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

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Question 1Air Brake System Components and Functions

On a vehicle with a dual air brake system, what is the purpose of the two separate air circuits?

  1. To allow one circuit to keep working as a back-up if the other circuit loses pressure Correct answer.
  2. To double the pressure delivered to the brake chambers during a hard stop
  3. To split front and rear service brakes so they apply at different pressures
  4. To supply air to the trailer while the tractor circuit stays isolated

Explanation

A dual system divides the brakes into two independent circuits so that a loss of pressure or a leak in one circuit still leaves a protected circuit able to stop the vehicle. The circuits are not designed to add pressure or to separate tractor from trailer supply.

Question 2Air Brake System Components and Functions

What does the supply pressure gauge on a vehicle with a dual air brake system show?

  1. The pressure being delivered to the brake chambers right now
  2. The pressure in the circuit that has the lowest reading
  3. The pressure available in the air storage tanks Correct answer.
  4. The pressure in the trailer service line only

Explanation

The supply gauge reports the air pressure stored in the tanks, telling you how much air is available to work the brakes. It is distinct from the application gauge, which shows the pressure you are sending to the brake chambers when you press the pedal.

Question 3Air Brake System Components and Functions

In a dual air brake system, the application pressure gauge tells the driver:

  1. How much air the trailer has used since the last stop
  2. How much air remains in the storage tanks
  3. How much air pressure is being applied to the brakes Correct answer.
  4. Whether the compressor is charging the system

Explanation

The application gauge shows the pressure you are sending to the brake chambers, so you can see how hard the brakes are being applied. It does not measure stored air or compressor output.

Question 4Air Brake System Components and Functions

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

  1. It stores compressed air for later use
  2. It regulates how much pressure reaches the brake chambers
  3. It pumps air into the storage tanks to build pressure Correct answer.
  4. It releases air from the system when pressure is too high

Explanation

The compressor is driven by the engine and pumps air into the storage tanks, building the pressure the system needs. The tanks store the air, and the governor and valves control pressure rather than the compressor itself.

Question 5Air Brake System Components and Functions

What is the job of the governor in an air brake system?

  1. It drains moisture from the air tanks
  2. It limits how fast the vehicle can decelerate
  3. It balances pressure between the two service circuits
  4. It controls when the compressor pumps air and when it stops Correct answer.

Explanation

The governor senses system pressure and signals the compressor to load, or pump, when pressure drops to the cut-in level, and to unload when pressure reaches the cut-out level. This keeps tank pressure within the proper operating range.

Question 6Air Brake System Components and Functions

The safety valve in an air brake system is designed to:

  1. Speed up the build-up of air pressure
  2. Warn the driver with a buzzer when pressure is low
  3. Release air if tank pressure climbs above a safe level Correct answer.
  4. Keep the trailer from rolling when it is parked

Explanation

The safety valve is a pressure-relief device that opens to vent air if system pressure rises above a safe maximum, protecting tanks and components. It is not a warning device or a parking brake.

Question 7Air Brake System Components and Functions

In a dual air brake system, what is the purpose of the alcohol evaporator or air dryer?

  1. To lubricate the brake chambers
  2. To cool the compressed air before it reaches the compressor
  3. To increase the pressure in the tanks
  4. To remove moisture and prevent it from freezing or corroding the system Correct answer.

Explanation

Moisture in compressed air can freeze in cold weather and block valves, and it promotes corrosion. An alcohol evaporator or an air dryer removes or controls that moisture so the system keeps working reliably.

Question 8Air Brake System Components and Functions

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

  1. It stores air for emergency brake applications
  2. It holds the brake shoes away from the drum when the brakes are released
  3. It regulates air pressure entering the brake chamber
  4. It converts the push rod stroke into rotation that applies the brake shoes Correct answer.

Explanation

The slack adjuster is a lever arm that connects the push rod to the brake camshaft. As the push rod moves, the slack adjuster converts that linear stroke into rotation that applies the brakes, and it also compensates for lining wear.

Question 9Air Brake System Components and Functions

A driver presses the brake pedal and notices the application pressure gauge moves but the vehicle slows weakly, and the slack adjusters show excessive push rod stroke. What is the most likely cause?

  1. The compressor is overcharging the tanks
  2. The safety valve is stuck open
  3. The brakes are out of adjustment, so the chamber stroke is used up before the shoes contact the drum Correct answer.
  4. The low-pressure warning buzzer has failed

Explanation

When push rod stroke is too long, the brake chamber runs out of travel before it can apply full force to the shoes, so braking is weak even though air pressure is being delivered. The fix is to have the brakes adjusted and inspected.

