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MS CDL Air Brakes study materials
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MS CDL Air Brakes practice questions

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Question 1Air Brake System Components and Function

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

  1. It drains moisture from the wet tank each time the brakes are applied.
  2. It regulates air pressure in the storage tanks by controlling when the compressor loads and unloads. Correct answer.
  3. It measures the amount of air flowing to the brake chambers during a stop.
  4. It converts hydraulic pressure from the brake pedal into air pressure.

Explanation

The governor senses reservoir pressure and tells the compressor when to build air (cut-in) and when to stop building (cut-out). This keeps system pressure within the designed operating range, typically about 100 to 125 psi. It does not meter air to the chambers or remove moisture.

Question 2Air Brake System Components and Function

What is the purpose of the supply reservoir (wet tank) in an air brake system?

  1. It stores air for the parking brake circuit only.
  2. It holds the reserve air used only when the trailer is disconnected.
  3. It supplies air directly to the brake chambers during a hard stop.
  4. It receives air directly from the compressor and allows moisture and oil to condense before air moves downstream. Correct answer.

Explanation

The supply or wet tank is the first tank after the compressor and acts as a collection point for condensed water and oil carryover. Air then passes through the air dryer or check valves to the primary and secondary tanks. This arrangement protects downstream components from contamination.

Question 3Air Brake System Components and Function

A driver applies the brakes and hears a steady hiss that continues after the pedal is held. Which component is most likely leaking?

  1. The compressor intake filter
  2. The governor cut-out setting
  3. A brake chamber diaphragm or a line in the applied circuit Correct answer.
  4. The low-pressure warning buzzer

Explanation

A continuous hiss while the brakes are held indicates a leak in the applied air circuit, such as a ruptured chamber diaphragm or a cracked line. Leaks that occur only when applied point to components pressurized by the treadle valve. This is a serious defect that must be corrected before driving.

Question 4Air Brake System Components and Function

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

  1. Automatically drains water from the tank at each compressor cycle.
  2. Balances pressure between the primary and secondary circuits.
  3. Releases air to the atmosphere if reservoir pressure exceeds a preset limit. Correct answer.
  4. Signals the driver when pressure drops below 60 psi.

Explanation

The safety valve is a pressure-relief device that opens if reservoir pressure rises above its set point, protecting tanks and lines from overpressure. It is normally set higher than the governor cut-out pressure. It does not drain moisture or warn the driver.

Question 5Air Brake System Components and Function

Which device prevents air from flowing backward from a reservoir to the compressor when the compressor unloads?

  1. The relay valve
  2. The slack adjuster
  3. The quick-release valve
  4. The one-way check valve Correct answer.

Explanation

Check valves allow air to flow in one direction only, so air already stored in the reservoirs cannot bleed back toward the compressor. This keeps reservoir pressure available for braking. Relay and quick-release valves control delivery to chambers, not compressor backflow.

Question 6Air Brake System Components and Function

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

  1. It automatically applies the spring brakes when pressure is lost.
  2. It uses reservoir air to apply the brakes faster at the chambers, shortening application and release time. Correct answer.
  3. It reduces reservoir pressure to a constant 60 psi.
  4. It dries the air before it reaches the treadle valve.

Explanation

A relay valve responds to a control signal from the treadle valve and delivers reservoir air directly to nearby brake chambers. This reduces lag caused by long lines and speeds both application and release. It does not set a fixed pressure or dry the air.

Question 7Air Brake System Components and Function

During a pre-trip inspection, where should a driver expect to find the slack adjusters?

  1. Inside the air dryer housing.
  2. Between the brake chamber push rod and the brake shoe mechanism at each wheel. Correct answer.
  3. On the firewall next to the treadle valve.
  4. Inside the supply reservoir.

Explanation

Slack adjusters connect the brake chamber push rod to the foundation brake at each wheel and take up clearance as linings wear. They are mechanical linkage parts located at the wheel ends. They are not part of the air supply or valve plumbing.

Question 8Air Brake System Components and Function

What is the function of the air dryer in a CDL air brake system?

  1. It converts air pressure into an electrical signal for the warning light.
  2. It stores reserve air for the trailer emergency circuit.
  3. It removes moisture and contaminants from compressed air before it reaches the reservoirs. Correct answer.
  4. It increases compressor output pressure above governor cut-out.

