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

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Question 1Air Brake System Components and Functions

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

  1. To cool the brake drums during heavy braking
  2. To lubricate the slack adjusters automatically
  3. To build and maintain air pressure in the storage tanks Correct answer.
  4. To release air from the spring brakes during parking

Explanation

The compressor is driven by the engine and pumps air into the storage reservoirs to keep system pressure at the governed range. It does not manage parking brakes, cool drums, or lubricate slack adjusters.

Question 2Air Brake System Components and Functions

The governor on an air brake system controls which of the following?

  1. The amount of air sent to the brake chambers
  2. The slack adjuster travel limit
  3. The cut-in and cut-out pressure of the compressor Correct answer.
  4. The rate the spring brakes apply

Explanation

The governor senses reservoir pressure and engages or disengages the compressor at the cut-in and cut-out points. It does not meter air to the chambers or set brake application rates.

Question 3Air Brake System Components and Functions

Air pressure in the storage tanks is protected from backflow by which component?

  1. The low pressure warning switch
  2. The alcohol evaporator
  3. The safety relief valve
  4. The one-way check valve Correct answer.

Explanation

Check valves allow air to flow into the reservoirs but prevent it from flowing back toward the compressor or other circuits. Safety valves vent excess pressure, and the warning switch only signals low pressure.

Question 4Air Brake System Components and Functions

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

  1. It vents air to the atmosphere if pressure exceeds a safe level Correct answer.
  2. It drains moisture from the bottom of the tank
  3. It warns the driver when pressure drops below 60 psi
  4. It balances pressure between the front and rear circuits

Explanation

The safety relief valve is a pressure-limiting device that opens to release air if reservoir pressure rises above its setting. It protects the system from overpressurization and component damage.

Question 5Air Brake System Components and Functions

The foot valve (brake valve) in an air brake system is operated by the driver to do what?

  1. Release the spring brakes automatically when the engine starts
  2. Apply air pressure to the brake chambers in proportion to pedal force Correct answer.
  3. Increase engine compression for downhill braking
  4. Drain the reservoirs after the vehicle is parked

Explanation

The foot valve meters air from the reservoirs to the brake chambers based on how far the pedal is pushed. This gives the driver proportional control over braking force.

Question 6Air Brake System Components and Functions

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

  1. To alert the driver when system pressure falls to an unsafe level Correct answer.
  2. To automatically apply the parking brake when the engine is off
  3. To release excess air from the tanks
  4. To increase compressor output at low RPM

Explanation

The warning device gives a visible and audible alert when reservoir pressure drops below the required threshold so the driver can stop safely. It does not apply brakes or change compressor output.

Question 7Air Brake System Components and Functions

Which component converts air pressure into mechanical force at the wheel?

  1. The brake chamber Correct answer.
  2. The supply reservoir
  3. The relay valve
  4. The governor

Explanation

The brake chamber contains a diaphragm and pushrod that convert air pressure into a mechanical push on the slack adjuster. That push then applies the foundation brake.

Question 8Air Brake System Components and Functions

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

  1. It regulates air pressure entering the brake chamber
  2. It drains condensation from the air system
  3. It converts pushrod stroke into rotation of the brake camshaft Correct answer.
  4. It stores reserve air for emergency braking

Explanation

The slack adjuster is a lever attached to the brake chamber pushrod and the camshaft; as the pushrod moves, it rotates the camshaft to apply the brakes. It also provides a point for adjusting brake clearance.

Question 9Air Brake System Components and Functions

A driver notices the low pressure warning buzzer sounds and the pressure gauge reads about 55 psi. What should the driver do?

  1. Stop safely and investigate before continuing Correct answer.
  2. Continue driving at reduced speed to the next stop
  3. Pump the brake pedal to build pressure
  4. Shift to a lower gear and use engine braking only

Explanation

When pressure falls below the warning threshold, braking performance is compromised and the spring brakes may apply. The safe action is to stop in a safe location and find the cause.

Question 10Air Brake System Components and Functions

What does the cut-out pressure of the governor represent?

