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KY CDL Air Brakes practice questions

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Question 1Air Brake System Components and Functions

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

  1. It controls when the compressor cuts in and out to maintain system air pressure within a set range Correct answer.
  2. It limits the maximum application pressure delivered to the service chambers
  3. It releases air from the spring brakes during a normal stop
  4. It converts mechanical energy into air pressure inside the supply tank

Explanation

The governor senses reservoir pressure and signals the unloader mechanism so the compressor stops pumping at the cut-out pressure and resumes at the cut-in pressure. This keeps the system operating in a safe band rather than pumping continuously. It does not itself generate air or apply the brakes.

Question 2Air Brake System Components and Functions

In a typical air brake system, what is the purpose of the supply reservoir (wet tank)?

  1. It stores air that has already passed through the foot valve for delivery to the chambers
  2. It provides the reference pressure that the governor uses to switch the compressor
  3. It receives air directly from the compressor and traps moisture and oil before the air moves downstream Correct answer.
  4. It holds the air that releases the spring brakes when the parking control is pushed in

Explanation

The supply or wet tank is the first reservoir after the compressor, so it collects the condensation and oil carryover that come with hot compressed air. Air then moves to the dry tanks that feed the brake circuits. This is why the wet tank is drained regularly.

Question 3Air Brake System Components and Functions

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

  1. It automatically drains accumulated water each time the compressor cycles
  2. It balances pressure between the primary and secondary circuits
  3. It signals the low pressure warning buzzer when pressure falls
  4. It vents air to the atmosphere if reservoir pressure rises above a preset limit Correct answer.

Explanation

The safety valve is a pressure relief device set above normal governor cut-out pressure. If the governor or unloader fails and pressure keeps climbing, the valve opens to protect the tank and lines from over-pressurization. It is not a drain or a warning device.

Question 4Air Brake System Components and Functions

A driver pushes in the parking brake control (yellow diamond knob) to release the spring brakes. Which component actually allows air to enter the spring brake chambers?

  1. The quick release valve
  2. The parking brake control valve Correct answer.
  3. The relay valve
  4. The tractor protection valve

Explanation

The parking brake control valve is the dash valve that directs reservoir air to the spring brake chambers to compress the springs and release the brakes. Pushing the knob in supplies air; pulling it out exhausts that air so the springs apply the brakes. Other valves may route the air but the control valve initiates the release.

Question 5Air Brake System Components and Functions

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

  1. It automatically bleeds moisture from the service line
  2. It limits how far the brake pedal can be depressed
  3. It converts the linear pushrod stroke from the chamber into rotary motion that applies the brake shoes Correct answer.
  4. It maintains a constant air pressure in the service chamber during a stop

Explanation

The slack adjuster is a lever arm splined to the camshaft. As the brake chamber pushrod extends, it rotates the slack adjuster, which twists the camshaft to spread the shoes. It also provides the mechanical advantage between chamber force and brake torque.

Question 6Air Brake System Components and Functions

Which component supplies reservoir air directly to the brake chambers when the driver presses the treadle valve, allowing a fast application at the rear axle?

  1. The low pressure warning switch
  2. The governor
  3. The relay valve Correct answer.
  4. The alcohol evaporator

Explanation

A relay valve uses a small control signal from the foot valve to open a large reservoir-fed passage close to the chambers. This shortens the distance air must travel, so rear brakes apply and release faster. The governor and warning switch do not deliver air to chambers.

Question 7Air Brake System Components and Functions

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

  1. To warn the driver before air pressure falls to a level that makes braking unsafe Correct answer.
  2. To indicate when the air dryer needs a new desiccant cartridge
  3. To release the spring brakes automatically when pressure drops
  4. To reduce compressor output when the system is overcharged

Explanation

The low pressure warning switch activates a buzzer and light when reservoir pressure drops below a safe threshold, typically around 60 psi. It alerts the driver in time to stop safely before the spring brakes apply from lost pressure. It does not itself apply or release brakes.

Question 8Air Brake System Components and Functions

What does the air dryer do in a CDL air brake system?