Explanation

The air dryer uses a desiccant to absorb water vapor and filter oil and dirt from the air leaving the compressor. Dry air reduces corrosion and prevents ice formation in lines during cold weather. It does not boost pressure or store air.

Question 9Air Brake System Components and Function

A driver notices the air pressure gauge rising to about 130 psi before the compressor stops building. What does this most likely indicate?

  1. The low-pressure warning switch is failing.
  2. The slack adjusters are out of adjustment.
  3. The quick-release valve is stuck open.
  4. The governor cut-out pressure is set too high or the governor is malfunctioning. Correct answer.

Explanation

Normal cut-out is roughly 125 psi, so a compressor that keeps building to 130 psi or beyond suggests a governor problem or misadjusted cut-out. The safety valve should eventually relieve excess pressure, but the condition needs repair. Slack adjusters and warning switches do not control compressor cycling.

Question 10Air Brake System Components and Function

How does a spring brake chamber apply the parking brake?

  1. A cable pulls the slack adjuster when the driver sets the valve.
  2. Air pressure pushes a piston to engage the brake shoes.
  3. A heavy spring applies the brake when air pressure is released from the chamber. Correct answer.
  4. An electric solenoid locks the push rod in place.

Explanation

Spring brakes use a large stored-energy spring that applies the brake when hold-off air pressure is removed, such as when the parking valve is set or pressure is lost. Air pressure compresses the spring to release the brake. This fail-safe design is why the brakes apply automatically on air loss.

Question 11Air Brake System Components and Function

What is the purpose of the tractor protection valve?

  1. It closes the tractor air lines to the trailer if pressure drops to a low level, preserving tractor braking. Correct answer.
  2. It regulates the compressor cut-in pressure.
  3. It drains the trailer reservoirs during a stop.
  4. It adjusts trailer slack adjusters automatically.

Explanation

The tractor protection valve senses supply line pressure and closes the tractor-to-trailer lines when pressure falls too low, preventing loss of the tractor's air supply. It also controls trailer emergency braking when lines separate. It does not regulate compressor cycling or adjust brakes.

Question 12Air Brake System Components and Function

During a static leak test, the driver fully applies the brakes and holds them. Pressure drops from 100 to 82 psi in one minute on a single vehicle. What should the driver conclude?

  1. The governor cut-in pressure is too low.
  2. The leak rate exceeds the allowable limit and the vehicle must be repaired before use. Correct answer.
  3. This is normal; applied leaks of 20 psi per minute are acceptable.
  4. The compressor is overcharging the system.

Explanation

For a single vehicle, a fully applied leak test should not lose more than about 3 psi per minute; 18 psi in a minute is far beyond that limit. An excessive applied leak means a defect in the applied circuit that must be fixed. The vehicle should be placed out of service until repaired.

Question 13Air Brake System Components and Function

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

  1. It releases the safety valve to vent excess air.
  2. It automatically applies the parking brake at 100 psi.
  3. It activates a buzzer and light before pressure drops below a safe level, typically around 60 psi. Correct answer.
  4. It shuts off the compressor to protect the tanks.

Explanation

The warning device must give an audible and visual alert before reservoir pressure falls below about 60 psi, giving the driver time to stop safely. On many vehicles the spring brakes begin to apply at low pressure as well. It does not control the compressor or the safety valve.

Question 14Air Brake System Components and Function

A driver performs a tug test after coupling to a trailer and the tractor moves forward while the trailer stays put. What does this indicate?

  1. The tractor protection valve has failed open.
  2. The air dryer is saturated with moisture.
  3. The trailer brakes are not receiving air or the coupling/air lines are not properly connected. Correct answer.
  4. The trailer spring brakes are functioning normally.

Explanation

In a tug test the driver applies the trailer brakes and gently pulls against them; if the trailer moves or the tractor pulls free, the trailer brakes are not holding. This suggests an air line connection problem or an uncoupled fifth wheel. The vehicle must not be driven until corrected.

Question 15Air Brake System Components and Function

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

  1. It speeds release of air from the brake chambers so the brakes disengage faster. Correct answer.
  2. It releases the fifth wheel latch when air pressure drops.
  3. It rapidly fills the trailer reservoirs at start-up.
  4. It quickly applies the spring brakes in an emergency.