  1. The pressure at which the spring brakes release
  2. The pressure at which the safety valve opens
  3. The pressure at which the compressor stops pumping Correct answer.
  4. The pressure at which the warning buzzer sounds

Explanation

Cut-out is the upper set point where the governor disengages the compressor to prevent overpressurization. Cut-in is the lower set point where the compressor resumes pumping.

Question 11Air Brake System Components and Functions

Why does an air brake system use a relay valve in the rear brake circuit?

  1. To reduce the total air pressure sent to the rear brakes
  2. To prevent the rear brakes from applying during a front circuit failure
  3. To cool the rear brake chambers during long descents
  4. To speed up air delivery to the rear chambers by using reservoir air locally Correct answer.

Explanation

A relay valve is mounted near the rear brakes and uses a small control signal from the foot valve to release reservoir air directly to the chambers. This shortens the distance air must travel and improves brake response.

Question 12Air Brake System Components and Functions

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

  1. To reduce air pressure before it reaches the chambers
  2. To prevent moisture in the air lines from freezing Correct answer.
  3. To lubricate the compressor pistons
  4. To increase the octane of the engine fuel

Explanation

The evaporator introduces alcohol vapor into the air system to lower the freezing point of condensed moisture. This helps prevent ice blockages in valves and lines in cold weather.

Question 13Air Brake System Components and Functions

A dual air brake system is designed so that a failure in one circuit will do what?

  1. Allow the other circuit to continue providing braking Correct answer.
  2. Automatically apply all spring brakes and stop the vehicle
  3. Vent all reservoir pressure to the atmosphere
  4. Cause the compressor to shut down to protect the tanks

Explanation

A dual circuit system separates front and rear (or split) circuits so a leak or failure in one does not disable the other. The driver retains braking ability from the intact circuit, though stopping distance increases.

Question 14Air Brake System Components and Functions

What does the supply reservoir do in an air brake system?

  1. It stores compressed air for use by the braking system Correct answer.
  2. It converts air pressure into mechanical force
  3. It signals the driver when pressure is low
  4. It meters air to the brake chambers based on pedal position

Explanation

The supply reservoir holds compressed air produced by the compressor so it is available for braking. Some systems also use a separate wet tank to collect moisture before air reaches the dry reservoirs.

Question 15Air Brake System Components and Functions

What is the function of the drain cock on an air reservoir?

  1. To release air pressure to the brake chambers
  2. To remove moisture and contaminants that collect in the tank Correct answer.
  3. To test the low pressure warning device
  4. To adjust the governor cut-in point

Explanation

Drain cocks are located at the low point of each reservoir so water and oil that condense from the air can be drained out. Regular draining helps prevent corrosion and freeze-ups.

Question 16Air Brake System Components and Functions

If the compressor is unable to build pressure because of a broken drive belt, which symptom will the driver most likely notice?

  1. The spring brakes release too quickly
  2. The slack adjusters over-tighten the brakes
  3. The pressure gauge fails to rise and the warning device activates Correct answer.
  4. The brake chambers apply unevenly on one side

Explanation

Without a driven compressor, no air is produced, so reservoir pressure will not build and the low pressure warning will sound. Braking performance then depends on remaining stored air.

Question 17Air Brake System Components and Functions

What is the purpose of the front brake limiting valve (where installed) on older air brake systems?

  1. To isolate the front circuit during parking
  2. To increase front brake force during hard stops
  3. To automatically drain the front reservoir
  4. To reduce front brake force on slippery surfaces Correct answer.

Explanation

A limiting valve reduces air pressure to the front chambers in the slippery setting to help prevent front wheel lockup. Many modern systems use automatic limiting or proportioning instead.

Question 18Air Brake System Components and Functions

How does the spring brake (parking/emergency brake) apply when air pressure is lost?