  1. It removes moisture and contaminants from compressed air before the air reaches the reservoirs Correct answer.
  2. It regulates the compressor cut-out pressure
  3. It cools the compressor discharge so the compressor can run continuously
  4. It supplies air to the trailer emergency line only

Explanation

The air dryer is installed between the compressor and the first reservoir. It uses a desiccant and often a coalescing filter to strip water vapor and oil from the hot compressed air, then purges collected moisture during the unload cycle. Dry air reduces corrosion and freeze-ups in valves and lines.

Question 9Air Brake System Components and Functions

During a pre-trip inspection, a driver notices the compressor keeps running and reservoir pressure climbs past the normal cut-out point. Which component is the most likely cause?

  1. A stuck unloader or faulty governor Correct answer.
  2. A misadjusted slack adjuster
  3. A leaking service chamber diaphragm
  4. A clogged alcohol evaporator

Explanation

The governor signals the unloader to stop the compressor at cut-out pressure. If the governor is faulty or the unloader sticks, the compressor keeps pumping and pressure rises above the normal range, which is what the safety valve protects against. Chamber leaks and slack adjuster issues would not cause over-pressurization.

Question 10Air Brake System Components and Functions

A tractor-trailer's service brakes apply slowly and release slowly, and the driver hears air escaping near the rear axle only when the brakes are released. Which valve is most likely restricted or faulty?

  1. The safety valve
  2. The quick release valve Correct answer.
  3. The compressor inlet filter
  4. The supply reservoir drain cock

Explanation

The quick release valve is mounted near the rear brake chambers and vents service air quickly when the foot valve is released. If it is clogged or stuck, air leaves the chambers slowly, causing slow release and sluggish brake behavior. A restricted quick release valve does not affect compressor intake or reservoir drains.

Question 11Air Brake System Components and Functions

Why does a CDL vehicle use a dual-circuit (split) air brake system?

  1. To provide a backup circuit so partial braking remains if one circuit fails Correct answer.
  2. To double the compressor output for faster buildup
  3. To allow the trailer to brake independently of the tractor at all times
  4. To eliminate the need for a parking brake

Explanation

A split system divides the vehicle into two separate air circuits, usually front and rear axles. If one circuit loses pressure, the other still provides braking and the driver can stop safely. This redundancy is why the low pressure warning and spring brakes are so important.

Question 12Air Brake System Components and Functions

What is the role of the tractor protection valve?

  1. It releases the trailer spring brakes during a normal stop
  2. It closes the tractor's air lines to the trailer if trailer pressure drops too low or the lines separate Correct answer.
  3. It regulates the pressure in the tractor's front brake circuit
  4. It dumps air from the trailer supply line whenever the compressor cycles

Explanation

The tractor protection valve monitors trailer supply pressure. If the trailer breaks away or its pressure falls too low, the valve closes to keep the tractor's air from escaping and to apply the trailer emergency brakes. This lets the tractor retain air for its own brakes.

Question 13Air Brake System Components and Functions

A driver performs a static leak test: with the engine off and the system fully charged, pressure drops from 120 psi to 100 psi in one minute. What does this indicate?

  1. An air leak that must be found and repaired before driving Correct answer.
  2. That the governor cut-in pressure is set too high
  3. That the safety valve is working correctly
  4. Normal behavior for a system with an air dryer

Explanation

With the engine off and brakes released, a charged system should lose only a small amount of pressure per minute, typically no more than about 2 psi. A 20 psi drop in one minute is excessive and signals a leak in the system. The leak must be repaired before the vehicle is operated.

Question 14Air Brake System Components and Functions

What is the function of the brake chamber return spring in a service brake chamber?

  1. It holds the pushrod out to keep the brakes applied while parked
  2. It limits the maximum air pressure entering the chamber
  3. It retracts the pushrod and releases the brake when chamber air is exhausted Correct answer.
  4. It applies the service brakes when air pressure is lost

Explanation

When the driver releases the foot valve, air exhausts from the service chamber and the return spring pushes the diaphragm and pushrod back to the released position. This retracts the slack adjuster and cam, letting the shoes return. The service brakes are applied by air pressure, not by this spring.

Question 15Air Brake System Components and Functions

In a spring brake chamber, what happens when air pressure is lost from the parking brake circuit?

  1. The governor raises compressor output to compensate
  2. The relay valve blocks all air to the chamber
  3. The service brakes are released and the vehicle rolls freely
  4. The power spring expands and applies the brakes mechanically Correct answer.