Explanation

The quick-release valve is mounted near the brake chambers and exhausts chamber air locally when the control pressure is released. This shortens release time compared with sending air all the way back to the treadle valve. It does not fill reservoirs or operate the fifth wheel.

Question 16Air Brake System Components and Function

A driver observes that the air pressure gauge takes an unusually long time to build from 85 to 100 psi. Which condition is most consistent with this symptom?

  1. A stuck quick-release valve in the released position
  2. An overfilled air dryer desiccant
  3. A governor cut-out set too low
  4. A worn compressor, a slipping compressor belt, or a significant supply-side leak Correct answer.

Explanation

Slow pressure buildup points to reduced compressor output or air escaping on the supply side, such as a worn compressor, loose belt, or leaking line. Normal buildup should reach operating pressure within a reasonable time. The cause must be found and repaired.

Question 17Air Brake System Components and Function

In a dual-circuit air brake system, what is the purpose of having separate primary and secondary reservoirs?

  1. To isolate circuits so a failure in one does not completely disable braking. Correct answer.
  2. To supply air only to the trailer.
  3. To provide two different pressure levels for front and rear brakes.
  4. To allow the compressor to run continuously without a governor.

Explanation

Dual circuits split the system so that a loss of pressure in one circuit still leaves the other able to provide braking. The primary and secondary tanks are protected by check valves. This redundancy is a core safety feature of modern air brake systems.

Question 18Air Brake System Components and Function

What does the brake pedal (treadle) valve do in an air brake system?

  1. It meters air pressure from the reservoirs to the brake circuits in proportion to pedal travel. Correct answer.
  2. It mechanically pulls the slack adjusters without using air.
  3. It sets the governor cut-out pressure each time it is pressed.
  4. It directly opens the reservoir drains when depressed.

Explanation

The treadle valve is the driver's control that regulates how much air pressure is delivered to the brake chambers based on pedal position. Greater pedal travel delivers more pressure for harder braking. It does not drain tanks or set governor limits.

Question 19Air Brake System Components and Function

A driver performs the compressor build-up check and finds pressure rises from 85 to 100 psi in about 20 seconds. How should this be interpreted?

  1. The governor is stuck in the unloaded position.
  2. The buildup is too slow and indicates a compressor defect.
  3. The buildup is acceptable and within typical performance expectations. Correct answer.
  4. The safety valve is leaking.

Explanation

A typical build-up check expects pressure to rise from 85 to 100 psi in roughly 45 seconds or less, so 20 seconds is a healthy result. Faster buildup indicates good compressor output. Slow buildup would be the concern.

Question 20Air Brake System Components and Function

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

  1. It converts air pressure into hydraulic pressure.
  2. It reverses the flow of air through the air dryer.
  3. It inverts the direction of the compressor pulley.
  4. It reverses the control signal so spring brakes can be applied or released during an emergency loss of pressure. Correct answer.

Explanation

The inversion valve provides a means to control spring brakes when normal control pressure is lost, allowing a modulated emergency application rather than a sudden full apply. It responds to loss of supply pressure in the circuit. It has nothing to do with compressor rotation or hydraulic conversion.

Question 21Air Brake System Components and Function

A tractor has a failed primary circuit but a functioning secondary circuit. Which statement best describes the expected braking behavior?

  1. The spring brakes will release automatically and prevent stopping.
  2. The secondary circuit still provides braking to its protected wheels, and the low-pressure warning alerts the driver. Correct answer.
  3. Only the trailer brakes will function.
  4. All braking is lost and the vehicle cannot be stopped.

Explanation

Dual-circuit design isolates the failure so the secondary circuit continues to supply its brakes, allowing a controlled stop. The low-pressure warning should alert the driver to the fault. Complete loss of braking is what the split system is designed to prevent.

Question 22Air Brake System Components and Function

Why must a driver drain moisture from air reservoirs, and when should this be done?

  1. To increase compressor output; drains should be opened while driving.
  2. To reduce reservoir pressure to zero before every trip; drains should be opened while the engine is running at high idle.
  3. To remove water that could freeze or corrode components; drains should be opened during pre-trip and after the vehicle has been parked long enough for moisture to settle. Correct answer.
  4. To test the safety valve; drains should be opened only when the governor cuts out.

