I85PREP
⌘ K
MT CDL Air Brakes study materials
MT CDL Air Brakes study resources

MT CDL Air Brakes practice questions

One question at a time. Build your MT CDL Air Brakes understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Air brake system components and their functions

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

  1. To release air from the system during a purge
  2. To apply the spring brakes when pressure drops
  3. To cool the air before it enters the wet tank
  4. To build and maintain air pressure in the storage tanks Correct answer.

Explanation

The compressor is engine-driven and pumps air into the storage reservoirs so the system always has enough pressure to apply the brakes. It runs whenever the engine runs and is regulated by the governor, which cuts it in and out at set pressures.

Question 2Air brake system components and their functions

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

  1. To control when the compressor loads and unloads to regulate system pressure Correct answer.
  2. To drain moisture from the supply reservoir
  3. To limit vehicle speed on downgrades
  4. To balance braking force between the front and rear axles

Explanation

The governor senses reservoir pressure and tells the compressor when to build air (cut-in) and when to stop (cut-out). This keeps system pressure within the designed operating range, typically around 100 to 125 psi.

Question 3Air brake system components and their functions

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

  1. Stores air that has already passed through the air dryer and holds the first moisture drop-out Correct answer.
  2. Provides air directly to the brake chambers without valving
  3. Holds the parking brake spring pressure
  4. Measures the application pressure at the treadle

Explanation

The supply reservoir receives air straight from the compressor and air dryer and serves as the first collection point for moisture and oil carryover. Air then flows to the primary and secondary reservoirs that feed the brake circuits.

Question 4Air brake system components and their functions

A dual air brake system uses separate primary and secondary circuits mainly to:

  1. Provide a backup circuit so a leak in one circuit still leaves braking ability Correct answer.
  2. Reduce the number of valves needed
  3. Allow the compressor to run at lower pressure
  4. Increase total air volume for long vehicles

Explanation

The two circuits are isolated so that a failure or leak in one still leaves the other with enough pressure to stop the vehicle. This redundancy is why the system has separate reservoirs and a dual-needle gauge.

Question 5Air brake system components and their functions

What is the function of the slack adjuster?

  1. It regulates the compressor cut-out pressure
  2. It converts the linear pushrod stroke into rotary motion to apply the brake shoes Correct answer.
  3. It releases moisture from the air dryer
  4. It stores air pressure for the spring brake

Explanation

The slack adjuster is a lever arm on the brake camshaft that turns the pushrod's linear stroke into rotation that expands the brake shoes. It also provides the adjustment point for setting brake chamber pushrod travel.

Question 6Air brake system components and their functions

What does the spring brake (parking brake) chamber do when air pressure is released from it?

  1. It sends air to the trailer supply line
  2. It opens the air dryer purge valve
  3. It retracts the pushrod and releases the brake
  4. A heavy spring applies the brake mechanically Correct answer.

Explanation

Spring brakes are held off by air pressure and applied by a large mechanical spring when pressure is released. This is why a loss of air pressure causes the parking brakes to set automatically.

Question 7Air brake system components and their functions

The low air pressure warning device must activate before pressure drops below what level?

  1. 120 psi
  2. 30 psi
  3. 90 psi
  4. 60 psi Correct answer.

Explanation

Federal standards require the low pressure warning to come on before system pressure falls below 60 psi. Drivers must stop and investigate when the warning activates because the spring brakes will soon apply automatically.

Question 8Air brake system components and their functions

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

  1. To increase compressor output pressure
  2. To cool the engine coolant
  3. To remove moisture and contaminants from compressed air before it reaches the reservoirs Correct answer.
  4. To lubricate the brake chambers

Explanation

The air dryer uses a desiccant cartridge to absorb water vapor and trap oil and dirt from the compressor discharge. Keeping air dry prevents corrosion and freeze-ups in valves and lines.

Question 9Air brake system components and their functions

During a static leak test with the engine off and brakes released, pressure should not drop more than what rate for a combination vehicle?