Explanation

Spring brakes use a large power spring that is held compressed by air pressure. If that air is lost, the spring extends and applies the brakes mechanically, which is why they are called emergency or parking brakes. This fail-safe design stops the vehicle even without air.

Question 16Air Brake System Components and Functions

A driver must manually adjust a type of slack adjuster that is not automatic. What is the correct goal of the adjustment?

  1. Set the pushrod stroke to the manufacturer's specified range so the brakes fully apply without over-stroking Correct answer.
  2. Adjust until the brake shoes drag continuously on the drum
  3. Set the pushrod stroke as long as possible to increase braking force
  4. Adjust until the chamber touches the axle housing

Explanation

Correct slack adjustment keeps pushrod stroke within the manufacturer's limit. Too much stroke means the chamber runs out of travel and braking force drops; too little causes drag and overheating. The specification is found in the vehicle's maintenance manual.

Question 17Air Brake System Components and Functions

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

  1. To lubricate the brake chamber diaphragms
  2. To introduce alcohol vapor into the air system to prevent moisture from freezing in valves and lines Correct answer.
  3. To remove oil from the compressor discharge
  4. To increase the compressor's cut-out pressure

Explanation

In cold weather, moisture in the air system can freeze and block valves or lines. An alcohol evaporator adds alcohol vapor to the air stream, lowering the freezing point of the moisture. It is a supplement to, not a replacement for, proper draining and a working air dryer.

Question 18Air Brake System Components and Functions

During a pre-trip test, the driver fanning the brakes observes that the low pressure warning buzzer sounds at about 60 psi and the spring brakes apply at about 45 psi. What is the correct interpretation?

  1. The governor cut-out is set too low
  2. The warning switch is faulty because it should sound only after the spring brakes apply
  3. The spring brakes are faulty because they applied too early
  4. The warning system is working as designed, alerting the driver before the spring brakes apply Correct answer.

Explanation

The low pressure warning should activate before the spring brakes apply from lost pressure, giving the driver time to stop safely. A warning around 60 psi followed by spring brake application at a lower pressure is the intended sequence. This confirms the warning system is functioning.

Question 19Air Brake System Components and Functions

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

  1. It controls the compressor unloader
  2. It meters air pressure from the reservoirs to the brake circuits in proportion to pedal movement Correct answer.
  3. It stores air for the trailer emergency line
  4. It automatically adjusts slack adjusters as the brakes wear

Explanation

The foot valve is the driver's primary control for service brakes. Its treadle movement opens a supply port and delivers increasing pressure to the brake circuits, while releasing the pedal exhausts that air. It meters application, not storage or adjustment.

Question 20Air Brake System Components and Functions

A driver applies the brakes and hears a steady hiss from under the dash that continues after the pedal is released. Which component should be suspected first?

  1. The foot valve or its exhaust port Correct answer.
  2. The slack adjuster clevis pin
  3. The compressor inlet filter
  4. The supply reservoir drain cock

Explanation

A continuous hiss near the dash after release points to a leaking foot valve or a stuck-open exhaust port. Air should stop flowing once the pedal returns. A leaking foot valve also causes slow pressure buildup and excessive compressor cycling.

Question 21Air Brake System Components and Functions

Why are the front and rear brake circuits in a dual air system usually separated rather than combined?

  1. So the front brakes can be applied without the rear brakes during normal driving
  2. So the compressor can fill both circuits at different pressures
  3. So the trailer can share air with the tractor's front circuit
  4. So a failure in one circuit still leaves the other circuit able to stop the vehicle Correct answer.

Explanation

Separating the circuits means a leak or failure in one does not drain the other. The driver retains partial braking and can bring the vehicle to a controlled stop. This redundancy is the central safety benefit of a split system.

Question 22Air Brake System Components and Functions

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

  1. To drain moisture from the spring brake chamber
  2. To switch the compressor between load and unload cycles
  3. To increase service brake pressure above reservoir pressure
  4. To allow a modulated spring brake application when the parking circuit loses pressure Correct answer.

Explanation

The inversion valve provides a controlled, modulated application of the spring brakes when the parking/emergency circuit loses pressure, rather than a sudden full application. This helps the driver bring the vehicle to a safer stop. It does not boost pressure or drain moisture.