Explanation

Water in the reservoirs can freeze in cold weather, block lines, and corrode valves, so manual drains should be opened to expel accumulated moisture. Draining during pre-trip inspection and after parking allows water to settle and be removed. Automatic drains and air dryers help but do not replace this check.

Question 23Air Brake System Components and Function

A vehicle's spring brakes fail to release after the parking valve is pushed in. Which component is most directly responsible for supplying the hold-off air that releases them?

  1. The spring brake control (parking) valve and its delivery circuit Correct answer.
  2. The air dryer purge valve
  3. The low-pressure warning switch
  4. The safety valve

Explanation

The parking brake control valve delivers hold-off air to compress the spring and release the brake. If that valve or its delivery circuit fails, the spring brakes stay applied. The safety valve and dryer purge are unrelated to hold-off pressure.

Question 24Air Brake System Components and Function

What is the function of the alcohol evaporator or equivalent cold-weather device in an air brake system?

  1. It heats the brake chambers to improve lining friction.
  2. It increases reservoir pressure in cold weather.
  3. It introduces alcohol vapor into the air system to help prevent moisture from freezing in lines and valves. Correct answer.
  4. It lubricates the compressor crankshaft.

Explanation

An alcohol evaporator adds alcohol vapor to the air stream so any remaining moisture is less likely to freeze in lines or valves during cold weather. It supplements, but does not replace, an air dryer and regular draining. It does not heat chambers or change pressure.

Question 25Air Brake System Components and Function

During a pre-trip inspection, a driver finds that the push rod stroke on one brake chamber exceeds the adjustment limit. What is the correct interpretation?

  1. The brake is properly adjusted and can be used as is.
  2. The air dryer needs to be replaced.
  3. The slack adjuster or foundation brake needs adjustment or repair before the vehicle is operated. Correct answer.
  4. The governor cut-out pressure is too high.

Explanation

Excessive push rod stroke means the slack adjuster has not taken up clearance properly, reducing braking force and indicating wear or misadjustment. The brake must be adjusted or repaired before driving. Stroke limits are specified by the manufacturer and must be checked during inspection.

Question 26Air Brake System Components and Function

Which component stores compressed air for brake applications and also serves as a collection point for moisture and oil carried over from the compressor?

  1. The treadle valve
  2. The governor
  3. The brake chamber
  4. The supply (wet) reservoir Correct answer.

Explanation

The supply or wet tank receives air directly from the compressor and is the first place where moisture and oil condense out, which is why it is drained regularly. It also feeds the primary and secondary circuits through a check valve arrangement. Brake chambers convert air pressure into mechanical force rather than storing system air.

Question 27Air Brake System Components and Function

Which device mechanically locks the brake push rod in the applied position when air pressure is deliberately released from its chamber?

  1. The low pressure indicator
  2. The relay valve
  3. The safety valve
  4. The spring brake (parking brake) chamber Correct answer.

Explanation

A spring brake chamber contains a large stored spring that applies the brake whenever air pressure is removed from the spring cavity, providing parking and emergency braking. Releasing air with the parking control valve lets the spring extend and lock the push rod. This is why a loss of air pressure results in brake application rather than release.

Question 28Air Brake System Components and Function

What is the purpose of the check valves installed between the supply reservoir and the primary and secondary circuits?

  1. They prevent air from flowing backward and losing pressure in one circuit when the other fails. Correct answer.
  2. They limit maximum system pressure to the governor cut-out setting.
  3. They release air quickly from the brake chambers during a stop.
  4. They automatically drain accumulated moisture from the reservoirs.

Explanation

Check valves allow air to flow in one direction only, so a failure or leak in one circuit cannot bleed pressure out of the other circuit or back into the supply tank. This isolation is central to the safety of a dual-circuit system. Without them, a single leak could disable both halves of the brake system.

Question 29Air Brake System Components and Function

How does the safety valve on an air reservoir protect the system?

  1. It vents air to the atmosphere if reservoir pressure exceeds its setting. Correct answer.
  2. It isolates a failed circuit from the rest of the brake system.
  3. It closes the compressor inlet to stop air from entering the system.
  4. It sounds a warning in the cab when pressure falls below 60 psi.

Explanation

The safety valve is a spring-loaded relief device set above normal governor cut-out pressure; if the governor fails and pressure keeps climbing, the valve opens and vents air to prevent damage or a burst component. It is a backup protection, not the primary pressure regulator. Normal pressure control belongs to the governor and compressor unloading system.