  1. A heavy spring forces the pushrod to apply the brake Correct answer.
  2. The governor signals the foot valve to apply
  3. The relay valve reverses airflow to the chamber
  4. A hydraulic piston pushes the slack adjuster

Explanation

Spring brakes use a large mechanical spring that is held compressed by air pressure during normal operation. When pressure is released or lost, the spring extends and applies the brake automatically.

Question 19Air Brake System Components and Functions

A driver hears a continuous hiss from a brake chamber after releasing the parking brake. What is the most likely cause?

  1. A leaking diaphragm or chamber seal Correct answer.
  2. A governor set too high
  3. A properly functioning relay valve
  4. A slack adjuster out of adjustment

Explanation

A constant hiss at a chamber usually indicates air escaping through a ruptured diaphragm or a bad seal. This leak will reduce available pressure and should be repaired before driving.

Question 20Air Brake System Components and Functions

Why must the compressor's governor cut-in pressure be high enough above the minimum operating pressure?

  1. To prevent the alcohol evaporator from cycling
  2. To allow the drain cocks to be opened while driving
  3. To keep the safety valve from opening during normal operation
  4. To ensure adequate reserve pressure remains for braking before the compressor restarts Correct answer.

Explanation

The cut-in point must leave enough stored air for safe braking before the compressor restarts. If cut-in is too low, the system may approach unsafe pressure between compressor cycles.

Question 21Air Brake System Components and Functions

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

  1. It converts hydraulic pressure to air pressure
  2. It reverses the direction of the compressor crankshaft
  3. It inverts the polarity of the low pressure warning circuit
  4. It provides a controlled way to release or modulate spring brakes during an emergency Correct answer.

Explanation

The inversion valve allows the spring brakes to be released or modulated in certain emergency or parking situations, for example to move a vehicle with a partial system failure. It works with the park control and relay valves.

Question 22Air Brake System Components and Functions

During a pre-trip inspection, a driver applies the brakes and observes that the pushrod stroke on one chamber is much longer than the others. What does this most likely indicate?

  1. The low pressure warning switch has failed
  2. That brake is out of adjustment or has a mechanical problem Correct answer.
  3. The alcohol evaporator is empty
  4. The governor cut-out is set too low

Explanation

Excessive pushrod stroke on one chamber points to a brake that is out of adjustment or has a worn component. It should be corrected before the vehicle is operated.

Question 23Air Brake System Components and Functions

Why is a wet tank used in some air brake systems?

  1. To collect moisture and oil before air reaches the dry reservoirs Correct answer.
  2. To regulate the compressor cut-out pressure
  3. To house the low pressure warning device
  4. To supply air directly to the brake chambers

Explanation

The wet tank receives air first from the compressor and allows moisture and oil to condense and be drained before the air moves to the dry reservoirs. This protects downstream valves and lines.

Question 24Air Brake System Components and Functions

A vehicle with air brakes has good pressure but the brakes apply slowly and weakly. Which component is most suspect?

  1. The drain cock left slightly open
  2. The alcohol evaporator overfilled
  3. The safety relief valve stuck open
  4. A restricted or leaking line between the valve and the chamber Correct answer.

Explanation

Slow, weak application with normal reservoir pressure points to restricted airflow or a leak between the control valve and the chamber. The air is available but not reaching the chamber promptly.

Question 25Air Brake System Components and Functions

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

  1. To quickly build pressure in the supply reservoir
  2. To release the parking brake automatically
  3. To exhaust air from the brake chambers rapidly when the brake is released Correct answer.
  4. To speed up the application of the spring brakes

Explanation

The quick release valve is mounted near the chambers and vents air to the atmosphere when the foot valve is released, shortening release time. Faster release helps prevent brake drag and improves response.

Question 26Air Brake System Components and Functions

What is the primary function of the air compressor in a vehicle air brake system?

  1. To build and maintain air pressure in the storage reservoirs Correct answer.
  2. To convert air pressure into mechanical braking force
  3. To release air from the system when pressure is too high
  4. To cool the air before it reaches the brake chambers

Explanation

The compressor is the source of air pressure; it draws in outside air, compresses it, and sends it to the reservoirs so the system has a constant supply. It does not apply the brakes or regulate system pressure itself.

