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

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Question 1Air Brake System Components and Functions

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

  1. It converts air pressure into mechanical force at the wheels
  2. It regulates the amount of air sent to the brake chambers
  3. It builds and maintains air pressure in the storage tanks Correct answer.
  4. It releases air from the system when pressure is too high

Explanation

The compressor is the source of air pressure; it pumps air into the storage tanks so the system has a supply to work with. It does not meter air to the chambers or release excess pressure, and it does not create mechanical braking force at the wheels. Those jobs belong to other components such as the governor, valves, and brake chambers.

Question 2Air Brake System Components and Functions

What does the governor do in an air brake system?

  1. It cuts the compressor in and out to keep tank pressure within a set range Correct answer.
  2. It limits how fast the brake chambers can release air
  3. It drains moisture from the tanks each time the brakes are applied
  4. It balances braking force between the front and rear axles

Explanation

The governor senses system pressure and signals the compressor to stop pumping when the tanks reach the cut-out pressure, then to resume when pressure falls to the cut-in point. This cycling keeps pressure within the designed operating range. Moisture removal, release timing, and brake force balance are handled by other components.

Question 3Air Brake System Components and Functions

What is the purpose of the safety valve on an air brake tank?

  1. It keeps the compressor from running continuously
  2. It supplies air to the trailer emergency line
  3. It releases air if tank pressure rises above a safe limit Correct answer.
  4. It automatically drains water from the tank

Explanation

The safety valve is a pressure relief device that opens if system pressure climbs above a preset maximum, protecting tanks and lines from over-pressurization. It is a backup protection device, not a routine pressure regulator. It does not drain moisture or control compressor cycling, and it is not part of the trailer supply circuit.

Question 4Air Brake System Components and Functions

What is the function of the slack adjuster?

  1. It converts pushrod stroke into rotation of the brake camshaft Correct answer.
  2. It stores compressed air for emergency braking
  3. It signals the driver when pressure is low
  4. It meters air pressure to each brake chamber

Explanation

The slack adjuster is a lever attached to the brake camshaft; as the pushrod extends, the slack adjuster rotates the camshaft to apply the brakes. It also provides a means of adjusting for lining wear. Storing air, metering pressure, and warning the driver are functions of other parts.

Question 5Air Brake System Components and Functions

What is the purpose of the low-pressure warning device on an air brake equipped vehicle?

  1. It alerts the driver when system pressure drops to an unsafe level Correct answer.
  2. It releases air from the spring brakes during normal driving
  3. It prevents the compressor from overcharging the tanks
  4. It automatically applies the parking brake when pressure falls

Explanation

The low-pressure warning system, usually a buzzer and light, activates when air pressure falls below a safe operating level so the driver can act before braking ability is lost. It is a warning device only; it does not apply the parking brake or control the compressor. Releasing spring brakes during normal driving is done by the parking brake valve.

Question 6Air Brake System Components and Functions

What does the foot valve (brake valve) do when the driver presses the brake pedal?

  1. It resets the governor cut-in pressure
  2. It sends air pressure from the tanks to the brake chambers in proportion to pedal force Correct answer.
  3. It exhausts all air from the brake chambers to apply the brakes
  4. It closes the supply line between tractor and trailer

Explanation

The foot valve is the driver's control for service braking; depressing the pedal admits air from the supply tanks toward the brake chambers, with more pedal force delivering more pressure. Releasing the pedal exhausts that air so the brakes release. It does not control the supply line or the governor.

Question 7Air Brake System Components and Functions

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

  1. They apply automatically when air pressure is lost Correct answer.
  2. They regulate air pressure delivered to the service chambers
  3. They hold the trailer in place only while the engine runs
  4. They provide extra braking force only during hard stops

Explanation

Spring brakes use heavy springs to push the brake shoes into the applied position when air pressure is removed, making them a fail-safe emergency and parking brake. Air pressure compresses the springs to release the brakes during normal operation. They are not a boost for hard stops and do not regulate service air pressure.