Question 23Air Brake System Components and Functions

A driver reports that the trailer brakes apply when the trailer supply line is disconnected, but the tractor's brakes remain fully usable. Which combination of components explains this behavior?

  1. The governor and the compressor unloader
  2. The foot valve and the quick release valve
  3. The tractor protection valve and the trailer spring brakes Correct answer.
  4. The relay valve and the safety valve

Explanation

When the supply line separates, the tractor protection valve closes to keep tractor air from escaping. At the same time, loss of trailer supply pressure causes the trailer spring brakes to apply. The tractor keeps its own air and brakes, which is the intended fail-safe response.

Question 24Air Brake System Components and Functions

During a compressor buildup test, the system takes far longer than normal to reach cut-out pressure and the compressor never seems to rest. Which combination of causes is most consistent with these symptoms?

  1. An over-adjusted slack adjuster and a clogged alcohol evaporator
  2. An air leak in the system or a worn compressor, combined with a governor that is not unloading Correct answer.
  3. A governor set too low and a stuck-open safety valve
  4. A faulty low pressure warning switch and a weak return spring

Explanation

Slow buildup with continuous running points to either air escaping faster than the compressor can supply it or a compressor that cannot produce enough air. A governor that fails to unload also keeps the compressor running past cut-out. Leak testing and compressor output checks isolate which cause applies.

Question 25Air Brake System Components and Functions

Why must the supply reservoir be drained more often than the dry reservoirs on many vehicles?

  1. Because it is the only tank connected to the safety valve
  2. Because it feeds the parking brake circuit directly
  3. Because it holds the highest pressure in the system
  4. Because it is the first tank after the compressor and collects most of the condensed moisture and oil Correct answer.

Explanation

Hot compressed air cools as it leaves the compressor, so water vapor condenses in the first tank it reaches. That makes the wet tank the main collection point for moisture and oil carryover. Draining it frequently protects downstream valves and reduces corrosion.

Question 26Air Brake System Components and Functions

In a typical air brake system, what is the function of the governor mounted on the compressor?

  1. It automatically drains moisture from the supply reservoir each cycle
  2. It meters the amount of air delivered to each brake chamber during a stop
  3. It unloads the compressor when reservoir pressure reaches cut-out and lets it resume pumping at cut-in pressure Correct answer.
  4. It limits the maximum application pressure the treadle valve can deliver

Explanation

The governor senses reservoir pressure and controls the compressor's loading and unloading cycle. It unloads the compressor at the cut-out pressure and allows pumping to resume at the cut-in pressure, keeping the system within its designed operating range. It does not meter air to the chambers or drain the reservoirs.

Question 27Air Brake System Components and Functions

Which statement best describes the purpose of a safety valve on an air reservoir?

  1. It releases air to the atmosphere if pressure rises above a preset level, protecting the system from overpressure Correct answer.
  2. It allows the driver to manually dump air from the reservoir during a pre-trip inspection
  3. It prevents air from flowing backward from the reservoir to the compressor
  4. It closes off the reservoir so a leak in one circuit cannot drain the other circuit

Explanation

The safety valve is a pressure-relief device set slightly above the governor cut-out pressure. If the governor fails and the compressor keeps pumping, the safety valve vents excess air to prevent bursting lines or tanks. It is a backup protection, not a normal operating control.

Question 28Air Brake System Components and Functions

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

  1. It reduces reservoir pressure to a steady level for the dash gauge
  2. It speeds up application and release of the brakes by sending reservoir air directly to the chambers near the axle Correct answer.
  3. It holds the parking brakes applied until the supply line is charged
  4. It divides supply air between the front and rear circuits

Explanation

A relay valve is mounted close to the chambers it serves and uses a small control signal to open a large passage from a nearby reservoir. This shortens the distance air must travel, so brakes apply and release faster than if all air came from the foot valve. It is essentially a remote, high-volume switch.

Question 29Air Brake System Components and Functions

A driver notices the air pressure gauge shows a rapid drop when the treadle is released after a full application. Which component is designed to quickly exhaust chamber air so the brakes release promptly?

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

Explanation

A quick release valve is placed near the brake chambers and vents chamber air to the atmosphere when the control signal is removed. This shortens the release path compared with sending air all the way back through the foot valve. Slow or incomplete release often points to a restricted or faulty quick release valve.