Question 27Air Brake System Components and Functions

Which device senses reservoir pressure and tells the compressor when to stop and resume pumping?

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

Explanation

The governor monitors air pressure and controls the compressor's loading and unloading, setting both the cut-out and cut-in pressures. It is the pressure regulator for the whole system.

Question 28Air Brake System Components and Functions

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

  1. To automatically drain water from the tank
  2. To balance pressure between the primary and secondary circuits
  3. To release air if system pressure rises above a safe maximum Correct answer.
  4. To open the circuit to the spring brakes during a failure

Explanation

The safety valve is a protective device that vents air to the atmosphere if pressure exceeds a preset limit, preventing tank or line damage. It is a backup for the governor.

Question 29Air Brake System Components and Functions

What does a check valve do in an air brake system?

  1. It reduces pressure delivered to the front brakes
  2. It permits air to flow in one direction only Correct answer.
  3. It stores a reserve of air for emergency use
  4. It converts air pressure into an electrical signal

Explanation

A check valve is a one-way flow device that prevents air from moving backward through the system, protecting reservoirs and circuits from losing pressure. It is common between the compressor and the first reservoir and between tanks.

Question 30Air Brake System Components and Functions

A driver wants to know which component actually pushes the brake shoes or pads against the drum or disc. Which component performs that mechanical movement?

  1. The brake chamber and pushrod Correct answer.
  2. The compressor
  3. The treadle valve
  4. The supply reservoir

Explanation

Air pressure enters the brake chamber and moves a diaphragm and pushrod, which transfers force through the slack adjuster to the foundation brake. This is where air pressure becomes mechanical movement.

Question 31Air Brake System Components and Functions

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

  1. It meters air pressure to the brake chambers according to how far the pedal is pushed Correct answer.
  2. It drains moisture from the air tanks
  3. It sets the compressor cut-in and cut-out pressures
  4. It automatically applies the parking brake when pressure drops

Explanation

The treadle valve is the driver's primary brake control; pedal travel regulates how much air pressure is delivered to the brake chambers, giving proportional braking. Releasing the pedal exhausts that air.

Question 32Air Brake System Components and Functions

Why does an air brake system include a low pressure warning switch?

  1. To equalize pressure between the two service circuits
  2. To increase compressor output when pressure drops
  3. To alert the driver before air pressure falls too low for safe braking Correct answer.
  4. To release the spring brakes automatically at low pressure

Explanation

The warning switch triggers a buzzer and light when reservoir pressure drops below a safe operating level, giving the driver time to stop before braking ability is lost. It is an alert, not a control.

Question 33Air Brake System Components and Functions

What is the role of the two-way check valve in a dual air brake system?

  1. It prevents air from flowing back into the compressor
  2. It limits application pressure to the front axle
  3. It allows the higher-pressure circuit to supply a device that is shared by both circuits Correct answer.
  4. It closes off the secondary circuit when the primary fails

Explanation

A two-way check valve lets whichever circuit has the greater pressure feed a shared component, such as the trailer supply line or a warning switch, while blocking backflow into the other circuit. This keeps the shared device working even if one circuit fails.

Question 34Air Brake System Components and Functions

What is the purpose of the tractor protection valve?

  1. To keep the tractor's air supply from being lost if the trailer breaks away or its lines separate Correct answer.
  2. To regulate the trailer's compressor output
  3. To apply the trailer spring brakes whenever the service brakes are used
  4. To boost brake pressure to the trailer during a hard stop

Explanation

The tractor protection valve closes the tractor-to-trailer air lines when trailer pressure drops sharply, as in a breakaway or line separation, preserving the tractor's air for its own brakes. It also controls the trailer supply line.

Question 35Air Brake System Components and Functions

A driver notices that the air pressure gauge climbs to about 125 psi and keeps rising instead of leveling off. Which component is most likely faulty?