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

Explanation

For a combination vehicle, the static leak rate with brakes released should be no more than 3 psi per minute; with the brakes applied the limit is 4 psi per minute. Excessive leakage means the vehicle must be taken out of service until repaired.

Question 10Air brake system components and their functions

What does the tractor protection valve do when air pressure in the tractor falls to a low level?

  1. It closes the trailer supply line to keep the trailer from losing its air Correct answer.
  2. It opens the trailer spring brake chambers
  3. It bypasses the foot valve
  4. It increases compressor output to recover pressure

Explanation

The tractor protection valve shuts off the trailer supply line when tractor pressure drops, preserving whatever air remains in the trailer. This helps prevent a total loss of trailer brakes and an uncontrolled breakaway.

Question 11Air brake system components and their functions

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

  1. It regulates compressor cut-in pressure
  2. It removes moisture from the air
  3. It stores air for the parking brake
  4. It senses control pressure and quickly delivers reservoir air to the brake chambers Correct answer.

Explanation

The relay valve uses a small control signal from the foot valve to open and send a large volume of reservoir air to the brake chambers. This shortens application and release time on long vehicles with many chambers.

Question 12Air brake system components and their functions

What does the quick-release valve do during a brake release?

  1. It holds the spring brake in the applied position
  2. It exhausts air from the brake chambers locally to speed release Correct answer.
  3. It increases the pressure applied to the chambers
  4. It shuts off the trailer supply line

Explanation

The quick-release valve vents air from the brake chambers near the axle instead of forcing it back through the long supply line. This makes the brakes release faster and improves response time.

Question 13Air brake system components and their functions

The foot valve (treadle valve) in an air brake system:

  1. Meters air pressure to the brake circuits based on how far the pedal is pressed Correct answer.
  2. Stores air for the parking brake
  3. Controls compressor cut-out pressure
  4. Removes moisture from the air

Explanation

The foot valve is the driver's primary control: pressing the treadle meters air pressure to the brake circuits in proportion to pedal travel. It also provides a feel of brake application and releases air when the pedal is let up.

Question 14Air brake system components and their functions

What does the dash valve (parking brake control) do when pushed in?

  1. It dumps air from the primary reservoir
  2. It charges the trailer supply line and releases the spring brakes Correct answer.
  3. It sets the governor cut-out pressure
  4. It applies the trailer service brakes only

Explanation

Pushing in the dash valve allows air to charge the trailer supply line and release the spring brakes. Pulling it out exhausts that air, which sets the spring brakes and closes the trailer supply.

Question 15Air brake system components and their functions

What is the purpose of the alcohol evaporator or air dryer in cold weather operation?

  1. To warm the brake drums
  2. To lubricate the brake shoes
  3. To keep moisture from freezing in air lines and valves Correct answer.
  4. To increase compressor discharge pressure

Explanation

Moisture in compressed air can freeze in lines and valves during cold weather, causing brake failure. The air dryer and alcohol evaporator reduce that moisture so the system stays functional in freezing temperatures.

Question 16Air brake system components and their functions

What does a brake chamber pushrod stroke that is too long indicate?

  1. The brakes are properly adjusted
  2. The slack adjuster is out of adjustment or the foundation brake is worn Correct answer.
  3. The compressor is overcharging the system
  4. The air dryer is saturated

Explanation

Excessive pushrod stroke means the slack adjuster is out of adjustment or the brake shoes or drum are worn beyond limits. Long stroke reduces braking force and is a common cause of out-of-service violations.

Question 17Air brake system components and their functions

During a pre-trip inspection, the driver applies the service brakes and hears a steady hiss that continues after the pedal is held. What does this most likely indicate?

  1. An air leak in the service brake system Correct answer.
  2. The governor cutting out
  3. The air dryer purging normally
  4. Normal air release

Explanation

A steady hiss that continues while the brake is held indicates a leak in the service brake circuit, such as a chamber diaphragm or line. The driver should perform a leak test and repair the fault before driving.

