster backwards.
  2. They should extend as the spring applies the brakes. Correct answer.
  3. They should retract fully into the chamber.
  4. They should remain in exactly the same position.

Explanation

Releasing air from the spring brake chamber lets the spring push the pushrod out, applying the brakes. The pushrod extension confirms the spring brake is functioning.

Question 12Air brake system components and their functions

Why must the air compressor have a way to unload (stop pumping) once reservoirs reach the cut-out pressure?

  1. To open the safety valve on each reservoir.
  2. To prevent overpressurization and reduce wear and heat when pressure is already sufficient. Correct answer.
  3. To allow the air dryer to reverse its airflow.
  4. To increase the volume of air delivered to the brake chambers.

Explanation

The governor unloads the compressor at cut-out pressure so it stops building pressure until it drops to cut-in pressure. This protects the system from overpressure and reduces unnecessary compressor wear and heat.

Question 13Air brake system components and their functions

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

  1. It quickly applies the spring brakes in an emergency.
  2. It speeds up the exhaust of air from the brake chambers when the brake is released. Correct answer.
  3. It rapidly recharges the supply reservoir after a stop.
  4. It quickly closes the safety valve after venting.

Explanation

A quick release valve is located near the brake chambers and vents their air locally when the control pressure drops, so brakes release faster than if air had to travel back to the foot valve.

Question 14Air brake system components and their functions

A driver notices that air pressure builds very slowly and the compressor runs almost constantly. Which component is the most likely cause?

  1. A slack adjuster that is out of adjustment
  2. A worn or leaking compressor or a serious leak in the supply system Correct answer.
  3. A faulty low pressure warning switch
  4. A misadjusted relay valve

Explanation

Slow pressure buildup with continuous compressor operation points to the compressor itself or to leaks that prevent the system from holding pressure. Slack adjusters and warning switches do not affect the rate of pressure buildup.

Question 15Air brake system components and their functions

What is the purpose of the alcohol evaporator or injector sometimes found in air brake systems?

  1. It introduces alcohol into the air system to help prevent moisture from freezing in valves and lines. Correct answer.
  2. It increases the octane rating of the fuel.
  3. It reduces the pressure needed to apply the brakes.
  4. It lubricates the compressor pistons.

Explanation

Alcohol evaporators add a small amount of alcohol to the air stream so that any remaining moisture is less likely to freeze in valves and lines in cold weather. They do not affect fuel or brake application pressure.

Question 16Air brake system components and their functions

During a static leak test, the driver charges the system, shuts off the engine, and watches the gauge. What does a steady pressure drop most directly indicate?

  1. Air is escaping somewhere in the system, such as a leaking line, fitting, or valve. Correct answer.
  2. The governor is stuck in the loaded position.
  3. The slack adjusters are over-adjusted.
  4. The compressor is overcharging the reservoirs.

Explanation

With the engine off and no air being consumed, pressure should hold steady; a steady drop means air is leaking out. The rate of drop helps judge whether the leak is acceptable.

Question 17Air brake system components and their functions

What does the brake chamber pushrod stroke measurement tell an inspector?

  1. Whether the air dryer desiccant is saturated.
  2. Whether the governor cut-in pressure is correct.
  3. Whether the brake adjustment is within the allowable limit for that chamber type. Correct answer.
  4. Whether the compressor is producing enough pressure.

Explanation

Pushrod stroke indicates how far the brake mechanism must travel to apply the brakes; excessive stroke means the brakes are out of adjustment. Each chamber type has a specified maximum stroke.

Question 18Air brake system components and their functions

Why is it important that the spring brake chambers be equipped with a release (caging) bolt?

  1. It locks the pushrod in the applied position for parking.
  2. It allows the spring to be mechanically compressed so the vehicle can be moved if air pressure is unavailable. Correct answer.
  3. It drains moisture from the brake chamber.
  4. It adjusts the brake clearance automatically.

Explanation

The caging bolt lets a technician manually compress and hold the spring so the brake can be released for towing or moving when there is no air pressure. It is a service aid, not a parking lock or adjustment device.

Question 19Air brake system components and their functions

What is the role of the supply reservoir in the air brake system?

  1. It converts air pressure into electrical signals for the warning system.
  2. It stores compressed air so the system has a reserve for braking and for charging other circuits. Correct answer.
  3. It measures the amount of air used during each stop.
  4. It meters air directly to the brake chambers when the pedal is pressed.

Explanation

The supply reservoir holds compressed air delivered by the compressor, providing a reserve that feeds the rest of the system. It does not itself meter air to the chambers; that is the job of valves.