Question 8Air Brake System Components and Functions

Why must air tanks be drained regularly?

  1. To remove moisture and oil that can damage valves and cause brake failure Correct answer.
  2. To keep the compressor from running too often
  3. To reset the safety valve after each trip
  4. To increase the volume of air the tanks can hold

Explanation

Compressed air carries moisture and oil that condense inside the tanks; draining removes this contamination before it can corrode tanks, freeze in lines, or damage valves. Tanks have a fixed volume, and draining does not change compressor cycling or reset the safety valve. Regular draining is a required part of pre-trip and daily maintenance.

Question 9Air Brake System Components and Functions

A driver notices the air pressure gauge climbing past the normal cut-out pressure and hears a loud hiss from the tank area. Which component is most likely doing its job?

  1. The safety valve Correct answer.
  2. The slack adjuster
  3. The foot valve
  4. The governor

Explanation

If pressure rises above the designed maximum, the safety valve opens and vents air to protect the system, producing a noticeable hiss. A correctly working governor would have already stopped the compressor at cut-out pressure, so continued rise points to a governor or compressor issue with the safety valve providing backup protection. The foot valve and slack adjuster do not vent tank overpressure.

Question 10Air Brake System Components and Functions

During normal driving, the compressor stops pumping at about 120 psi and restarts at about 100 psi. What component establishes these two pressure points?

  1. The governor Correct answer.
  2. The relay valve
  3. The safety valve
  4. The air dryer

Explanation

The governor is the device that senses system pressure and switches the compressor between cut-out (stop pumping) and cut-in (resume pumping) pressures. These settings keep the system within a safe operating range. The safety valve only protects against overpressure, the air dryer removes moisture, and the relay valve speeds brake application at the wheels.

Question 11Air Brake System Components and Functions

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

  1. To reduce system pressure so the compressor can rest
  2. To keep the trailer from rolling when the parking brake is set
  3. To warn the driver of low air pressure
  4. To speed up application and release of brakes at distant chambers using a nearby air supply Correct answer.

Explanation

A relay valve uses a small control signal from the foot valve to open a larger local supply of air to nearby brake chambers, shortening the time it takes for pressure to build and drop at distant wheels. This improves brake response and balance. It does not rest the compressor, hold the trailer, or provide warnings.

Question 12Air Brake System Components and Functions

What does the tractor protection valve do?

  1. It prevents the tractor's spring brakes from releasing until the engine starts
  2. It limits compressor output to the tractor's front axle
  3. It closes the tractor's air lines to the trailer if pressure drops to a low level Correct answer.
  4. It drains moisture from the trailer tanks automatically

Explanation

The tractor protection valve guards the tractor's air supply; if pressure drops too low, or if the trailer breaks away, it closes off the lines to the trailer so the tractor retains enough air to stop. It does not control spring brake release on start-up or limit compressor output to an axle, and it does not drain trailer tanks.

Question 13Air Brake System Components and Functions

A driver releases the parking brake control and hears air escaping, but the spring brakes do not release. What is the most likely explanation?

  1. The low-pressure warning buzzer has failed
  2. The slack adjusters have rotated too far
  3. The safety valve has opened
  4. Insufficient air pressure has reached the spring brake chambers to compress the springs Correct answer.

Explanation

Spring brakes stay applied until enough air pressure enters the spring brake chamber to compress the springs and release the brakes. Hearing air escape but no release suggests pressure is not building sufficiently in those chambers, such as from low system pressure or a restriction. Slack adjuster position, the safety valve, and the warning buzzer do not prevent spring brake release.

Question 14Air Brake System Components and Functions

What is the function of the quick release valve?

  1. It speeds up compressor cut-in after a hard stop
  2. It exhausts air quickly from brake chambers so the brakes release faster Correct answer.
  3. It rapidly applies the spring brakes in an emergency
  4. It vents the supply tanks when the engine is shut off

Explanation

A quick release valve is placed near the brake chambers and dumps their air locally when the foot valve is released, so the brakes release faster than if air had to travel all the way back to the foot valve. It works on the release side of a service application. It does not apply spring brakes, vent supply tanks, or affect compressor cut-in.