Question 30Air Brake System Components and Function

A driver applies the brakes and hears a sharp exhaust of air from a valve mounted near the rear axle immediately after releasing the pedal. Which component is most likely operating as designed?

  1. The relay valve exhausting air from the brake chambers Correct answer.
  2. The safety valve relieving overpressure
  3. The governor unloading the compressor
  4. The alcohol evaporator purging the system

Explanation

Relay valves are mounted close to the brake chambers they serve and exhaust air locally when the pedal is released, which shortens release time on long wheelbases. A quick burst of air at release is normal relay valve operation. The sound location near the rear axle and its timing with pedal release point to that valve rather than to a compressor or relief function.

Question 31Air Brake System Components and Function

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

  1. It opens the drain cocks to purge the reservoirs.
  2. It stops the compressor from pumping by unloading it. Correct answer.
  3. It closes the check valves to seal the wet tank.
  4. It directs compressor output to the spring brake chambers.

Explanation

The governor monitors reservoir pressure and, at cut-out, causes the compressor to unload so it stops building pressure even though the engine keeps turning it. When pressure falls to the cut-in setting, the governor reloads the compressor. This cycling keeps system pressure within the designed operating range.

Question 32Air Brake System Components and Function

Which valve controls the amount of air pressure delivered to the trailer brakes in proportion to how hard the driver presses the pedal, while also permitting the trailer supply line to be charged?

  1. The bobtail proportioning valve
  2. The tractor protection valve
  3. The hand control (trailer parking) valve
  4. The trailer spring brake valve Correct answer.

Explanation

The trailer spring brake valve, often called the trailer relay valve, receives the control signal and delivers proportional pressure to the trailer brakes while managing the trailer supply and service lines. It applies the trailer spring brakes when supply pressure is lost. Its proportional delivery is what lets the trailer brake in step with the tractor.

Question 33Air Brake System Components and Function

A driver notes that the front service brakes apply, but the rear service brakes on the same tractor do not. Which arrangement best explains this behavior?

  1. The rear chambers are plumbed to the supply tank, which is drained first.
  2. The two service circuits are supplied from separate reservoirs, so a failure in the circuit feeding the rear brakes can leave the front circuit working. Correct answer.
  3. The rear brakes are applied only by spring force, so they never receive service air.
  4. The treadle valve delivers air only to the front axle by design.

Explanation

In a dual-circuit system the primary and secondary circuits feed different groups of brake chambers from separate reservoirs, so a leak or failure in one circuit does not necessarily disable the other. Loss of the circuit feeding the rear service brakes can therefore leave the front service brakes functioning. This partial braking capability is the safety purpose of the split system.

Question 34Air Brake System Components and Function

What is the function of the automatic drain valve fitted to some air reservoirs?

  1. It equalizes pressure between the primary and secondary reservoirs.
  2. It releases collected moisture and contaminants from the tank without manual operation. Correct answer.
  3. It adds alcohol vapor to the air to prevent freezing.
  4. It maintains a minimum pressure in the tank during heavy brake use.

Explanation

Automatic drain valves cycle open briefly, typically when the compressor unloads, to expel condensed water and oil from the reservoir so it does not accumulate and corrode the tank or travel downstream. They supplement, rather than replace, the driver's responsibility to check that drains are working. Manual drain cocks perform the same job by hand.

Question 35Air Brake System Components and Function

Which component converts the air pressure signal from the foot valve into the mechanical force that moves the slack adjuster and applies the brake shoes?

  1. The air compressor
  2. The pressure gauge
  3. The brake chamber and push rod Correct answer.
  4. The inversion valve

Explanation

Air entering the brake chamber acts on a diaphragm or piston that pushes the push rod outward; the push rod moves the slack adjuster, which rotates the camshaft or applies the wedge to press the linings against the drum. This is the point where pneumatic energy becomes mechanical braking force. The push rod stroke is also the measurement used to judge adjustment.

Question 36Air Brake System Components and Function

A driver notices that the low pressure warning buzzer sounds and the spring brakes begin to apply while the vehicle is moving. Which condition is most consistent with these symptoms?

