Mississippi · Air Brakes CDL Practice Test

The same federal Air Brakes question pool used by the Mississippi Department of Public Safety, with 50 practice questions and detailed explanations.

About the Air Brakes exam in Mississippi

To add the Air Brakes endorsement to a CDL issued by Mississippi, drivers complete the same federal knowledge exam covered on this site, then pay Mississippi's endorsement add-on fee of approximately $16 per endorsement. The state schedules the written test at most full-service driver license offices and accepts walk-ins at many locations, though appointments are encouraged in metropolitan areas.

The Air Brakes (AB) test in Mississippi draws from the same federal question pool used nationwide. Practice with the questions on this page reflects the topics, format, and difficulty you will encounter at the counter. Some states also require an additional state-specific supplement for certain endorsements; check Mississippi's CDL manual cover page for any state-specific addenda before your test date.

Once the Air Brakes endorsement is added, it appears as the letter AB on the back of your physical license and in the state CDLIS record visible to employers and motor carrier safety auditors. The endorsement remains valid as long as the underlying CDL stays current and any federal prerequisites — such as the TSA security threat assessment for Hazmat — remain in good standing.

For drivers preparing for the Air Brakes endorsement test in Mississippi, additional context — including federal manual excerpts, employer hiring practices, and DOT medical guidance — is widely available from industry resources. Continue reading on a recommended industry resource for further detail. Always cross-check anything you read with the current edition of your state CDL manual, since enforcement guidance is updated periodically.

How to schedule the written test in Mississippi

Most the Mississippi Department of Public Safety offices accept walk-in CDL written-test applicants during posted hours, though appointments are strongly recommended in larger metro areas to avoid long waits. Bring your current driver's license, a current DOT medical examiner's certificate, and any documents required to satisfy the federal REAL ID identity-and-residency check if you have not already done so.

If you fail the Air Brakes exam in Mississippi, you can retake it after a state-determined waiting period (commonly one business day, but in some counties, longer). Most jurisdictions cap the number of retakes within a single application cycle; check with your local the Mississippi Department of Public Safety office to confirm.

Air Brakes · Practice Quiz

Same federal question pool. Read the question, choose the best answer, then expand the explanation.

Question 1

The air compressor is connected to the engine:
- A By the alternator belt only
- B By gears, V-belt, or direct mounting Correct answer
- C By the transmission output
- D By the steering linkage
Show explanation

The compressor is engine-driven via gears, belt, or direct mount. It builds the air pressure that the brake system uses.
Question 2

The air compressor governor controls:
- A The transmission shifting
- B The alternator output
- C When the compressor pumps air into the storage tanks Correct answer
- D The fuel injection rate
Show explanation

The governor cycles the compressor between cut-in (~100 psi) and cut-out (~125 psi) to maintain reservoir pressure within range.
Question 3

Air storage tanks are used to:
- A Hold diesel fuel
- B Store transmission fluid
- C Cool the engine oil
- D Hold compressed air for braking Correct answer
Show explanation

Reservoirs (storage tanks) hold pressurized air ready for delivery to the brake chambers when the foot valve is applied.
Question 4

Why must you drain the air tanks?
- A To reduce engine load
- B To save fuel
- C Compressed air contains water and oil that can damage the system if allowed to accumulate Correct answer
- D To increase braking pressure
Show explanation

Daily draining removes accumulated moisture and oil that would otherwise corrode tanks and damage valves and seals.
Question 5

The supply pressure gauge shows:
- A How much pressure is being applied to the brakes
- B Engine RPM
- C How much fuel is in the tank
- D How much pressure is in the air tanks Correct answer
Show explanation

The supply pressure gauge reads reservoir (tank) pressure. The application gauge reads brake-application pressure.
Question 6

The low-pressure warning device must activate at:
- A 20 psi
- B 60 psi or above Correct answer
- C 40 psi
- D 100 psi
Show explanation

Federal regulation requires the low-pressure warning to activate before reservoir pressure drops below 60 psi.
Question 7