Question 15Air Brake System Components and Functions

How does an air dryer benefit the air brake system?

  1. It removes moisture and contaminants from the air before it reaches the tanks Correct answer.
  2. It increases the compressor's cut-out pressure
  3. It converts moisture into extra air pressure
  4. It lubricates the brake chambers automatically

Explanation

An air dryer uses a desiccant and filters to strip moisture and oil from compressed air before it enters the storage tanks, reducing corrosion and the risk of frozen lines in cold weather. It does not change governor settings, generate pressure, or lubricate brake chambers. Draining tanks is still necessary even with a dryer.

Question 16Air Brake System Components and Functions

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

  1. To introduce alcohol into the air system to help prevent air line freeze-up Correct answer.
  2. To boost system pressure during cold starts
  3. To lubricate the compressor pistons
  4. To clean the brake linings of oil contamination

Explanation

In cold weather, moisture in the air lines can freeze and block airflow; an alcohol evaporator adds alcohol vapor to the air to lower the freezing point and help keep lines clear. It does not clean brake linings, raise system pressure, or lubricate the compressor. Modern systems often rely on air dryers instead.

Question 17Air Brake System Components and Functions

What does the application pressure gauge on some vehicles show the driver?

  1. The pressure in the supply tanks at all times
  2. The pressure in the trailer emergency line
  3. The air pressure being delivered to the brake chambers during a brake application Correct answer.
  4. The compressor cut-out setting

Explanation

An application pressure gauge indicates how much air pressure is being sent to the brake chambers as the driver applies the brakes, helping the driver judge braking effort. It is distinct from the supply gauge that shows tank pressure. It does not show governor settings or trailer line pressure.

Question 18Air Brake System Components and Functions

A driver performs a static leak test and finds that air pressure drops faster than allowed after the brakes are applied and held. What does this most likely indicate?

  1. A governor set too low
  2. An air leak somewhere in the applied brake circuit Correct answer.
  3. A safety valve stuck open
  4. A slack adjuster out of adjustment

Explanation

A pressure drop beyond the allowable limit during a static leak test signals air escaping somewhere in the system, often in the applied brake circuit or lines. Governor settings affect when the compressor cycles, not leak rate. Slack adjustment is mechanical and does not cause pressure loss, and a stuck safety valve would vent air continuously rather than only when brakes are applied.

Question 19Air Brake System Components and Functions

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

  1. It inverts the signal from the foot valve to the relay valve
  2. It reverses the rotation of the slack adjuster for adjustment
  3. It provides a controlled way to release spring brakes when normal air pressure is lost Correct answer.
  4. It drains air from the supply tanks during parking

Explanation

The inversion valve allows a modulated air signal to release or control spring brakes in an emergency when normal supply pressure is lost, so the vehicle can still be moved if needed. It does not adjust slack adjusters, invert foot valve signals to relay valves, or drain supply tanks. It is part of the emergency braking circuit.

Question 20Air Brake System Components and Functions

Why does a spring brake chamber have both a service chamber and a spring chamber?

  1. So the slack adjuster can work on both sides of the wheel
  2. So the safety valve can vent each chamber separately
  3. So the compressor can fill two tanks at once
  4. So normal service braking and fail-safe spring braking can operate independently in one unit Correct answer.

Explanation

A spring brake assembly combines a service chamber, which applies brakes with air during normal driving, and a spring chamber, which applies the brakes by spring force when air is lost. Keeping the functions separate lets the service brake work normally while the spring side provides fail-safe parking and emergency braking. The other options describe unrelated components.

Question 21Air Brake System Components and Functions

A vehicle has a relay valve that is not receiving its control signal. What symptom would the driver most likely notice?