  1. System air pressure has dropped into the warning and emergency application range. Correct answer.
  2. The air dryer purge cycle is running frequently.
  3. The compressor is building pressure faster than normal.
  4. The governor cut-out pressure is set slightly high.

Explanation

The low pressure warning activates at roughly 60 psi, and further loss of pressure causes the spring brakes to apply automatically as the spring cavities lose their holding air. Together these symptoms indicate a serious pressure loss that requires the driver to stop safely. They are not normal operating variations.

Question 37Air Brake System Components and Function

What is the purpose of the front brake limiting valve found on some older air brake vehicles?

  1. It prevents the front brakes from ever applying during a stop.
  2. It increases front braking force when the vehicle is fully loaded.
  3. It reduces front brake application pressure under light loads to lessen the risk of front wheel lockup. Correct answer.
  4. It automatically adjusts the front slack adjusters.

Explanation

The limiting valve reduces the air pressure delivered to the front brake chambers, typically in a 'normal' or 'slippery' setting, to reduce the chance of front wheel lockup on low-friction surfaces. It does not eliminate front braking entirely, since steering control depends on the front wheels continuing to roll. Modern vehicles more often use proportioning or antilock systems for this purpose.

Question 38Air Brake System Components and Function

A driver reports that the parking brake valve must be pulled before the spring brakes will release, and the valve will not stay pushed in. Which explanation best fits this behavior?

  1. The supply pressure reaching the dash valve is too low to hold the valve in and release the springs. Correct answer.
  2. The relay valve is delivering too much pressure to the service chambers.
  3. The safety valve is set below governor cut-out pressure.
  4. The slack adjusters are adjusted too tightly.

Explanation

The parking brake control valve is designed so that it will not remain in the released position unless sufficient air pressure is available to keep the spring brakes fully released. Low supply pressure causes the valve to pop back out, which is a built-in safety feature. The driver should treat this as a low pressure or leak problem and investigate before moving.

Question 39Air Brake System Components and Function

Which statement best describes how a quick-release valve and a relay valve differ in function?

  1. The quick-release valve meters pedal force, while the relay valve adjusts slack.
  2. The quick-release valve exhausts chamber air rapidly on release, while the relay valve both delivers reservoir air on application and exhausts it on release. Correct answer.
  3. The quick-release valve regulates system pressure, while the relay valve warns the driver of low pressure.
  4. The quick-release valve controls trailer supply, while the relay valve controls tractor protection.

Explanation

A quick-release valve is placed close to the brake chambers so that, when the pedal is released, chamber air exhausts locally instead of traveling all the way back to the foot valve, speeding release. A relay valve performs a broader job: it uses a control signal to deliver air from a nearby reservoir on application and to exhaust it on release. Both shorten response time, but only the relay valve also supplies the application air.

Question 40Air Brake System Components and Function

During diagnosis, a technician finds that releasing the parking brake control does not release the spring brakes, although reservoir pressure is normal and the dash valve stays in. Which component should be examined first?

  1. The low pressure warning switch
  2. The spring brake control valve or its delivery lines to the spring chambers Correct answer.
  3. The trailer light circuit
  4. The compressor discharge line

Explanation

If supply pressure is normal and the dash valve remains in the released position, the likely fault is downstream: the spring brake control valve may not be delivering air, or the lines to the spring cavities may be blocked or leaking. Air must reach the spring cavity to compress the spring and release the brake. The compressor and warning devices are not in that delivery path.

Question 41Dual Air Brake System and Safety Features

On a dual air brake system, which reservoir circuit normally supplies the rear (service) brakes on a typical Class B CDL vehicle?

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

Explanation

In a dual (split) system the primary circuit feeds the rear service brakes, while the secondary circuit feeds the front service brakes. This split allows one circuit to keep working if the other loses pressure. The parking brake and trailer supply are separate functions, not the reservoir circuit that feeds rear service brakes.

Question 42Dual Air Brake System and Safety Features

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

  1. It lets the higher-pressure circuit supply a shared component Correct answer.
  2. It automatically applies the spring brakes on a leak
  3. It drains moisture from both reservoirs at once
  4. It limits maximum system pressure to 150 psi

Explanation

The double check valve compares pressure from both circuits and passes the higher pressure to a shared component such as the low-pressure warning switch or the stop lamps. This ensures the shared device still works when one circuit fails. It does not manage moisture, set maximum system pressure, or apply spring brakes.