Question 10Air Brake System Components and Functions

What does the low air pressure warning signal indicate to a driver?

  1. The trailer has disconnected from the tractor
  2. The parking brake is fully released
  3. The system pressure has dropped to an unsafe level and braking may soon be lost Correct answer.
  4. The compressor has reached its cut-out pressure

Explanation

The warning buzzer and light activate when air pressure falls below a safe operating level, alerting the driver that the system is losing pressure and that a stop should be made safely. It is not a trailer-connection or brake-release indicator.

Question 11Air Brake System Components and Functions

What is the role of the tractor protection valve?

  1. It limits the top speed of the tractor when a trailer is attached
  2. It applies the trailer spring brakes whenever the foot pedal is released
  3. It keeps the tractor and trailer tanks at identical pressure at all times
  4. It prevents air loss from the tractor if the trailer breaks away or the trailer line is cut Correct answer.

Explanation

The tractor protection valve closes off the tractor's air supply to the trailer couplers when pressure drops or the trailer separates, protecting the tractor's ability to brake. It is a protection device, not a speed limiter or a pressure equalizer.

Question 12Air Brake System Components and Functions

During a pre-trip inspection, a driver hears air escaping at the rear of the tractor when the trailer is connected. Which component is most directly responsible for sealing the connection between tractor and trailer air lines?

  1. The relay valve
  2. The glad hands and their coupler seals Correct answer.
  3. The governor
  4. The quick release valve

Explanation

Glad hands are the couplers that join the tractor and trailer air lines, and their rubber seals keep the connection airtight. A leak at the rear of the tractor when the trailer is hooked up points to the couplers or their seals.

Question 13Air Brake System Components and Functions

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

  1. It reduces pressure to the front brakes only
  2. It speeds up the application and release of air at the brake chambers by using tank air close to the chambers Correct answer.
  3. It converts electrical signals into air pressure
  4. It drains water from the lowest point in the system

Explanation

On vehicles with long air lines, the relay valve takes a small control signal from the foot valve and uses air from a nearby tank to apply the brakes quickly. This shortens the time needed to fill and exhaust the brake chambers.

Question 14Air Brake System Components and Functions

How do spring brakes apply on a vehicle with air brakes?

  1. Mechanical spring force applies the brakes when air pressure is released Correct answer.
  2. Hydraulic pressure from the master cylinder applies the brakes
  3. Air pressure pushes the springs to apply the brakes
  4. An electric motor winds the springs to apply the brakes

Explanation

Spring brakes use heavy springs to push the brakes on, and air pressure holds the springs compressed to keep the brakes released. When air pressure is lost or the parking valve is set, the springs expand and apply the brakes.

Question 15Air Brake System Components and Functions

A driver sets the parking brake control and hears air exhaust, and the vehicle does not roll. Which principle explains this?

  1. The service brake pedal was locked in the applied position
  2. Air pressure was applied to hold the brakes on
  3. The compressor was turned off to hold the brakes
  4. Air pressure was released from the spring chambers, letting the springs apply the brakes Correct answer.

Explanation

Setting the parking brake control releases air from the spring brake chambers, allowing the springs to expand and apply the brakes mechanically. This is why spring brakes are a fail-safe design.

Question 16Air Brake System Components and Functions

What does the foot valve, or treadle valve, do in an air brake system?

  1. It releases air from the spring chambers when the parking brake is set
  2. It stores air for emergency use
  3. It controls the amount of air pressure sent to the brake chambers when the driver presses the pedal Correct answer.
  4. It regulates compressor cut-out pressure

Explanation

The foot valve is the driver's primary control: pressing the pedal meters air pressure to the brake chambers in proportion to pedal travel. It does not store air or control the compressor.

Question 17Air Brake System Components and Functions

A driver notices the supply gauge on a dual air brake vehicle drops steadily while the vehicle is parked with the engine off and no brakes applied. What does this most likely indicate?

  1. The application gauge is faulty
  2. The compressor is working correctly
  3. Normal cooling of compressed air in the tanks
  4. An air leak somewhere in the system that should be located and repaired Correct answer.

Explanation

With the engine off and no brakes applied, tank pressure should hold steady. A steady drop in the supply gauge points to a leak that must be found and repaired before the vehicle is driven.

Question 18Air Brake System Components and Functions

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

  1. It quickly applies the spring brakes in an emergency
  2. It opens the safety valve when pressure spikes
  3. It rapidly drains the air tanks after shutdown
  4. It speeds up the release of air from the brake chambers so the brakes let off faster Correct answer.