Question 30Air Brake System Components and Functions

What does a check valve installed between the compressor and the supply reservoir accomplish?

  1. It limits the pressure delivered to the front brake circuit
  2. It regulates the compressor's cut-in and cut-out pressures
  3. It allows air to flow toward the reservoir but blocks it from flowing back to the compressor Correct answer.
  4. It removes oil and water from the compressed air

Explanation

A check valve is a one-way valve. Placed downstream of the compressor, it lets charged air enter the reservoir while preventing reservoir air from draining back through the compressor when it unloads or stops. This keeps stored pressure available for braking.

Question 31Air Brake System Components and Functions

A driver is inspecting the system and finds a component that senses pressure in the service line and applies the trailer spring brakes if that pressure is lost. What is this component?

  1. The tractor protection valve
  2. The relay valve
  3. The spring brake priority valve
  4. The inversion valve Correct answer.

Explanation

An inversion valve reacts to a loss of service-line pressure by directing reservoir air to release or modulate the spring brakes, allowing controlled stopping. It effectively inverts a loss-of-signal condition into a spring brake application. It is common on vehicles with spring brakes on the trailer or rear axle.

Question 32Air Brake System Components and Functions

What is the purpose of the supply line (emergency line) in a tractor-trailer air brake hookup?

  1. It carries the driver's service application signal to the trailer relay valve
  2. It vents trailer chamber air to speed release
  3. It charges the trailer reservoirs and, when disconnected, triggers a trailer emergency brake application Correct answer.
  4. It carries the trailer's brake light signal back to the tractor

Explanation

The supply line, often color-coded red, delivers air to charge the trailer's reservoirs. If it is disconnected or ruptured, the loss of supply pressure causes the trailer spring brakes to apply, providing a fail-safe breakaway function. The service line, usually blue, carries the application signal.

Question 33Air Brake System Components and Functions

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

  1. It stores pressurized air for later use during braking
  2. It pumps air into the system to build and maintain reservoir pressure Correct answer.
  3. It converts air pressure into mechanical force at the wheel
  4. It regulates the pressure shown on the dash gauge

Explanation

The compressor is driven by the engine and pumps atmospheric air into the system to build and maintain reservoir pressure. Storage is the reservoir's job, and converting pressure into mechanical force happens in the brake chamber. The compressor is the source of the air supply.

Question 34Air Brake System Components and Functions

A driver finds that the front service brakes apply but the rear service brakes do not, and the rear circuit gauge reads normal. Which component should be suspected first?

  1. The air dryer
  2. The low pressure warning switch
  3. The rear circuit relay valve or its control line Correct answer.
  4. The compressor

Explanation

With normal reservoir pressure, a circuit that fails to apply points to the valving or control path for that circuit. The relay valve or its control line delivers reservoir air to the rear chambers, so a fault there would prevent rear application while leaving the front circuit working. Compressor or dryer faults would affect pressure system-wide, not one circuit.

Question 35Air Brake System Components and Functions

A driver observes that the spring brakes will not release even though the parking control valve is pushed in and system pressure is normal. Which component is most directly responsible for supplying air to release the spring brakes?

  1. The compressor governor
  2. The safety valve
  3. The quick release valve
  4. The spring brake control valve and its delivery line to the spring brake chambers Correct answer.

Explanation

Spring brakes are held applied by mechanical springs and are released by air pressure delivered through the parking/spring brake control valve. If that valve or its delivery line fails to pass air, the springs stay applied even with a fully charged system. The other listed components do not supply release air to the spring brake chambers.

Question 36Air Brake System Components and Functions

What is the purpose of the pressure gauge (dash air gauge) in a CDL vehicle?

  1. It indicates the amount of air flowing to the trailer
  2. It displays the parking brake spring force
  3. It measures the compressor's oil temperature
  4. It shows the pressure available in the brake system so the driver can monitor buildup and loss Correct answer.

Explanation

The dash gauge displays system air pressure, usually for each circuit on a dual system. The driver uses it to confirm the compressor is building pressure and to detect leaks or abnormal loss. It is a monitoring instrument, not a control device.