A "wig-wag" is:
- A A fifth-wheel slider
- B An exhaust brake
- C A mechanical low-pressure warning that drops into the driver's view Correct answer
- D A trailer hand valve
Show explanation

Wig-wags are a mechanical alternative to (or addition to) the audible/visual low-pressure warning. They drop into the field of view at low pressure.
Question 8

Spring brakes activate automatically when air pressure drops to:
- A 125 psi
- B 100 psi
- C Between 20 and 45 psi Correct answer
- D 60 psi
Show explanation

When air pressure falls into the 20–45 psi range, the spring brakes apply automatically as a fail-safe parking and emergency brake.
Question 9

The parking brake control on most trucks is:
- A A red triangular handle
- B A foot pedal
- C A blue lever on the floor
- D A yellow, diamond-shaped knob that pulls out to apply Correct answer
Show explanation

Standard parking brake control is a yellow diamond pull knob. Trailer air supply is the red eight-sided knob.
Question 10

The trailer air supply control is colored and shaped:
- A Blue triangle
- B Green round
- C Red, eight-sided (octagonal) Correct answer
- D Yellow diamond
Show explanation

Trailer air supply (sometimes called the trailer parking brake) is a red eight-sided knob — pushed in to charge, pulled out to set trailer brakes.
Question 11

The maximum allowable air loss for a single vehicle with the engine off and brakes fully applied is:
- A 10 psi per minute
- B 5 psi per minute
- C 2 psi per minute Correct answer
- D 15 psi per minute
Show explanation

A single vehicle may lose no more than 2 psi per minute. Combination vehicles may lose 3 psi per minute.
Question 12

For combination vehicles, maximum air loss is:
- A 5 psi per minute
- B 6 psi per minute
- C 8 psi per minute
- D 3 psi per minute Correct answer
Show explanation

Combination vehicles allow up to 3 psi per minute of leakage during the static test.
Question 13

When the brake pedal is pressed, air pressure is sent to:
- A The engine to slow it
- B The alternator
- C The transmission to downshift
- D The brake chambers, which apply the brakes Correct answer
Show explanation

Foot valve application sends air pressure to brake chambers; the chambers convert pressure into mechanical force on the slack adjusters and brake shoes.
Question 14

Brake fade is caused by:
- A Excessive heat from prolonged or heavy brake use Correct answer
- B Cold temperatures
- C Moisture in the lines
- D Underinflated tires
Show explanation

Heat reduces friction between brake shoes and drums, dramatically increasing stopping distance. Use engine braking and gear selection on long descents.
Question 15

Riding the brakes downhill:
- A Is required by federal regulation
- B Causes brake fade and is dangerous Correct answer
- C Improves gear shifts
- D Saves fuel
Show explanation

Light continuous brake pressure heats brakes without giving them time to cool — the textbook cause of fade and runaway.
Question 16

On a vehicle without ABS, the proper emergency braking technique is:
- A Pumping rapidly
- B Releasing brakes entirely
- C Slamming and holding the brakes
- D Either stab or controlled braking Correct answer
Show explanation

Without ABS, use stab (apply-release-apply) or controlled (just below lockup) braking to keep the wheels rolling and steerable.
Question 17

On a vehicle WITH ABS, the proper emergency braking technique is:
- A Pump the brakes rapidly
- B Apply the brakes firmly and hold; ABS prevents lockup Correct answer
- C Release brakes
- D Use only the parking brake
Show explanation

ABS modulates pressure automatically. Apply firmly and steer; do not pump.
Question 18

Before driving with air brakes, you should wait for the air pressure to build to at least:
- A 20 psi
- B 40 psi
- C 100 psi Correct answer
- D 60 psi
Show explanation

Federal regulation requires at least 100 psi before driving. Below that, braking performance is inadequate and the low-pressure warning may activate.
Question 19

A leak in the air brake system can be detected by:
- A Pressing the accelerator
- B Listening with the engine off and brakes fully applied Correct answer
- C Checking the dashboard fuel gauge
- D Driving the vehicle and listening
Show explanation