  1. The compressor running without stopping
  2. Continuous venting from the supply tanks
  3. Delayed or weak brake application at the wheels served by that relay valve Correct answer.
  4. Immediate lockup of the front wheels

Explanation

The relay valve depends on a control signal from the foot valve to open and deliver supply air to its brake chambers. Without that signal, those chambers receive little or no pressure, so braking at those wheels is delayed or weak. It would not cause lockup, continuous tank venting, or continuous compressor operation.

Question 22Air Brake System Components and Functions

In a dual air brake system, why are the circuits separated?

  1. So the compressor can run at two different pressures
  2. So a failure in one circuit still leaves the other circuit able to provide braking Correct answer.
  3. So the parking brake can be released without air pressure
  4. So the trailer can be supplied with air independently of the tractor

Explanation

Dual circuits divide the system, typically front and rear, so that a loss of pressure in one circuit does not disable the other; the remaining circuit still provides braking. This redundancy is a key safety feature. The compressor does not run at two pressures, trailer supply is a separate concern, and spring brakes require air to release.

Question 23Air Brake System Components and Functions

What is the purpose of the trailer supply valve (red button) in the cab?

  1. It adjusts the trailer slack adjusters remotely
  2. It controls air pressure to the trailer supply line to charge or release the trailer brakes Correct answer.
  3. It drains the trailer air tanks
  4. It sets the governor cut-out pressure for the trailer

Explanation

The trailer supply valve controls air to the trailer supply (emergency) line; pushed in, it charges the trailer system and allows the trailer spring brakes to release, and pulled out, it removes that air so the trailer brakes apply. It does not set governor pressures, drain tanks, or adjust slack adjusters.

Question 24Air Brake System Components and Functions

A driver notices that the air pressure builds very slowly and the compressor seems to run constantly. Which component should be checked first?

  1. The quick release valves
  2. The low-pressure warning light
  3. The compressor and its drive belt Correct answer.
  4. The slack adjusters

Explanation

Slow pressure buildup with the compressor running continuously points to the compressor or its drive, such as a slipping or broken belt, worn compressor, or intake restriction. Slack adjusters, warning lights, and quick release valves do not affect how fast the compressor builds pressure. Checking the compressor and belt is the logical first step.

Question 25Air Brake System Components and Functions

What is the function of the parking brake control valve (yellow button) in a vehicle with spring brakes?

  1. It releases air from the spring brake chambers so the springs apply the brakes Correct answer.
  2. It opens the safety valve to test it
  3. It supplies air to the service chambers for normal braking
  4. It sets the compressor cut-in pressure

Explanation

Pulling the yellow parking brake control valve exhausts air from the spring brake chambers, allowing the springs to apply the brakes for parking or emergency stopping. Pushing it in restores air to release the springs. It does not supply service air, set governor pressures, or test the safety valve.

Question 26Air Brake System Components and Functions

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

  1. It prevents air from flowing back toward the compressor
  2. It releases air if tank pressure rises above a set limit Correct answer.
  3. It drains moisture from the tank automatically
  4. It divides the tank output between the two service circuits

Explanation

The safety valve is a pressure-relief device that vents air to the atmosphere when tank pressure exceeds a preset value, protecting the tank and lines from over-pressurization if the governor or compressor malfunctions. It reseats once pressure falls back to a safe level.

Question 27Air Brake System Components and Functions

On a vehicle with air brakes, what is the purpose of the low pressure warning device?

  1. To hold the compressor at a steady output
  2. To measure the pressure applied to the brake chambers
  3. To alert the driver before air pressure drops too low for safe braking Correct answer.
  4. To release spring brakes automatically at low pressure

Explanation

The warning device, which may be a buzzer, light, or both, activates when system pressure falls below a safe operating level so the driver can stop before braking ability is lost. It must warn before pressure drops below about 60 psi.

Question 28Air Brake System Components and Functions

What is the purpose of a check valve installed in the air brake system?

  1. To remove oil and water from the air supply
  2. To reduce the pressure delivered to the brake chambers
  3. To speed up brake release at the wheels
  4. To allow air to flow in one direction only Correct answer.