Explanation

The quick release valve is mounted near the brake chambers and exhausts air quickly when the pedal is released, so the brakes release faster than if air had to travel all the way back to the foot valve.

Question 19Air Brake System Components and Functions

A driver is testing the air brake system and finds that pressure builds from 85 to 100 psi in about 45 seconds at fast idle. What does this result suggest?

  1. The safety valve is stuck closed
  2. The system is operating normally and no action is needed
  3. The compressor is building pressure too quickly and should be derated
  4. The system is building pressure more slowly than it should, indicating a possible leak or compressor problem Correct answer.

Explanation

A healthy system typically builds from 85 to 100 psi in a short time, roughly 45 seconds or less at fast idle. Taking much longer than that signals a leak, a weak compressor, or a governor problem that should be checked.

Question 20Air Brake System Components and Functions

Why must the air tanks be drained regularly?

  1. To make room for more air in the tanks
  2. To remove moisture and oil that can freeze, corrode, or damage valves Correct answer.
  3. To reset the low-pressure warning system
  4. To reduce the pressure so the compressor can rest

Explanation

Compressed air carries moisture and some oil, which collect in the tanks. Draining the tanks removes that contamination so it cannot freeze in cold weather, corrode components, or foul valves.

Question 21Air Brake System Components and Functions

On a dual air brake system, one circuit loses pressure and the other remains charged. What should the driver expect?

  1. The supply gauge will read zero on both circuits
  2. Both circuits will fail at the same time
  3. The remaining circuit will still provide braking, but stopping distance may increase and the vehicle should be stopped safely Correct answer.
  4. The spring brakes will release automatically

Explanation

The dual system is designed so a failure in one circuit leaves the other able to apply brakes, though performance is reduced and stopping distance may grow. The driver should slow down and stop in a safe place for repair.

Question 22Air Brake System Components and Functions

A driver shuts off the engine, releases the parking brake, and applies the foot brake firmly. The application gauge shows pressure but the vehicle rolls slowly on a grade. Which combination best explains this?

  1. The low-pressure warning buzzer is defective
  2. The governor cut-in pressure is set too high
  3. The brakes are out of adjustment or the foundation brakes are worn, so chamber pressure is not producing enough shoe force Correct answer.
  4. The compressor has failed and no air is reaching the chambers

Explanation

Air pressure reaching the chambers is only part of the picture; the foundation brake components must convert that force into friction at the drum. Excessive stroke or worn linings reduce braking force even when chamber pressure looks normal.

Question 23Air Brake System Components and Functions

Why is the spring brake chamber designed to apply the brakes when air pressure is lost?

  1. Because it lets the driver release the brakes without air pressure
  2. Because it creates a fail-safe system in which loss of air results in braking rather than loss of braking Correct answer.
  3. Because it allows the compressor to shut off automatically
  4. Because the springs are stronger than any air pressure the system can produce

Explanation

The spring brake is a fail-safe design: air pressure holds the springs compressed to keep the brakes released, so any loss of pressure lets the springs apply the brakes. This means a major air loss results in the vehicle braking rather than rolling freely.

Question 24Air Brake System Components and Functions

A vehicle with a dual air brake system has a leak in the front service circuit only. Which statement best describes the system's response when the foot brake is applied?

  1. Both circuits lose pressure at the same rate because they share one tank
  2. The compressor immediately overcharges the remaining circuit to compensate
  3. The rear circuit continues to supply its brakes, and the front circuit loses pressure until the system is repaired Correct answer.
  4. The trailer brakes apply automatically and the tractor brakes release

Explanation

In a dual system the circuits are isolated by check valves and valving, so a leak in the front circuit does not drain the rear circuit. The rear brakes remain available, though overall braking is reduced.

Question 25Air Brake System Components and Functions

A driver performs a static leak test by charging the system, shutting off the engine, and watching the gauges with the brakes released and then applied. What is the purpose of this test?

  1. To verify that the compressor can reach cut-out pressure
  2. To confirm that the system holds pressure within acceptable limits both when released and when applied Correct answer.
  3. To check that the low-pressure warning buzzer sounds
  4. To measure how quickly the spring brakes apply

Explanation

A static leak test checks that pressure loss over a set time stays within acceptable limits in both the released and applied states. Excessive loss in either state indicates a leak that must be repaired.