Static leakage testing (engine off, full brake application, listening and watching the gauge) is the standard detection method.
Question 20

Slack adjusters with too much travel indicate:
- A The brakes are out of adjustment Correct answer
- B The driver is too heavy
- C Tires are over-inflated
- D The compressor is too strong
Show explanation

Excessive pushrod travel means the brake shoes are not being pushed against the drum with full force. Adjust or repair before driving.
Question 21

ABS is required on all air-braked vehicles built after:
- A January 1, 2000
- B March 1, 1998 Correct answer
- C January 1, 2010
- D July 1, 1995
Show explanation

Federal regulation required ABS on all air-braked tractors built after March 1, 1997 and on trailers built after March 1, 1998.
Question 22

If ABS fails on a tractor:
- A You can no longer steer
- B You lose all braking
- C You must stop immediately
- D You still have normal brakes — drive normally and have it repaired Correct answer
Show explanation

ABS failure does not affect normal braking. The yellow ABS warning lamp will illuminate; have the system serviced as soon as practical.
Question 23

Most heavy trucks have:
- A A single air system
- B A single hydraulic system
- C A combination of air and hydraulic
- D Two separate air brake systems with their own air tanks and valves Correct answer
Show explanation

Federal regulation requires dual air brake systems on heavy trucks so that a failure in one circuit still leaves brakes on at least some axles.
Question 24

On a dual air brake system, both systems should build to operating pressure within:
- A 10 minutes
- B 2 minutes
- C 5 seconds
- D About 45 seconds Correct answer
Show explanation

Pressure should build from 85 to 100 psi within about 45 seconds; longer build time may indicate a leak or a weak compressor.
Question 25

When the parking brake is "set," the spring brakes:
- A Use battery power
- B Apply through spring force, not air pressure Correct answer
- C Are held by air pressure
- D Are disabled
Show explanation

When set, spring brakes apply mechanically through their internal springs. Air pressure is what holds them OFF during driving.
Question 26

You should never use the trailer parking brake to:
- A Park overnight
- B Use during inspection
- C Hold a parked combination if the tractor brakes are released Correct answer
- D Set during loading
Show explanation

Trailer brakes can leak off air over time and release. Always use the tractor parking brake — or both — to secure a parked combination.
Question 27

Compared to hydraulic brakes, air brakes have:
- A No braking power
- B A slight delay before they activate (brake lag) Correct answer
- C Faster response time
- D No delay
Show explanation

Air must travel through the lines before reaching the chambers. The result is a small but noticeable lag of about half a second.
Question 28

Total stopping distance for an air-braked vehicle is approximately:
- A Half the speed in feet
- B Reaction time only
- C Just braking distance
- D Driver perception + reaction + brake lag + braking distance Correct answer
Show explanation

Air-braked stopping distance includes brake lag in addition to the standard perception, reaction, and braking distances.
Question 29

How do you check the slack adjuster?
- A Listen for hissing only
- B Park on level ground, chock wheels, release brakes, and pull on the slack adjuster — it should not move more than about an inch Correct answer
- C Push the brake pedal
- D Check the dashboard
Show explanation

With brakes released and wheels chocked, pulling the slack adjuster by hand should reveal less than approximately 1 inch of free travel.
Question 30

During the air brake check, the air-pressure-build test verifies that:
- A Pressure builds from 85 to 100 psi within 45 seconds at the engine's normal operating RPM Correct answer
- B The compressor is removed
- C The mirrors are clean
- D The horn works
Show explanation

Slow build indicates a weak compressor or major leak. The build test is part of every pre-trip air brake check.
Question 31

The modulator valve in an air brake system:
- A Adjusts the alternator output
- B Modulates trailer brake application during ABS events Correct answer
- C Regulates fuel injection
- D Controls the engine governor
Show explanation

Modulator valves rapidly cycle brake pressure during ABS activation to prevent wheel lockup.
Question 32