Question 18Air brake system components and their functions

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

  1. It drains water automatically each day
  2. It meters air to the brake chambers
  3. It signals the driver when pressure is low
  4. It releases air if pressure exceeds a preset level to protect the system Correct answer.

Explanation

The safety valve is a pressure relief device that opens if the governor fails and pressure climbs too high. This prevents reservoir or line damage and possible rupture.

Question 19Air brake system components and their functions

How does the compressor unloader mechanism work when the governor reaches cut-out pressure?

  1. It shuts off the engine
  2. It holds the compressor valves open so air is not pumped into the system Correct answer.
  3. It dumps air from the wet tank
  4. It closes the foot valve

Explanation

At cut-out, the governor sends air to the unloader, which holds the compressor intake valves open so the compressor stops pumping. When pressure falls to cut-in, the unloader releases and the compressor resumes building air.

Question 20Air brake system components and their functions

What does the application pressure gauge on the dash show?

  1. The compressor cut-in pressure
  2. The pressure being delivered to the brake chambers Correct answer.
  3. The trailer spring brake pressure
  4. The pressure in the supply reservoir

Explanation

The application pressure gauge shows how much air pressure the driver's foot valve is sending to the brake chambers. This helps the driver judge how hard the brakes are being applied.

Question 21Air brake system components and their functions

Which component prevents the trailer from rolling away if it separates from the tractor?

  1. The emergency (breakaway) gladhand and trailer spring brakes Correct answer.
  2. The quick-release valve
  3. The tractor protection valve
  4. The air dryer purge valve

Explanation

If the trailer separates, the supply line breaks and pressure is lost, causing the trailer spring brakes to apply automatically. This breakaway protection keeps the trailer from rolling uncontrolled.

Question 22Air brake system components and their functions

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

  1. It purges moisture from the lines
  2. It sets the governor cut-in pressure
  3. It reverses the direction of the compressor
  4. It allows a modulated spring brake application when service air is lost Correct answer.

Explanation

The inversion valve provides a controlled, modulated application of the spring brakes if service brake air is lost. This gives the driver some braking control instead of an abrupt full spring brake application.

Question 23Air brake system components and their functions

Which statement best describes the function of the double check valve in a dual circuit system?

  1. It allows the higher of two pressure sources to pass through Correct answer.
  2. It stops all air flow when pressure drops
  3. It drains the reservoirs daily
  4. It sets the slack adjuster free play

Explanation

The double check valve compares two pressure sources and lets the higher one pass to the downstream component. This ensures a signal can come from either circuit, preserving function if one circuit fails.

Question 24Air brake system components and their functions

During the compressor build-up test, which result indicates the system is performing properly?

  1. Pressure builds from 85 to 100 psi within about 45 seconds Correct answer.
  2. Pressure rises to 150 psi and stops
  3. Pressure stays at 85 psi indefinitely
  4. Pressure drops while the engine runs

Explanation

A properly functioning system should build from 85 to 100 psi in roughly 45 seconds at fast idle. Slower build-up points to a weak compressor, a leak, or a restricted line.

Question 25Air brake system components and their functions

What is the purpose of the trailer supply line (emergency line) gladhand color coding?

  1. Both lines are black and interchangeable
  2. Red identifies the service line and blue identifies the supply line
  3. Blue identifies the service line and red identifies the supply line Correct answer.
  4. Green identifies the supply line and yellow the service line

Explanation

Standard color coding uses blue for the service line and red for the supply (emergency) line so they are not crossed. Crossing them prevents proper charging and brake release on the trailer.

Question 26Air brake system components and their functions

Which component stores compressed air so that the system has a reserve for repeated brake applications?

  1. The air reservoir (supply tank) Correct answer.
  2. The governor
  3. The slack adjuster
  4. The brake chamber

Explanation

Air reservoirs hold compressed air delivered by the compressor and act as the system's storage volume, allowing several brake applications even after the compressor stops. Brake chambers convert air pressure into mechanical force and do not store a reserve.