Question 26Air Brake System Components and Functions

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

  1. It converts electrical signals from the brake pedal into air pressure
  2. It limits the maximum pressure delivered to the trailer service line
  3. It removes moisture and oil vapor from the compressed air
  4. It regulates whether the compressor pumps air or idles, based on system pressure Correct answer.

Explanation

The governor senses air tank pressure and cuts the compressor in at the low setting and out at the high setting, keeping system pressure within its normal operating range. It does not itself generate or clean the air; it only controls when the compressor loads and unloads.

Question 27Air Brake System Components and Functions

A driver wants to check that the compressor is building pressure properly. Which gauge reading should be watched during the buildup check?

  1. The low air pressure warning buzzer
  2. The supply pressure gauge Correct answer.
  3. The application pressure gauge
  4. The trailer air supply control knob

Explanation

The supply gauge shows the pressure available in the tanks, so it is the reading that rises during the compressor buildup test. The application gauge shows only the pressure being sent to the brake chambers and does not indicate tank pressure.

Question 28Air Brake System Components and Functions

What is the purpose of the safety valve mounted on the air tank?

  1. It signals the driver when pressure drops below 60 psi
  2. It releases air if tank pressure rises above a preset limit Correct answer.
  3. It prevents air from flowing back from the trailer into the tractor
  4. It drains accumulated water each time the brakes are applied

Explanation

The safety valve is a pressure relief device that vents air to the atmosphere if tank pressure exceeds its preset limit, protecting the system from overpressure. It is a backup protection device and does not operate during normal brake use.

Question 29Air Brake System Components and Functions

On a dual air brake system, which components are supplied by the two separate circuits?

  1. One circuit supplies the front axle brakes and the other supplies the rear axle and trailer brakes Correct answer.
  2. One circuit supplies the compressor and the other supplies the governor
  3. One circuit supplies the parking brakes and the other supplies the service brakes
  4. One circuit supplies the gauges and the other supplies the warning devices

Explanation

In a dual system the primary circuit typically serves the rear axle and trailer brakes while the secondary circuit serves the front axle brakes, so a failure in one circuit still leaves some braking. Each circuit has its own reservoir and check valve protection.

Question 30Air Brake System Components and Functions

A driver applies the brakes and the application pressure gauge shows pressure rising, but the vehicle decelerates far less than expected and no air is heard escaping. Which component should be suspected?

  1. A brake chamber with a ruptured diaphragm leaking internally Correct answer.
  2. A drain cock left open on the supply tank
  3. A governor stuck in the unloaded position
  4. A safety valve that has popped open

Explanation

If pressure is being delivered but braking force is weak and no external leak is heard, a chamber diaphragm may have failed so the pushrod does not stroke fully. A governor or supply-side fault would show up as low or falling supply pressure rather than normal application pressure.

Question 31Air Brake System Components and Functions

What is the purpose of the check valve located between the compressor and the first air tank?

  1. It limits the compressor's maximum output pressure
  2. It keeps air from flowing back out of the tank toward the compressor Correct answer.
  3. It automatically drains water from the tank
  4. It removes oil carried over from the compressor

Explanation

The check valve allows air to flow from the compressor into the supply tank but blocks reverse flow, so pressure is retained when the compressor unloads or fails. Pressure limiting is done by the governor, not by this valve.

Question 32Air Brake System Components and Functions

Why does the spring brake chamber have a mechanical spring in addition to the service brake diaphragm?

  1. To increase the maximum service brake application pressure
  2. To hold the pushrod retracted while the vehicle is moving under power
  3. To cushion the diaphragm against pressure spikes
  4. To apply the brakes automatically when air pressure is lost Correct answer.

Explanation

The spring is held compressed by air pressure while the system is charged; if pressure is lost, the spring extends and applies the brakes, providing a fail-safe parking and emergency brake. The service diaphragm works separately and does not depend on the spring for normal braking.

Question 33Air Brake System Components and Functions

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

  1. The low pressure warning switch has failed
  2. The spring brake chambers are not receiving enough pressure to compress the springs Correct answer.
  3. The service brake chambers have fully stroked pushrods
  4. The relay valve is delivering too much pressure to the trailer

Explanation

Spring brakes release only when air pressure is admitted to the spring chamber to compress the spring; if that pressure is insufficient or blocked, the brakes stay applied even though air is heard. The sound of air alone does not confirm that the spring chambers are being charged.

Question 34Air Brake System Components and Functions

What does the inversion valve do in a spring brake system?