Question 20Air brake system components and their functions

A truck's brakes release slowly after the driver lifts off the pedal. Which component is the most likely cause?

  1. A restricted or faulty quick release valve or exhaust passage Correct answer.
  2. A slack adjuster that is too tight
  3. A governor set to a low cut-out pressure
  4. An overfilled supply reservoir

Explanation

Slow release usually means air cannot exhaust quickly from the chambers, often because of a clogged quick release valve or blocked exhaust passage. Governor settings affect charging, not release speed.

Question 21Air brake system components and their functions

Why does a dual circuit system use a foot valve with two separate delivery sections?

  1. So a loss of pressure in one circuit still leaves the other circuit controllable by the pedal. Correct answer.
  2. So the driver can apply front and rear brakes independently by choice.
  3. So the compressor can charge both circuits simultaneously.
  4. So the parking brake can be released without air pressure.

Explanation

The dual foot valve has separate supply and delivery sections so that a failure in one circuit does not disable the other. The driver still applies both circuits with a single pedal motion.

Question 22Air brake system components and their functions

An inspector finds that governor cut-in pressure is 85 psi and cut-out is 105 psi on a system specified for 100 psi cut-out. What is the main concern?

  1. The compressor may be cycling at pressures outside the manufacturer's specification, which can affect braking performance and component life. Correct answer.
  2. The safety valve will open every time the compressor unloads.
  3. The spring brakes will release automatically at 85 psi.
  4. The air dryer will stop working below 105 psi.

Explanation

Governor settings must match the manufacturer's specification so the system maintains adequate pressure without overpressurizing. Incorrect cut-in and cut-out points can reduce braking reserve or stress components.

Question 23Air brake system components and their functions

Why must a vehicle with spring brakes have enough air pressure to fully release them before it can be moved normally?

  1. The spring brakes only release when the compressor is unloaded.
  2. The spring brakes stay applied until air pressure compresses the spring, so without enough air the wheels remain braked. Correct answer.
  3. The spring brakes release only when the low pressure warning sounds.
  4. The spring brakes require the safety valve to be open to release.

Explanation

Spring brakes are applied by spring force and released by air pressure acting against the spring. If system pressure is too low to compress the spring, the brakes stay applied and the vehicle cannot roll.

Question 24Air brake system components and their functions

A driver reports that the low pressure warning does not activate even when reservoir pressure falls well below the specified threshold. Why is this a serious safety concern?

  1. The driver may lose braking ability without any warning, so a failed warning device removes a critical early alert. Correct answer.
  2. The spring brakes will apply too early.
  3. The compressor will overheat because it cannot unload.
  4. The air dryer will stop removing moisture.

Explanation

The warning device is the driver's early alert that pressure is dropping toward an unsafe level. If it fails, the driver may continue driving until service braking is compromised.

Question 25Air brake system components and their functions

What is the function of check valves in the air brake supply system?

  1. They limit the maximum pressure in the brake chambers.
  2. They allow air to flow in one direction only, protecting charged reservoirs from backflow losses. Correct answer.
  3. They convert control signals into brake applications.
  4. They automatically drain water from the reservoirs.

Explanation

Check valves permit air to flow into a reservoir but prevent it from flowing back out, so a loss in one part of the system does not drain other charged reservoirs. They are one-way flow devices.

Question 26Air brake system components and their functions

On a heavy vehicle air brake system, what is the primary job of the compressor governor?

  1. It meters the amount of air delivered to each brake chamber during a stop.
  2. It removes moisture and oil vapor from the air before it reaches the reservoirs.
  3. It regulates when the compressor pumps and when it idles by sensing reservoir pressure. Correct answer.
  4. It limits the maximum pushrod travel of the slack adjuster during braking.

Explanation

The governor is a pressure-sensitive control that monitors reservoir pressure and signals the compressor to load (pump) at cut-in pressure and unload (idle) at cut-out pressure. It does not meter air to the chambers, dry the air, or limit pushrod stroke; those are handled by other components.

Question 27Air brake system components and their functions

Which statement best describes the function of the safety valve installed on an air reservoir?

  1. It releases air to atmosphere if reservoir pressure rises above a preset limit. Correct answer.
  2. It supplies a reduced-pressure signal to the low pressure warning switch.
  3. It prevents air from flowing backward from the reservoir to the compressor.
  4. It automatically drains accumulated water each time the compressor cycles.

Explanation

The safety valve is a pressure relief device set above governor cut-out so that a failed governor cannot overpressurize the reservoir. It vents excess air to atmosphere rather than controlling water, backflow, or warning signals.