Explanation

A check valve permits air to pass in one direction and blocks it from flowing back, which keeps air from escaping a charged tank when the compressor unloads or another circuit loses pressure. This protects stored air needed for braking.

Question 29Air Brake System Components and Functions

Which component uses engine coolant to warm the air as it passes through, helping prevent freeze-ups in the system?

  1. The air compressor discharge line
  2. The alcohol evaporator
  3. The air dryer with a heated purge valve Correct answer.
  4. The moisture ejector on the supply tank

Explanation

Some air dryers use engine coolant routed through a heat exchanger to warm the air and desiccant, reducing the chance that condensed moisture freezes and blocks valves. The heated purge valve is part of that freeze-protection approach.

Question 30Air Brake System Components and Functions

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

  1. It receives air directly from the compressor and traps the first moisture and oil Correct answer.
  2. It delivers modulated pressure directly to the front brake chambers
  3. It stores air only for the parking brake release
  4. It converts air pressure into hydraulic pressure at each wheel

Explanation

The supply reservoir is the first tank after the compressor, so it collects much of the condensed water and oil carried in the hot compressed air. Air then passes through check valves to the other reservoirs that feed the brake circuits.

Question 31Air Brake System Components and Functions

A driver notices that the front service brakes apply weakly compared with the rear, even though pressure at the foot valve seems normal. Which component most directly controls the pressure delivered to each front brake chamber?

  1. The low pressure switch
  2. The governor
  3. The front brake limiting valve or proportioning arrangement Correct answer.
  4. The compressor unloader

Explanation

A limiting or proportioning valve on the front axle reduces or modulates the pressure sent to the front brake chambers, which can explain weaker front braking. It is the component between the foot valve and the front chambers that shapes delivery pressure.

Question 32Air Brake System Components and Functions

What is the function of the two-way check valve used in some air brake circuits?

  1. It splits one pressure source equally into two outputs
  2. It vents excess pressure to the atmosphere
  3. It stops all air flow when either source drops
  4. It lets the higher of two pressure sources pass on to the next component Correct answer.

Explanation

A two-way check valve compares two incoming pressure signals and allows whichever is higher to pass downstream, while blocking the lower one. This lets a circuit be controlled by either of two sources, such as a hand valve or foot valve, without backfeeding.

Question 33Air Brake System Components and Functions

In a system with a hand valve (trailer trolley valve), what does operating it allow the driver to do?

  1. Reduce compressor output on long downgrades
  2. Apply the trailer service brakes independently of the tractor brakes Correct answer.
  3. Drain the trailer supply tank
  4. Release the trailer spring brakes for parking

Explanation

The hand valve sends a service brake application signal to the trailer brakes alone, which helps the driver check trailer brake operation and control a trailer during certain maneuvers. It does not affect the tractor service brakes.

Question 34Air Brake System Components and Functions

A driver applies the brakes and hears a continuous leak near a wheel that stops when the brakes are released. Which component is the most likely source?

  1. A leaking brake chamber diaphragm or hose at that wheel Correct answer.
  2. A faulty low pressure warning buzzer
  3. A stuck compressor unloader
  4. A clogged air dryer cartridge

Explanation

A leak that appears only when the brakes are applied and stops on release points to a component in the applied-pressure path at the wheel, such as a chamber diaphragm, hose, or fitting. These parts hold pressure only during an application.

Question 35Air Brake System Components and Functions

What is the purpose of the tractor protection valve's automatic function when the trailer breaks away?

  1. It closes the trailer supply line and applies the trailer spring brakes Correct answer.
  2. It dumps all tractor air to the atmosphere
  3. It opens the trailer supply line to keep the trailer charged
  4. It increases pressure to the trailer service brakes

Explanation

If the trailer separates from the tractor, the sudden loss of pressure causes the tractor protection valve to close the trailer supply line, which vents the trailer's emergency line and lets its spring brakes apply. This keeps the trailer from rolling away uncontrolled.

Question 36Air Brake System Components and Functions

What does a pressure gauge labeled in the cab as the supply or primary circuit show?