  1. It converts supply pressure into electrical signals for the warning lamp
  2. It reverses the direction of air flow in the supply line
  3. It inverts the trailer service signal for the front axle
  4. It provides a modulated spring brake application when service air pressure is lost Correct answer.

Explanation

The inversion valve senses loss of service air and allows a controlled, modulated application of the spring brakes instead of a sudden full application, improving control during an emergency. It is a safety device and does not reverse normal air flow.

Question 35Air Brake System Components and Functions

A driver notices that the air pressure in both circuits builds very slowly and the compressor runs almost constantly. Which condition best explains this?

  1. The low pressure warning buzzer is stuck on
  2. A leak in the system or a worn compressor is preventing normal buildup Correct answer.
  3. The governor cut-out setting is too low
  4. The application gauge is reading incorrectly

Explanation

Slow buildup with the compressor running continuously points to air escaping faster than it can be replaced or to a compressor that cannot keep up, such as a leak or worn compressor. A governor setting issue would change the cut-out point but not cause constant running with slow buildup.

Question 36Air Brake System Components and Functions

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

  1. It lubricates the compressor with an alcohol-based oil
  2. It measures the alcohol content of the compressed air
  3. It removes alcohol from the system before it reaches the brake chambers
  4. It adds alcohol vapor to the air to help prevent moisture from freezing in the lines Correct answer.

Explanation

The evaporator introduces alcohol vapor into the air system so that any moisture present is less likely to freeze in lines and valves during cold weather. It is a freeze-protection device, not a measurement or filtration device.

Question 37Air Brake System Components and Functions

A driver performs the applied leak test and finds that pressure drops faster than the allowable limit. What does this indicate?

  1. The compressor is building pressure too quickly
  2. The low pressure warning device is out of adjustment
  3. There is a leak in the service brake system that must be repaired before driving Correct answer.
  4. The governor cut-out setting is too high

Explanation

The applied leak test checks for leakage with the brakes applied; an excessive drop means air is escaping from the service side of the system, such as a leaking chamber, line, or valve. The vehicle should not be placed in service until the leak is found and corrected.

Question 38Air Brake System Components and Functions

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

  1. It sets the parking brake when the trailer is disconnected
  2. It balances pressure between the primary and secondary circuits
  3. It regulates the compressor output to the wet tank
  4. It closes the trailer lines if tractor pressure drops, protecting the tractor's air supply Correct answer.

Explanation

The tractor protection valve senses tractor supply pressure and closes off the trailer lines when pressure falls too low, so the tractor retains enough air to stop safely. It also closes the lines when the trailer is disconnected.

Question 39Air Brake System Components and Functions

A driver observes that the supply pressure gauge falls while the application gauge stays steady during a long grade descent with repeated brake applications. Which statement best explains this?

  1. The application gauge is faulty and should be replaced
  2. The spring brakes are releasing automatically
  3. The trailer relay valve is overcharging the service line
  4. Repeated applications are consuming air faster than the compressor can replenish it Correct answer.

Explanation

Each brake application uses air from the tanks, and on a long descent repeated use can draw down supply pressure faster than the compressor can recover it, which is why proper gear selection and controlled braking are required. A steady application gauge does not mean the supply is not being consumed.

Question 40Air Brake System Components and Functions

Why must a driver allow the air system to build to normal pressure before performing the static leak test?

  1. The low pressure warning must be sounding during the test
  2. The test compares pressure loss over time from a fully charged system Correct answer.
  3. The spring brakes must be released before the test
  4. The compressor must be running during the test to supply air

Explanation

The static leak test measures how much pressure is lost over a set time after the system is fully charged and the engine is shut off, so the starting point must be at normal operating pressure. Testing from a partially charged system would give meaningless results.

Question 41Dual Air Brake System and Safeguards

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

  1. To double the braking force at the wheels
  2. To separate the front and rear tires' inflation systems
  3. To allow the compressor to run at two different speeds
  4. To provide a backup supply so one circuit can still brake if the other fails Correct answer.

Explanation

A dual air brake system splits the air supply into two independent circuits so that a failure or leak in one circuit does not leave the vehicle without brakes. The remaining circuit still provides braking capability, though possibly reduced. This redundancy is the core safety purpose of the dual system.

Question 42Dual Air Brake System and Safeguards

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

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

Explanation

The primary circuit normally supplies the rear axle service brakes, while the secondary circuit supplies the front axle brakes. This arrangement lets the rear brakes, which carry more stopping responsibility, remain available if the front circuit fails. Knowing which circuit serves which axle helps a driver interpret warning signals.