Question 28Air brake system components and their functions

What is the purpose of the slack adjuster in an air brake assembly?

  1. It converts the linear push of the chamber rod into rotation that applies the foundation brake. Correct answer.
  2. It stores compressed air for release of the spring brakes.
  3. It filters contaminants from the air before it enters the foot valve.
  4. It senses reservoir pressure and signals the compressor to load.

Explanation

The slack adjuster is a lever arm mounted on the brake camshaft; as the chamber pushrod extends it rotates the camshaft to apply the brake shoes. It also provides the adjustment point for pushrod stroke. It does not store air, sense pressure, or filter air.

Question 29Air brake system components and their functions

In a typical truck air brake system, what does the foot valve (treadle valve) do when the driver applies the brake?

  1. It pumps air into the supply reservoir to maintain pressure.
  2. It closes the exhaust port so spring brakes can release.
  3. It opens a path that admits reservoir air to the delivery lines at a pressure set by pedal travel. Correct answer.
  4. It mechanically pulls the brake shoes against the drum through a cable.

Explanation

The foot valve is the driver's pressure-control device: depressing the treadle admits air from the supply circuit to the delivery lines in proportion to pedal movement. The air then does the work in the chambers; there is no cable linkage and the valve does not generate pressure.

Question 30Air brake system components and their functions

A driver applies the brakes and the chamber pushrods extend, but the brakes feel weak and the pushrods travel much farther than the readjustment limit. What is the most likely explanation?

  1. The slack adjusters are out of adjustment, so much of the chamber stroke is used taking up clearance. Correct answer.
  2. The safety valve is opening too early and dumping reservoir pressure.
  3. The alcohol evaporator has run out of fluid.
  4. The compressor governor cut-out pressure is set too high.

Explanation

Excessive pushrod stroke means the chamber is spending its limited stroke on clearance rather than applying force to the shoes, so braking force drops. This is the classic symptom of brakes needing adjustment. Safety valve, governor setting, and alcohol supply affect pressure or moisture, not stroke length.

Question 31Air brake system components and their functions

What is the function of the relay valve in a system where it is used to speed up brake application at the rear axles?

  1. It converts electrical signals from the tractor into air pressure.
  2. It reduces the pressure sent to the front brakes so they do not lock.
  3. It automatically cages the spring brakes when pressure is lost.
  4. It uses a small control signal from the foot valve to deliver reservoir air directly to nearby brake chambers. Correct answer.

Explanation

A relay valve shortens the air path: a small control pressure from the foot valve actuates the relay, which then supplies air from a nearby reservoir straight to the chambers. This speeds application and release on long vehicles. It does not proportion front brake pressure, cage springs, or convert electrical signals.

Question 32Air brake system components and their functions

Why is a drain cock or automatic drain valve provided at the bottom of each air reservoir?

  1. To release excess pressure when the governor fails.
  2. To allow the reservoir to be used as a jacking point.
  3. To supply air to the low pressure warning buzzer.
  4. To remove condensed water and oil that collect at the low point of the tank. Correct answer.

Explanation

Moisture and oil carried by compressed air condense and settle at the bottom of the reservoir, so a drain at the low point lets the driver or an automatic valve expel them. Keeping the tank dry protects valves and prevents freeze-ups. Overpressure relief is the safety valve's job, not the drain's.

Question 33Air brake system components and their functions

During a pre-trip inspection the driver finds that the spring brakes will not release even though the system is charged to normal pressure. Which component should be suspected first?

  1. The compressor drive belt.
  2. The low pressure warning switch.
  3. The air dryer desiccant cartridge.
  4. The spring brake control valve or its delivery line to the chambers. Correct answer.

Explanation

Spring brakes release only when air is delivered to the spring brake side of the chamber. If the system is at normal pressure but the brakes stay set, the control valve or the line feeding the spring brake chambers is the likely fault. A compressor belt or dryer problem would show as low pressure, and the warning switch does not control release.

Question 34Air brake system components and their functions

What does the inversion (anti-compounding) valve do on a vehicle equipped with spring brakes?

  1. It converts spring brake air into a signal for the stop lamps.
  2. It reverses the direction of pushrod travel when the parking brake is set.
  3. It prevents service brake air and spring brake force from being applied to the same chamber at the same time. Correct answer.
  4. It inverts the air flow so the compressor can draw from the reservoir.

Explanation

The inversion valve is designed so that service brake application is prevented when the spring brakes are applied, avoiding compounding of forces that could damage the chamber or linkage. It is a control logic device, not a flow reverser or lamp switch.