  1. The low pressure warning switch
  2. The governor Correct answer.
  3. The quick release valve
  4. The slack adjuster

Explanation

The governor is responsible for unloading the compressor at the cut-out pressure. If it fails to signal the compressor to stop pumping, pressure continues to rise until the safety valve vents.

Question 36Air Brake System Components and Functions

What does the supply line (emergency line) between tractor and trailer do?

  1. It carries the air that applies the trailer service brakes
  2. It charges the trailer reservoirs and signals the trailer to release its spring brakes Correct answer.
  3. It carries electrical signals for the trailer lights
  4. It returns exhausted air from the trailer brake chambers

Explanation

The supply line charges the trailer's air tanks and, when charged, provides the pressure that holds off the trailer spring brakes. Losing pressure in this line triggers a trailer emergency brake application.

Question 37Air Brake System Components and Functions

What is the function of the spring brake control valve (parking brake valve)?

  1. It meters service brake pressure to the rear wheels
  2. It releases air from the spring brake chambers to apply the parking brakes and admits air to release them Correct answer.
  3. It limits compressor output during parking
  4. It drains condensation from the spring brake chambers

Explanation

The parking brake valve controls air to the spring brake chambers: exhausting that air lets the springs apply the brakes, and supplying air compresses the springs to release them. It is the driver's control for parking and emergency braking.

Question 38Air Brake System Components and Functions

During a walk-around inspection, a driver finds that the compressor's discharge line is hot and the system pressure will not build. Which combination of components should be checked first?

  1. The trailer service line and the glad hands
  2. The slack adjusters and the brake chambers
  3. The governor and the compressor's unloading mechanism Correct answer.
  4. The low pressure warning switch and the buzzer

Explanation

If the compressor runs but pressure does not build, the governor may be stuck in the unloaded position or the compressor's unloading or intake mechanism may be faulty, so no air is being pumped into the reservoirs. These are the first items to inspect for a no-build condition.

Question 39Air Brake System Components and Functions

A driver applies the service brakes and the application is slow and weak at all wheels even though reservoir pressure is normal. Which component is the most likely cause?

  1. The drain cock left partly open
  2. The treadle valve delivering insufficient air Correct answer.
  3. The safety valve
  4. The compressor drive belt

Explanation

With normal reservoir pressure, a system-wide slow, weak application points to the control side, most often the treadle valve not delivering full pressure to the chambers. A restricted or faulty foot valve limits air to every wheel.

Question 40Air Brake System Components and Functions

Why must the air system be able to hold pressure with the engine off and the brakes released during a leakage test?

  1. To confirm the compressor is still pumping air
  2. To prove the governor is cycling correctly
  3. To verify that check valves, lines, and seals are not allowing air to escape Correct answer.
  4. To ensure the spring brakes remain released indefinitely without air

Explanation

A static leak test isolates the stored-air side of the system; if pressure drops faster than the allowed rate, a check valve, line, fitting, or seal is leaking. This test confirms the system can retain the air needed for braking.

Question 41Dual Air Brake Systems and Safety Valves

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

  1. To increase the total braking force on the vehicle
  2. To provide a backup supply of air if one circuit fails Correct answer.
  3. To allow the trailer to brake independently of the tractor
  4. To reduce the amount of air the compressor must produce

Explanation

A dual air brake system splits braking into two independent circuits so that a failure in one circuit still leaves the other circuit able to supply air and apply brakes. This redundancy is the reason dual systems are required on modern heavy vehicles.

Question 42Dual Air Brake Systems and Safety Valves

On a dual air brake system, which components are typically operated by the primary circuit?

  1. The front steering axle brakes
  2. The rear drive axle brakes Correct answer.
  3. The parking brake chambers only
  4. The trailer service brakes only

Explanation

The primary circuit usually operates the rear (drive) axle brakes, while the secondary circuit operates the front steering axle brakes. This split lets the vehicle retain stopping ability if either circuit loses pressure.

Question 43Dual Air Brake Systems and Safety Valves

When the air pressure in one circuit of a dual air brake system drops to a low level, what should the warning device do?