Question 37Air Brake System Components and Functions

A driver hears a continuous leak at the base of the treadle valve only while the brake pedal is held down, and the leak stops when the pedal is released. Which internal condition best explains this?

  1. The supply reservoir drain cock is open
  2. The low pressure warning buzzer is stuck on
  3. The compressor is unloaded
  4. The treadle valve's exhaust valve seat is leaking only during application Correct answer.

Explanation

A leak that occurs only during application points to a fault in the treadle valve's delivery or exhaust valving, such as a worn exhaust seat that does not seal when the valve is shifted. When the pedal is released, the valve returns to a position where that seat no longer leaks. Leaks present all the time would suggest a different source.

Question 38Air Brake System Components and Functions

What does the alcohol evaporator do in an air brake system?

  1. It increases compressor output during cold weather
  2. It injects alcohol vapor into the air system to help prevent moisture from freezing in valves and lines Correct answer.
  3. It lubricates the compressor pistons
  4. It removes alcohol from the air before it reaches the chambers

Explanation

The alcohol evaporator adds alcohol vapor to the compressed air so that any condensed moisture is less likely to freeze in valves, lines, and other components in cold weather. It is a freeze-prevention aid, not a filter or a compressor performance device.

Question 39Air Brake System Components and Functions

A driver reports that the parking brake will not hold on a grade even though the control valve is fully applied and system pressure is normal. Which condition is the most likely cause?

  1. The air dryer purge valve is stuck open
  2. The governor cut-out pressure is set too high
  3. The spring brake chamber's spring is broken or the parking brake linkage is out of adjustment Correct answer.
  4. The relay valve is applying too much service air

Explanation

Spring brakes hold by mechanical spring force when air is released from the chamber. If the spring is broken or the linkage is misadjusted, the brake cannot hold even with correct control valve position and normal system pressure. Service air and compressor settings do not create parking brake holding force.

Question 40Air Brake System Components and Functions

A driver performs a static leak test and finds that air pressure drops noticeably at the spring brake chambers on the rear axle only while the parking brake control valve is in the released (driving) position. Which component is most likely leaking?

  1. The relay valve's service delivery port
  2. The service brake diaphragm inside the service chamber
  3. The spring brake piston seal inside the parking/emergency chamber Correct answer.
  4. The safety valve on the supply reservoir

Explanation

In a spring brake chamber the parking section is pressurized to compress the power spring and release the brakes, so it holds air whenever the control valve is released for driving. A leak that appears only in the released position points to the seal or diaphragm of that spring (parking) section. The service diaphragm is only pressurized during a brake application, and the supply reservoir safety valve is not part of the spring brake release circuit.

Question 41Dual Air Brake System and Safety Features

On a dual air brake system, what is the purpose of the two separate reservoirs (one for the front circuit and one for the rear circuit)?

  1. To balance air pressure between the left and right sides of the vehicle
  2. To provide a backup supply so one circuit can still stop the vehicle if the other loses pressure Correct answer.
  3. To store extra air for the horn and other accessories
  4. To increase the total braking force at the wheels

Explanation

A dual air brake system splits the service brakes into two independent circuits, each with its own reservoir and check valve. If one circuit fails and loses pressure, the other circuit remains charged and can still apply part of the brakes, allowing a controlled stop. This redundancy is the core safety feature of the split system.

Question 42Dual Air Brake System and Safety Features

Where is the supply reservoir, also called the wet tank, located in the air charging system?

  1. After the treadle valve and before the brake chambers
  2. Downstream of the relay valve on the rear axle
  3. Between the compressor and the dual-circuit reservoirs Correct answer.
  4. Inside the cab next to the parking brake valve

Explanation

The compressor discharges into the supply (wet) reservoir first, and the air then passes through check valves into the individual circuit reservoirs. Because it is the first tank to receive compressed air, it collects most of the moisture and oil carryover. This protects the downstream service reservoirs from contamination.

Question 43Dual Air Brake System and Safety Features

What is the function of the check valves installed between the supply reservoir and the dual service reservoirs?

  1. They release excess pressure to the atmosphere when the compressor overcharges
  2. They automatically drain accumulated moisture each time the brakes are applied
  3. They allow air to flow into each service reservoir while preventing backflow that could drain a good circuit Correct answer.
  4. They equalize pressure between the front and rear brake chambers during a stop

Explanation

Check valves permit one-way flow from the wet tank into each circuit reservoir. If one circuit develops a leak, the check valve keeps that circuit from bleeding air back out of the wet tank or the other circuit. This isolates the failure so the remaining circuit stays charged.