Question 35Air brake system components and their functions

A driver reports that the parking brake will not hold on a grade even though the spring brake chambers are new and the system holds pressure. What is the most likely cause?

  1. The spring brake chamber is not receiving full release air, so the spring cannot fully set. Correct answer.
  2. The compressor is building too much pressure.
  3. The air dryer purge valve is stuck open.
  4. The brake drums are too cool.

Explanation

Spring brakes apply by spring force when air is exhausted; if release air is trapped or only partly exhausted, the spring cannot apply fully and holding power drops. New chambers do not help if the control or exhaust path is restricted. Temperature, excess pressure, and dryer purge do not explain weak parking force.

Question 36Air brake system components and their functions

What is the role of the quick release valve in a system where it is installed near the brake chambers?

  1. It prevents the spring brakes from applying automatically.
  2. It admits air to the chambers faster than the foot valve could.
  3. It limits the maximum pressure the chambers can receive.
  4. It exhausts chamber air locally so the brakes release faster than if air had to travel back to the foot valve. Correct answer.

Explanation

A quick release valve senses the drop in control pressure and dumps the chamber air right at the axle, shortening the release path. It does not boost application pressure, cap chamber pressure, or disable spring brake application.

Question 37Air brake system components and their functions

An inspector finds that the low pressure warning activates at 55 psi on a system where the specification requires activation before pressure falls below 60 psi. What does this indicate?

  1. The air dryer is not purging.
  2. The compressor is producing too much air.
  3. The warning switch is out of calibration and should be adjusted or replaced. Correct answer.
  4. The slack adjusters are over-adjusted.

Explanation

The warning device must come on before reservoir pressure drops below the specified threshold so the driver has time to react. Activating late means the pressure switch is out of calibration or faulty. Compressor output, slack adjustment, and dryer purge do not change the warning set point.

Question 38Air brake system components and their functions

What does the application (service) portion of a combination spring brake chamber do?

  1. It applies the parking brake when air is exhausted.
  2. It applies the service brake when air pressure pushes the diaphragm and pushrod. Correct answer.
  3. It regulates the compressor cut-in and cut-out pressures.
  4. It stores air for the trailer emergency circuit.

Explanation

A combination chamber has two sections: the service section, which applies the brake when air pressure acts on the diaphragm, and the spring section, which applies the parking brake when air is exhausted. The service side does not store trailer air or set governor pressures.

Question 39Air brake system components and their functions

A driver reports that the compressor runs continuously and system pressure climbs past the normal cut-out point until the safety valve vents. Which component has most likely failed?

  1. The governor has failed to unload the compressor at cut-out pressure. Correct answer.
  2. The slack adjusters are out of adjustment.
  3. The low pressure warning switch has failed closed.
  4. The foot valve has an internal leak.

Explanation

The governor is responsible for unloading the compressor at cut-out pressure; if it fails to do so, pressure keeps rising until the safety valve relieves it. A leaking foot valve would cause pressure loss, not overpressure. The warning switch and slack adjusters do not control compressor loading.

Question 40Air brake system components and their functions

Why does a dual circuit air brake system use a foot valve with two separate supply and delivery sections?

  1. So that the compressor can supply two different pressures at once.
  2. So that the driver can select front-only or rear-only braking from the cab.
  3. So that the parking brake can be released without air pressure.
  4. So that a loss of pressure in one circuit still leaves the other circuit able to apply some braking. Correct answer.

Explanation

Splitting the system into two circuits, each with its own foot valve section, means a failure in one circuit does not disable the other, preserving a degree of braking. The driver does not select circuits manually, spring brakes still need air to release, and the compressor produces one pressure level.

Question 41Dual air brake systems and safety features (low pressure warning, spring brakes, parking brakes)

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

  1. To provide a backup circuit so partial braking remains if one circuit fails Correct answer.
  2. To reduce the number of air tanks needed
  3. To increase the top speed of the vehicle
  4. To allow the vehicle to run two trailers at once

Explanation

A dual air brake system splits the brakes into two independent circuits, each with its own reservoir, so that a failure or leak in one circuit still leaves the other circuit able to apply some braking. This redundancy is the central safety feature of the design.

Question 42Dual air brake systems and safety features (low pressure warning, spring brakes, parking brakes)

On a dual air brake system, the pressure gauge or gauges are used to monitor what?