  1. Flash a light only when the parking brake is set
  2. Sound a buzzer and show a low-pressure warning before pressure falls too far Correct answer.
  3. Release the spring brakes to allow the vehicle to roll
  4. Automatically shut off the engine to protect the compressor

Explanation

A dual system must have a warning device that alerts the driver when pressure in either circuit drops below a safe level, typically a buzzer and light that activate before pressure is too low to brake safely. The warning gives the driver time to stop before losing braking ability.

Question 44Dual Air Brake Systems and Safety Valves

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

  1. To hold the spring brakes applied until pressure builds
  2. To route air to the trailer emergency line
  3. To release air if system pressure exceeds a safe maximum Correct answer.
  4. To drain moisture from the air tanks automatically

Explanation

The safety valve (pop-off valve) is a pressure-relief device that opens if system pressure rises above a set maximum, protecting tanks and lines from over-pressurization. It is a safety backup, not a normal operating control.

Question 45Dual Air Brake Systems and Safety Valves

What does the tractor protection valve do when the trailer air supply is lost or the trailer breaks away?

  1. It dumps all tractor air overboard to stop the vehicle
  2. It opens the trailer supply line to atmosphere permanently
  3. It increases pressure to the trailer to keep it braking
  4. It closes the trailer lines to keep tractor air from escaping Correct answer.

Explanation

The tractor protection valve isolates the tractor's air supply from the trailer lines when pressure drops or the trailer separates, preventing the tractor from losing its own air through an open trailer line. This keeps the tractor's brakes operational.

Question 46Dual Air Brake Systems and Safety Valves

In a dual air brake system, the front and rear circuits are protected from each other by which device?

  1. The governor
  2. The double check valve Correct answer.
  3. The alcohol evaporator
  4. The compressor unloader

Explanation

A double check valve (two-way check valve) lets either circuit supply air to a shared component while preventing air from one circuit from bleeding back into the other. This isolation keeps a failure in one circuit from draining the other.

Question 47Dual Air Brake Systems and Safety Valves

A driver notices the low-pressure warning buzzer sounds and the pressure gauge for the primary circuit reads low while the secondary circuit reads normal. What is the most likely condition?

  1. The gauge is reading normal and no action is needed
  2. The trailer emergency line is overpressurized
  3. One circuit has a leak or failure while the other circuit still holds pressure Correct answer.
  4. The compressor has failed completely and both circuits are losing air

Explanation

Because the circuits are isolated, one circuit can lose pressure while the other holds normal pressure. The driver should recognize this as a partial system failure and stop safely to investigate.

Question 48Dual Air Brake Systems and Safety Valves

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

  1. To regulate when the compressor loads and unloads to maintain system pressure Correct answer.
  2. To measure the amount of air flowing to the trailer
  3. To apply the spring brakes when pressure is low
  4. To filter oil and water out of the compressed air

Explanation

The governor senses system pressure and signals the compressor to load (build air) at cut-in pressure and unload (stop building) at cut-out pressure. This keeps system pressure within the designed operating range.

Question 49Dual Air Brake Systems and Safety Valves

During a pre-trip inspection, the driver builds air pressure to the governor cut-out, then shuts off the engine and watches the gauge. What is the driver checking?

  1. The operation of the alcohol evaporator
  2. The setting of the trailer spring brakes
  3. The compressor's ability to build pressure
  4. The air loss rate of the system with the engine off Correct answer.

Explanation

With the engine off and the system fully charged, watching the gauge reveals the static leak rate of the air system. Excessive loss indicates leaks that must be fixed before driving.

Question 50Dual Air Brake Systems and Safety Valves

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

  1. To apply the brakes automatically when air pressure is lost Correct answer.
  2. To provide normal service braking during driving
  3. To release the parking brake when the engine starts
  4. To increase air pressure during hard braking

Explanation

Spring brakes use heavy springs to apply the brakes when air pressure is released or lost, making them a fail-safe emergency and parking brake. Air pressure must be present to compress the springs and release the brakes.