Question 27Air brake system components and their functions

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

  1. It converts air pressure into mechanical braking force at the wheel.
  2. It releases excess air pressure to the atmosphere during a brake application.
  3. It regulates the amount of air sent to each brake chamber.
  4. It builds and maintains the supply of compressed air in the reservoirs. Correct answer.

Explanation

The compressor is driven by the engine and pumps atmospheric air into the reservoirs to keep system pressure at the governed level. It is the source of supply, not an actuator or a control valve.

Question 28Air brake system components and their functions

A driver notices the air pressure gauge creeping down while the engine is off and no brakes are applied. Which component is most directly responsible for replacing that lost air when the engine runs?

  1. The drain cock
  2. The low pressure warning switch
  3. The safety valve
  4. The compressor Correct answer.

Explanation

The compressor is the only component that generates new compressed air to replace losses, so it is the part that restores pressure during normal operation. Leak detection and pressure protection are handled by other devices.

Question 29Air brake system components and their functions

Which component converts the lengthwise pushrod stroke into rotation of the brake camshaft?

  1. The slack adjuster Correct answer.
  2. The supply line gladhand
  3. The relay valve
  4. The brake chamber diaphragm

Explanation

The slack adjuster is mounted on the camshaft and is moved by the pushrod, turning the camshaft so the brake shoes expand. It also provides the adjustment that keeps pushrod stroke within limits.

Question 30Air brake system components and their functions

During a brake application, air pressure in the service chamber pushes against a diaphragm. What does the diaphragm do as a result?

  1. It sends a signal to the dash gauge.
  2. It locks the pushrod in place to hold the parking brake.
  3. It vents the air to the atmosphere through the chamber's exhaust port.
  4. It moves the pushrod outward to apply the brake. Correct answer.

Explanation

In a service chamber the diaphragm is a flexible piston: pressure on one side deflects it and drives the pushrod out, applying force to the slack adjuster. The chamber's return spring retracts the pushrod when pressure is released.

Question 31Air brake system components and their functions

What does the governor do when reservoir pressure reaches the cut-in level?

  1. It allows the compressor to resume pumping air into the reservoirs. Correct answer.
  2. It vents air from the service chambers.
  3. It applies the spring brakes automatically.
  4. It opens the safety valve to protect the reservoir.

Explanation

The governor monitors reservoir pressure and switches the compressor between loaded and unloaded states. At cut-in it permits the compressor to pump again; at cut-out it unloads the compressor so pressure stops rising.

Question 32Air brake system components and their functions

A driver is checking that the system holds air with the engine off and the brakes released. Which component's condition is most directly being evaluated by that test?

  1. The function of the low pressure warning buzzer
  2. The integrity of the reservoirs, lines, and valves that must seal Correct answer.
  3. The adjustment of the slack adjusters
  4. The compressor drive belt

Explanation

A static leak test with the brakes released checks whether the pressurized components seal properly, since any leaking reservoir, hose, fitting, or valve will show up as pressure loss. Mechanical adjustment and warning devices are checked by other tests.

Question 33Air brake system components and their functions

What is the purpose of the front brake limiting valve (ratio valve) found on some older vehicles?

  1. It balances air pressure between the tractor and trailer.
  2. It increases front braking force to shorten stopping distance.
  3. It reduces front brake application pressure to lessen the chance of front wheel lockup on slippery surfaces. Correct answer.
  4. It holds the front brakes released so the vehicle can be towed.

Explanation

The limiting valve can be set to deliver only a portion of the control pressure to the front chambers, reducing lockup tendency on low-friction surfaces. On dry pavement the normal position allows full front brake pressure.

Question 34Air brake system components and their functions

What is the function of the two-way check valve in a dual-circuit brake system?

  1. It prevents air from escaping the reservoirs when the engine is shut down.
  2. It allows the higher-pressure control signal from either circuit to pass to the spring brake valve. Correct answer.
  3. It automatically applies the trailer brakes if the tractor loses air.
  4. It divides supply air equally between the front and rear circuits at all times.