  1. The trailer spring brake status
  2. The pressure currently applied to the brake chambers
  3. The air pressure available in that reservoir circuit Correct answer.
  4. The compressor discharge temperature

Explanation

The primary and secondary gauges show the stored air pressure available in each of the two service circuits. The driver watches them to confirm pressure is adequate before driving and that both circuits build and hold pressure.

Question 37Air Brake System Components and Functions

A driver reports that the spring brakes will not release until pressure builds well above the normal range, and the yellow control button seems to do nothing at low pressure. Which explanation fits best?

  1. The spring brakes release only when the engine is off
  2. The yellow button only controls the trailer brakes
  3. The relay valve must be replaced to release the parking brake
  4. The system must reach a minimum pressure before the spring brakes can be released Correct answer.

Explanation

Spring brakes are held applied by mechanical springs and need sufficient air pressure to compress them for release. If system pressure is below the release threshold, pushing the yellow control cannot release the brakes until the compressor builds enough pressure.

Question 38Air Brake System Components and Functions

What is the purpose of the double check valve in a spring brake control circuit?

  1. It prevents the parking brake from being released when the engine is off
  2. It limits compressor output during high demand
  3. It ensures the spring brakes apply if either the parking control or the emergency line loses pressure Correct answer.
  4. It allows the compressor to feed both the primary and secondary tanks

Explanation

In a spring brake control circuit, a double check valve lets the spring brakes be controlled by either the parking brake valve or the emergency supply line, and it applies them if either source loses pressure. This provides a fail-safe path to application.

Question 39Air Brake System Components and Functions

Why does the compressor have an unloading mechanism?

  1. To cool the compressed air before it enters the tank
  2. To separate oil from the air before it reaches the dryer
  3. To let the compressor stop pumping when the reservoirs reach cut-out pressure Correct answer.
  4. To increase compressor output when pressure is low

Explanation

When the governor senses cut-out pressure, it signals the unloader to hold the compressor valves open so the compressor stops building pressure and runs unloaded. This prevents over-pressurization and reduces wear until pressure falls to cut-in.

Question 40Air Brake System Components and Functions

Which component is responsible for releasing air from the trailer's emergency line when the trailer supply valve is pulled out (closed)?

  1. The trailer spring brake control valve in the tractor Correct answer.
  2. The front brake limiting valve
  3. The compressor discharge valve
  4. The air dryer purge valve

Explanation

Closing the trailer supply valve stops charging the trailer emergency line, and the trailer spring brake control arrangement then vents that line so the trailer spring brakes apply. This is how the red button secures the trailer.

Question 41Dual Air Brake System and Air Supply

In a dual air brake system, how are the two braking circuits kept independent so that a failure in one does not drain the other?

  1. By a mechanical linkage joining both foot valves
  2. By a common relay valve serving both circuits
  3. By check valves that isolate the circuits from each other Correct answer.
  4. By a single shared reservoir feeding both circuits

Explanation

A dual air brake system uses two separate circuits supplied by separate reservoirs, and one-way check valves prevent air from escaping a healthy circuit into a failed one. This isolation is what allows a vehicle to retain braking ability after a single-circuit failure. The circuits share the compressor but not a common storage volume.

Question 42Dual Air Brake System and Air Supply

What is the purpose of the supply reservoir in an air brake system?

  1. To measure the system pressure for the gauge
  2. To hold compressed air that feeds the other circuits Correct answer.
  3. To store air for the parking brakes only
  4. To cool the air before it reaches the compressor

Explanation

The supply reservoir receives compressed air from the compressor and serves as the main storage tank that feeds the primary and secondary circuits. It acts as a buffer so that compressor cycling does not cause rapid pressure swings at the brakes. It is not dedicated to parking brakes and does not cool or measure air.

Question 43Dual Air Brake System and Air Supply

The governor on an air compressor serves which function?