Question 43Dual Air Brake System and Safety Features

During a static leak test on a dual air brake system, the vehicle is charged to governor cut-out and the engine is shut off. What maximum pressure loss is allowed in one minute with the brakes released?

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

Explanation

With a fully charged system and the engine off, a single vehicle should lose no more than 3 psi in one minute with the brakes released. This static leak check confirms the compressor and lines hold pressure. A larger drop points to a leak that must be repaired before driving.

Question 44Dual Air Brake System and Safety Features

When the low air pressure warning activates on a dual air brake vehicle, at what pressure must it come on before the pressure drops any lower?

  1. Before pressure falls below 60 psi Correct answer.
  2. Before pressure falls below 100 psi
  3. Before pressure falls below 20 psi
  4. Before pressure falls below 45 psi

Explanation

The low air pressure warning device must activate before the system pressure drops below 60 psi. This gives the driver time to stop safely before spring brakes apply. The warning is a required safety feature on air-brake-equipped commercial vehicles.

Question 45Dual Air Brake System and Safety Features

A driver hears the low air pressure buzzer and watches the gauge fall. At approximately what pressure will the spring brakes begin to apply automatically on most vehicles?

  1. Around 45 psi Correct answer.
  2. Around 110 psi
  3. Around 150 psi
  4. Around 90 psi

Explanation

Spring brakes are held off by air pressure; when pressure falls to roughly 45 psi, the springs overcome the remaining air and start to apply the brakes. This provides a fail-safe stop if pressure keeps dropping. The exact value varies by manufacturer but is in that range.

Question 46Dual Air Brake System and Safety Features

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

  1. Adds air to the system when pressure is low
  2. Releases excess pressure to the atmosphere at a set limit Correct answer.
  3. Prevents the spring brakes from ever applying
  4. Keeps the compressor running continuously

Explanation

The safety valve opens and vents air if system pressure rises above its setting, protecting components from overpressure. It is a passive protective device, not a pressure source. It does not control compressor cycling or block spring brake operation.

Question 47Dual Air Brake System and Safety Features

The governor on an air brake system serves which function?

  1. It meters air to the brake chambers during a stop
  2. It controls when the compressor loads and unloads to maintain system pressure Correct answer.
  3. It releases the parking brake when the button is pulled
  4. It filters oil from the compressor discharge

Explanation

The governor senses reservoir pressure and signals the compressor to load (pump) or unload (stop pumping) to keep pressure within the cut-in and cut-out range. This prevents overpressurization and keeps adequate air available. It does not meter braking air, release parking brakes, or filter oil.

Question 48Dual Air Brake System and Safety Features

What is the normal function of the alcohol evaporator or air dryer in a dual air brake system?

  1. It removes moisture and contaminants from the compressed air Correct answer.
  2. It reduces the spring brake holding force
  3. It increases reservoir pressure above governor cut-out
  4. It lubricates the brake shoes

Explanation

An alcohol evaporator or air dryer removes moisture and contaminants from compressed air before it reaches the reservoirs, helping prevent freeze-ups and corrosion. Moisture in air lines can cause brake failure in cold weather. These devices do not raise pressure, lubricate shoes, or affect spring brake force.

Question 49Dual Air Brake System and Safety Features

A vehicle has a dual air brake system and the primary circuit fails. What should the driver expect?

  1. All braking is lost immediately
  2. Only the parking brake remains, with no service braking
  3. The secondary circuit still provides braking, but stopping distance increases Correct answer.
  4. The compressor will overheat and shut down

Explanation

A split system is designed so that loss of one circuit leaves the other circuit able to provide service braking, though performance and stopping distance are affected. The driver should still be able to stop and should move the vehicle to a safe location. Total loss of braking does not occur from a single circuit failure.

Question 50Dual Air Brake System and Safety Features

Before driving, a driver performs the applied leak test. With the brakes fully applied and the engine off, what maximum loss is generally allowed in one minute for a single vehicle?

  1. 3 psi
  2. 5 psi
  3. 4 psi Correct answer.
  4. 8 psi

Explanation

With the brakes fully applied and the system charged, a single vehicle should lose no more than 4 psi in one minute. This checks for leaks in the applied side of the system, including chambers and valves. A greater loss means the vehicle should be taken out of service for repair.