Explanation

A two-way check valve compares two pressure signals and lets the higher one through while blocking the lower, so a control signal can reach a device from either circuit. This lets the spring brake valve be controlled even if one circuit fails.

Question 35Air brake system components and their functions

Why must the compressor's discharge air pass through the air dryer before entering the supply reservoir?

  1. To raise system pressure above the governor cut-out point
  2. To cool the air so the compressor runs less often
  3. To remove water vapor and oil that would otherwise collect and freeze or corrode system parts Correct answer.
  4. To lubricate the brake chamber diaphragms

Explanation

Compressed air carries moisture and oil that can condense in reservoirs and lines, causing corrosion and winter freeze-ups. The dryer removes most of that contamination before the air reaches the supply tank.

Question 36Air brake system components and their functions

A driver presses the brake pedal and the vehicle stops normally, but the pedal feels spongy and the gauge shows a slower-than-normal rise in application pressure. Which explanation best fits a failing foot valve?

  1. The foot valve has been bypassed, so the brakes cannot be applied at all.
  2. The foot valve is stuck open, so the brakes stay applied after release.
  3. The foot valve is leaking internally, so some control pressure escapes instead of reaching the chambers. Correct answer.
  4. The foot valve is delivering full pressure too quickly to the chambers.

Explanation

An internal leak in the foot valve lets part of the control pressure escape, so the chambers receive less air than the pedal position calls for. This shows up as sluggish application and a spongy feel rather than a complete failure.

Question 37Air brake system components and their functions

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

  1. It keeps the trailer supply line open so the trailer can be pulled to a shop.
  2. It applies only the tractor's front brakes.
  3. It closes the tractor's trailer supply line and vents the trailer emergency line, applying the trailer spring brakes. Correct answer.
  4. It dumps all tractor reservoir air to stop the tractor quickly.

Explanation

When supply pressure is lost, the tractor protection valve closes to keep the tractor's air from escaping and lets the trailer emergency line lose pressure. The loss of pressure causes the trailer spring brakes to apply.

Question 38Air brake system components and their functions

A driver wants to test that the trailer spring brakes apply when the trailer supply line is disconnected. Which action correctly performs this check?

  1. Pull the trailer air supply (emergency) valve knob out and watch for the trailer brakes to set. Correct answer.
  2. Step on the foot valve and hold it while someone disconnects a gladhand.
  3. Push in the trailer air supply valve and listen for a leak.
  4. Open a reservoir drain cock and watch the gauge fall.

Explanation

Pulling the trailer air supply valve out simulates a loss of supply pressure to the trailer, which should cause the trailer spring brakes to apply. This verifies the emergency line and trailer spring brake system work as designed.

Question 39Air brake system components and their functions

During a pre-trip inspection a driver finds that the spring brake chambers apply slowly and unevenly when the parking control is pulled. Which component is most likely restricting the release of air from the spring brake chambers?

  1. The air dryer purge valve
  2. The compressor discharge line
  3. The front brake limiting valve
  4. The quick-release valve or the exhaust passage in the spring brake control circuit Correct answer.

Explanation

Spring brakes apply when air is exhausted from their chambers, so a restricted release path makes application slow. A clogged quick-release valve or blocked exhaust passage in the control circuit is the usual cause of sluggish spring brake application.

Question 40Air brake system components and their functions

Why is a dual-circuit (split) air brake system required rather than a single circuit?

  1. It provides two separate circuits so a failure in one still leaves a portion of the service brakes available. Correct answer.
  2. It lets the driver choose between front-only and rear-only braking.
  3. It allows the compressor to run less often, saving fuel.
  4. It eliminates the need for spring brakes on the vehicle.

Explanation

A split system divides the service brakes into two isolated circuits fed from separate reservoirs, so a loss of pressure in one circuit still leaves the other able to apply part of the brakes. This redundancy is the reason the design is required.