  1. The pressure in the fuel and hydraulic systems
  2. The air pressure in each of the two service brake circuits Correct answer.
  3. Engine oil pressure and coolant temperature
  4. Tire pressure in the steer and drive axles

Explanation

The air gauge (often a two-needle gauge) shows the pressure in each service brake circuit separately so the driver can spot a loss in either circuit. The driver must watch these readings because a dropping needle signals a leak or failure.

Question 43Dual air brake systems and safety features (low pressure warning, spring brakes, parking brakes)

What does the low air pressure warning device do on a vehicle with air brakes?

  1. It shuts off the engine to protect the compressor
  2. It releases the parking brakes automatically
  3. It automatically applies the trailer brakes only
  4. It warns the driver when system pressure drops below a safe operating level Correct answer.

Explanation

The low pressure warning device activates when air pressure falls into an unsafe range, alerting the driver before braking ability is lost. It may be a buzzer, a light, or both, and it must be working for the vehicle to be operated.

Question 44Dual air brake systems and safety features (low pressure warning, spring brakes, parking brakes)

At approximately what pressure must the low air pressure warning signal activate on a vehicle with air brakes?

  1. Below 20 psi
  2. Below 90 psi
  3. Below 120 psi
  4. Below 60 psi Correct answer.

Explanation

The warning must come on before the pressure drops below roughly 60 psi, giving the driver time to stop safely before the spring brakes apply. Drivers should know this threshold so they recognize a developing air loss.

Question 45Dual air brake systems and safety features (low pressure warning, spring brakes, parking brakes)

What type of brakes are the spring brakes on an air brake vehicle?

  1. The normal service brakes used with the foot pedal
  2. A hydraulic backup to the air system
  3. The front-wheel steering brakes
  4. A mechanical emergency and parking brake held off by air pressure Correct answer.

Explanation

Spring brakes are applied by heavy springs and held in the released position by air pressure. If air pressure is lost, the springs apply the brakes automatically, which is why they serve as both an emergency and parking brake.

Question 46Dual air brake systems and safety features (low pressure warning, spring brakes, parking brakes)

Why will the spring brakes apply automatically if air pressure is lost?

  1. Because the compressor reverses direction
  2. Because the trailer disconnects from the tractor
  3. Because the foot pedal is spring-loaded to the floor
  4. Because the springs are held back by air pressure and release when pressure drops Correct answer.

Explanation

Spring brakes are designed so that air pressure keeps the springs compressed and the brakes released. When pressure is lost, nothing holds the springs back, so they apply the brakes. This fail-safe design is why a vehicle will not roll freely after a major air loss.

Question 47Dual air brake systems and safety features (low pressure warning, spring brakes, parking brakes)

Before driving, a driver performs a test in which the parking brake valve is pulled and the vehicle should not move. What does this confirm?

  1. That the service brakes are worn out
  2. That the compressor is fully charged
  3. That the spring brakes can hold the vehicle when parked Correct answer.
  4. That the trailer lights are working

Explanation

Pulling the parking brake valve releases air from the spring brake chambers so the springs apply and hold the vehicle. If the vehicle moves with the valve pulled, the spring brakes are not holding and the vehicle must not be driven.

Question 48Dual air brake systems and safety features (low pressure warning, spring brakes, parking brakes)

How is the parking brake normally released on a vehicle with spring brakes?

  1. By pulling the trailer supply valve
  2. By turning off the engine
  3. By pushing in the parking brake valve to supply air to the spring chambers Correct answer.
  4. By pressing the foot pedal firmly

Explanation

Pushing in the parking brake control valve restores air pressure to the spring brake chambers, compressing the springs and releasing the brakes. The vehicle must have enough air pressure for this release to occur.

Question 49Dual air brake systems and safety features (low pressure warning, spring brakes, parking brakes)

A driver notices that the air pressure in one circuit is dropping steadily while the other circuit holds steady. What is the most appropriate first action?

  1. Stop safely and investigate the leak before continuing Correct answer.
  2. Release the parking brake to reset the system
  3. Increase engine speed to build pressure faster
  4. Continue driving but use only the good circuit

Explanation

A steady pressure drop in one circuit indicates a leak or failure that could worsen and eventually cause the spring brakes to apply. The safe response is to stop in a safe location and find the problem before continuing.

Question 50Dual air brake systems and safety features (low pressure warning, spring brakes, parking brakes)

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

  1. To control when the compressor builds air and when it stops Correct answer.
  2. To regulate the engine coolant flow
  3. To apply the spring brakes
  4. To measure vehicle speed

Explanation

The governor senses reservoir pressure and cuts the compressor in when pressure falls to the cut-in level and out at the cut-out level. This keeps system pressure within the proper operating range without overloading the compressor.