Question 44Dual Air Brake System and Safety Features

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

  1. 45 psi
  2. 90 psi
  3. 110 psi
  4. 60 psi Correct answer.

Explanation

Federal standards require a visible and audible warning to come on before the air pressure in the service brake system falls below 60 psi. The warning alerts the driver while enough pressure remains to stop safely. A driver who sees the warning should stop and find the leak before continuing.

Question 45Dual Air Brake System and Safety Features

Why must the low air pressure warning be both visible and audible?

  1. Because the warning must repeat the same information shown on the governor
  2. Because a driver may not be looking at the gauge when pressure begins to fall Correct answer.
  3. Because the audible alarm also serves as the parking brake release signal
  4. Because the gauge is only accurate above 90 psi

Explanation

A pressure loss can develop while the driver is watching the road rather than the gauge. A red warning lamp alone could be missed, so an audible buzzer or alarm is required in addition. Together they give the driver immediate notice to stop before braking ability is lost.

Question 46Dual Air Brake System and Safety Features

During the static leak test on a dual air brake system, what is the maximum allowable pressure loss with the engine off and the brakes released?

  1. 8 psi in two minutes
  2. 5 psi in one minute
  3. 3 psi in one minute Correct answer.
  4. 10 psi in one minute

Explanation

With a fully charged system, the engine off, and the service brakes released, pressure should not drop more than 3 psi in one minute for a single vehicle. A larger drop indicates a leak that must be repaired before driving. The leak test is normally done before the applied-brake test.

Question 47Dual Air Brake System and Safety Features

When performing the applied-brake leak test, how much pressure loss is allowed in one minute with the brakes fully applied and the engine off?

  1. 3 psi
  2. 6 psi
  3. 4 psi Correct answer.
  4. 2 psi

Explanation

With the system charged and the service brake held fully applied, pressure loss must not exceed 4 psi in one minute. Holding the brakes applied pressurizes more of the system, so the allowable loss is slightly higher than in the released test. Any greater drop means a leak must be located and fixed.

Question 48Dual Air Brake System and Safety Features

What does the safety valve on the air brake system do if the governor fails and the compressor keeps pumping?

  1. It dumps all air from the service reservoirs and applies the spring brakes
  2. It redirects excess air into the brake chambers to increase holding force
  3. It shuts off the compressor drive belt automatically
  4. It opens at a preset pressure and releases air to prevent overpressurization of the system Correct answer.

Explanation

The safety valve is a pressure relief device set slightly above the governor cut-out pressure. If the governor sticks and the compressor does not unload, the safety valve vents air to the atmosphere to protect reservoirs and lines from bursting. It is a backup protection, not a normal pressure regulator.

Question 49Dual Air Brake System and Safety Features

The governor on a dual air brake system normally cuts the compressor in at about 100 psi and out at about 125 psi. What does the cut-out function accomplish?

  1. It opens the safety valve to vent air to the atmosphere
  2. It unloads the compressor so it stops building pressure once the reservoirs are fully charged Correct answer.
  3. It sends air directly to the brake chambers to hold them ready
  4. It applies the spring brakes when pressure reaches 125 psi

Explanation

When reservoir pressure reaches the cut-out setting, the governor signals the compressor to unload so it circulates air without building further pressure. This prevents overpressurization and reduces wear. When pressure falls to the cut-in setting, the governor reloads the compressor to recharge the system.

Question 50Dual Air Brake System and Safety Features

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

  1. It cleans the air of carbon deposits before it reaches the reservoirs
  2. It lubricates the compressor pistons during operation
  3. It introduces alcohol vapor into the air system to prevent moisture from freezing in valves and lines Correct answer.
  4. It increases reservoir pressure during cold-weather braking

Explanation

An alcohol evaporator adds alcohol vapor to the compressed air so that any moisture present will not freeze in lines, valves, or the treadle valve during cold weather. Freezing moisture can block air flow and cause brake failure or drag. The evaporator supplements, but does not replace, regular draining of the reservoirs.