  1. It limits engine speed to protect the compressor
  2. It drains moisture from the reservoirs automatically
  3. It controls when the compressor loads and unloads Correct answer.
  4. It regulates the pressure applied to the brake chambers

Explanation

The governor senses reservoir pressure and signals the compressor to load, building air, or unload, stopping the build, at preset cut-in and cut-out pressures. This cycling keeps system pressure within the designed operating range. It does not govern engine speed, drain moisture, or set brake application pressure.

Question 44Dual Air Brake System and Air Supply

During a pre-trip inspection, the air compressor should build pressure from 85 to 100 psi within what approximate time in a typical Class A vehicle?

  1. About 10 minutes
  2. About 3 minutes
  3. About 30 seconds
  4. About 45 seconds Correct answer.

Explanation

A common manufacturer and test standard is that the compressor should raise system pressure from 85 to 100 psi in about 45 seconds at a fast idle. This check confirms the compressor and governor are performing adequately. Much longer build times suggest a weak compressor, leaks, or a restricted intake.

Question 45Dual Air Brake System and Air Supply

Air pressure in the system should not drop more than a specified amount with the engine off and the brakes released. This test is checking for what?

  1. Compressor cut-out pressure
  2. Brake chamber stroke
  3. Governor cut-in pressure
  4. Static air leaks Correct answer.

Explanation

With the engine off and brakes released, watching the gauge over time reveals static leaks in the tanks, lines, and valves. A pressure loss greater than the allowed rate means the vehicle should be repaired before use. The test does not measure governor settings or chamber stroke.

Question 46Dual Air Brake System and Air Supply

A vehicle with a dual air brake system has one circuit fail and lose pressure. What should the driver expect?

  1. The remaining circuit still provides braking Correct answer.
  2. All braking is lost immediately
  3. The compressor stops building air
  4. The parking brake releases by itself

Explanation

The central safety feature of a dual system is that a failure in one circuit leaves the other circuit able to apply brakes, though stopping distance may increase. The driver should still have partial braking and should stop safely for repair. Total loss of braking does not occur from a single-circuit failure.

Question 47Dual Air Brake System and Air Supply

What does the low air pressure warning device signal the driver to do?

  1. Increase engine RPM to build air faster
  2. Shift to a lower gear for engine braking
  3. Release the parking brake manually
  4. Stop and investigate before continuing Correct answer.

Explanation

The low pressure warning alerts the driver that system pressure has fallen below a safe level, and the correct response is to stop safely and find the cause. Continuing to drive risks losing braking ability. Raising RPM or downshifting does not fix a leak or faulty compressor.

Question 48Dual Air Brake System and Air Supply

Which component removes moisture and contaminants from the compressed air before it enters the reservoirs?

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

Explanation

The air dryer is installed in the discharge line between the compressor and the reservoirs and removes water vapor and oil contaminants from the air. Dry air reduces corrosion and prevents moisture from freezing in lines during cold weather. Safety valves, evaporators, and quick release valves serve different purposes.

Question 49Dual Air Brake System and Air Supply

A driver notices that system pressure rises to about 130 psi and the compressor keeps running. Which component is most likely failing to unload the compressor?

  1. The relay valve
  2. The slack adjuster
  3. The governor Correct answer.
  4. The safety valve

Explanation

The governor signals the compressor to unload at the cut-out pressure, so a compressor that keeps building past the normal range points to a faulty governor. The safety valve may then release air to protect the system, which is a symptom rather than the cause. Relay valves and slack adjusters are unrelated to compressor unloading.

Question 50Dual Air Brake System and Air Supply

During an applied-air leak test with the brakes fully applied and the engine off, pressure drops faster than the allowed limit. What does this most likely indicate?

  1. A governor cut-in set too low
  2. An overfilled supply reservoir
  3. A normal condition caused by the compressor
  4. A leaking brake chamber or line in the applied circuit Correct answer.

Explanation

The applied leak test pressurizes the brake chambers and lines, so an excessive drop points to a leak in those components, such as a diaphragm or hose. This is different from the static test with brakes released, which checks the supply side. A leak here can reduce braking force and must be repaired.