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

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

  1. To allow the vehicle to maintain braking capability if one circuit loses pressure Correct answer.
  2. To let the driver choose between front-only and rear-only braking
  3. To reduce the compressor workload during normal driving
  4. To increase the total reservoir capacity for longer downhill grades

Explanation

A dual air brake system divides the brakes into two independent circuits so that a failure or pressure loss in one circuit still leaves the other circuit able to apply brakes. This redundancy is the central safety reason the system exists.

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

A CMV with a dual air brake system must have a low air pressure warning that activates before pressure drops below what level?

  1. 60 psi Correct answer.
  2. 80 psi
  3. 100 psi
  4. 45 psi

Explanation

Federal standards require a warning signal to activate before the air pressure in either service reservoir falls below 60 psi. The warning alerts the driver while enough pressure remains to stop safely.

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

What type of device typically provides the low air pressure warning on a modern air brake vehicle?

  1. A manual valve the driver opens
  2. A gauge needle that turns red
  3. A mechanical pop-out button on the dash
  4. A red light and buzzer activated by a pressure switch Correct answer.

Explanation

A pressure switch senses reservoir pressure and closes to sound a buzzer and illuminate a red warning lamp when pressure is too low. The driver gets both an audible and visual alert.

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

What is the normal function of spring brakes on a heavy vehicle?

  1. They apply automatically when air pressure is lost Correct answer.
  2. They replace the service brakes during normal stops
  3. They release automatically whenever the engine starts
  4. They serve only as a parking brake when air is applied

Explanation

Spring brakes use heavy springs to push the brake shoes into the applied position; air pressure holds the springs compressed to release them. If air pressure is lost, the springs apply the brakes automatically.

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

In a dual air brake system, which circuit typically supplies the front brakes?

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

Explanation

By common design, the primary circuit supplies the rear service brakes and the secondary circuit supplies the front brakes. Each circuit has its own reservoir so a failure is isolated.

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

When air pressure in the system drops into the low warning range, what should the driver do first?

  1. Turn off the engine to preserve air
  2. Pump the brake pedal rapidly to build pressure
  3. Move to a safe location and stop as soon as possible Correct answer.
  4. Shift to a lower gear and continue to the next stop

Explanation

A low pressure warning means braking capacity is at risk, so the driver should stop safely before spring brakes apply on their own. Continuing to drive risks a sudden automatic brake application.

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

Spring brakes begin to apply automatically when system air pressure falls to approximately what range?

  1. 120 to 140 psi
  2. 20 to 45 psi Correct answer.
  3. 90 to 110 psi
  4. 60 to 80 psi

Explanation

Spring brakes are designed to start applying when pressure drops into roughly the 20 to 45 psi range, depending on the vehicle. This automatic application is a fail-safe feature.

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

What component compresses the springs to release the spring brakes during normal operation?

  1. Air pressure from the reservoirs Correct answer.
  2. Hydraulic fluid
  3. A mechanical cable
  4. Engine vacuum

Explanation

Air pressure from the reservoirs pushes against the spring brake chambers to compress the springs and release the brakes. When that air pressure is lost, the springs take over.

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

A driver notices the low air pressure warning buzzer sounds while driving and the pressure gauge is falling. What is the most likely result if the driver ignores it?

  1. The compressor will overheat and shut down
  2. The spring brakes will eventually apply automatically Correct answer.
  3. The trailer will disconnect from the tractor
  4. The service brakes will gradually become stronger

Explanation

As pressure continues to fall, spring brakes reach their apply point and engage automatically, bringing the vehicle to a stop. Ignoring the warning removes the driver's control over when that stop happens.

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

During a pre-trip inspection, how can a driver check that the low air pressure warning works?

  1. Fan the brakes to reduce pressure and watch for the warning at the specified level Correct answer.
  2. Turn on the headlights and check the dash
  3. Rev the engine and watch the gauge rise
  4. Apply the parking brake and listen for a click

Explanation

By fanning off air pressure with the brake pedal, the driver lowers reservoir pressure and confirms the warning activates before the required threshold. This verifies the safety device functions.