Question 43Dual Air Brake System and Safeguards

What does the low air pressure warning device on a dual air brake vehicle signal to the driver?

  1. That the air pressure has dropped below a safe operating level Correct answer.
  2. That the parking brake is fully released
  3. That the compressor has just finished charging the tanks
  4. That the trailer has been disconnected

Explanation

The low air pressure warning alerts the driver when system pressure falls to an unsafe level, typically around 60 psi or the manufacturer's specified setting. It warns that braking performance may soon be compromised. The driver should stop and investigate before continuing.

Question 44Dual Air Brake System and Safeguards

At approximately what pressure should the low air pressure warning device activate on most commercial vehicles?

  1. 100 psi
  2. 10 psi
  3. 150 psi
  4. 60 psi Correct answer.

Explanation

Federal standards require the low pressure warning to activate before pressure drops below roughly 60 psi, giving the driver time to react. Normal operating pressure is much higher, commonly 100 to 125 psi. A warning that comes on at normal operating pressure indicates a fault.

Question 45Dual Air Brake System and Safeguards

What is the function of the safety valve on an air brake system reservoir?

  1. It releases air if tank pressure exceeds a safe maximum Correct answer.
  2. It regulates compressor cut-in pressure
  3. It supplies air to the trailer emergency line
  4. It drains moisture from the bottom of the tank

Explanation

The safety valve is a pressure-relief device that opens if reservoir pressure rises above a safe maximum, protecting the tank and components from over-pressurization. It is a backup safeguard if the governor fails. It does not perform routine pressure regulation.

Question 46Dual Air Brake System and Safeguards

A driver notices the low air pressure warning buzzer and light activate while driving. What is the correct immediate action?

  1. Pump the brake pedal repeatedly to rebuild pressure
  2. Stop safely as soon as possible and investigate the cause Correct answer.
  3. Shift to a lower gear and use engine braking only
  4. Continue at reduced speed to the next scheduled stop

Explanation

When the low pressure warning activates, the driver should bring the vehicle to a safe stop as soon as practical and find the cause before proceeding. Continuing to drive risks losing braking capability. Pumping the pedal does not rebuild supply pressure and can worsen the situation.

Question 47Dual Air Brake System and Safeguards

What does the governor do in a dual air brake system?

  1. It meters air to the trailer service line
  2. It controls when the compressor loads and unloads to maintain proper tank pressure Correct answer.
  3. It applies the spring brakes when pressure is low
  4. It divides air between the primary and secondary circuits

Explanation

The governor senses reservoir pressure and signals the compressor to load (pump) when pressure falls to the cut-in point and to unload at the cut-out point. This keeps system pressure within the proper operating range. It does not itself distribute air to the circuits.

Question 48Dual Air Brake System and Safeguards

On a dual air brake system, what is the role of the check valves located between the compressor and the reservoirs?

  1. They automatically drain condensate from the tanks
  2. They reduce the pressure delivered to the front brakes
  3. They connect the primary and secondary circuits together
  4. They prevent air from flowing back out of the reservoirs toward the compressor Correct answer.

Explanation

Check valves allow air to flow into the reservoirs but prevent it from flowing back toward the compressor or between circuits. This one-way flow protects stored pressure and keeps the circuits independent. That independence is what makes the dual system a safeguard.

Question 49Dual Air Brake System and Safeguards

What is the purpose of the tractor protection valve on a combination vehicle with dual air brakes?

  1. It increases air pressure to the trailer brakes during hard stops
  2. It releases the trailer spring brakes automatically at highway speed
  3. It closes the tractor air lines if the trailer breaks away or pressure drops Correct answer.
  4. It balances air pressure between the primary and secondary circuits

Explanation

The tractor protection valve guards the tractor's air supply by closing the lines to the trailer when trailer pressure drops too low or the trailer separates. This preserves tractor braking if the trailer line fails. It also helps apply the trailer emergency brakes during a breakaway.

Question 50Dual Air Brake System and Safeguards

During a pre-trip inspection, a driver builds normal pressure and then shuts off the engine. The pressure drops from 120 psi to 100 psi in one minute with the brakes released. What does this most likely indicate?

  1. An air leak that should be found and repaired Correct answer.
  2. A normal and acceptable pressure loss
  3. That the compressor is overcharging the tanks
  4. That the low pressure warning is malfunctioning

Explanation

With the engine off and brakes released, system pressure should hold fairly steady; a rapid loss such as 20 psi in a minute signals a leak. The driver should locate and repair the leak before operating. A small loss is normal, but this rate is excessive.