In cold weather, alcohol evaporators in the air system:
- A Reduce stopping distance
- B Increase engine power
- C Improve braking force
- D Help prevent ice buildup in the air lines Correct answer
Show explanation

Alcohol introduced into the air system reduces the freezing point of moisture, helping prevent ice in valves and lines.
Question 33

In freezing weather, you should drain the air tanks:
- A Only when the truck is hot
- B Once per week
- C Daily to remove water before it freezes Correct answer
- D Never in cold weather
Show explanation

Daily draining is essential in cold weather; ice in the air system can disable warning devices and brake valves.
Question 34

The application pressure gauge shows:
- A How much pressure is being applied to the brakes Correct answer
- B Fuel level
- C Engine oil pressure
- D How much pressure is in the tanks
Show explanation

The application gauge reads how hard you are pressing the brakes; the supply gauge reads tank pressure.
Question 35

The most common type of foundation brake on heavy trucks is:
- A S-cam drum brake Correct answer
- B Wedge brake
- C Disc brake
- D Band brake
Show explanation

S-cam drum brakes use an S-shaped cam to push the brake shoes outward against the drum. They are the standard on most heavy trucks, though disc brakes are increasingly common.
Question 36

Air disc brakes:
- A Use air pressure to apply caliper-mounted pads to a rotor Correct answer
- B Use exhaust pressure
- C Use spring force only
- D Use hydraulic fluid
Show explanation

Air disc brakes use air to actuate the caliper, which clamps pads onto a rotor — like a car brake but air-actuated.
Question 37

The brake chamber:
- A Stores air for the parking brake
- B Filters air entering the system
- C Cools the brake drum
- D Converts air pressure into mechanical force on the pushrod Correct answer
Show explanation

Brake chambers convert pneumatic energy to mechanical motion on the pushrod, which moves the slack adjuster and rotates the S-cam.
Question 38

The trailer hand valve (also called the trolley valve) should be used only:
- A To stop the truck
- B To test the trailer brakes Correct answer
- C To park the trailer
- D In an emergency
Show explanation

The hand valve was historically used for trailer-only brake control but should now only be used for testing. Never use it for routine stopping.
Question 39

Brake lag for air brakes is approximately:
- A 5 seconds
- B Half a second Correct answer
- C 2 seconds
- D 1 second
Show explanation

Air must travel from the foot valve through the lines to the chambers — a delay of roughly half a second on a typical heavy truck.
Question 40

A typical loaded tractor-trailer at 55 mph stops in approximately:
- A 50 feet
- B 1000 feet
- C 100 feet
- D 419 feet Correct answer
Show explanation

The textbook figure for a loaded combination at 55 mph on dry pavement is approximately 419 feet — about the length of a 14-story building tipped over.
Question 41

Spring brakes provide:
- A Only parking
- B Both parking brake and emergency brake function Correct answer
- C Service braking
- D Only emergency
Show explanation

Spring brakes serve as both the parking brake (set manually) and the emergency brake (apply automatically when air pressure drops dangerously low).
Question 42

If the air pressure warning light comes on while driving:
- A Increase speed
- B Stop safely as soon as possible — spring brakes will apply automatically when pressure falls further Correct answer
- C Continue to your destination
- D Open the windows for cooling
Show explanation

Pressure dropping below 60 psi triggers the warning. If pressure continues to fall, spring brakes apply automatically — possibly without warning. Stop safely now.
Question 43

The trailer service air line is the air line that:
- A Supplies constant air to charge the trailer reservoirs
- B Carries hydraulic fluid
- C Carries air pressure from the tractor to the trailer brakes during application Correct answer
- D Carries electrical power
Show explanation

The service line carries variable application pressure during braking; the supply (emergency) line carries constant pressure to charge the trailer reservoirs.
Question 44

The trailer supply (emergency) line:
- A Carries water
- B Carries fuel
- C Only carries air during braking
- D Carries constant air pressure to charge the trailer's reservoirs and release its spring brakes Correct answer
Show explanation

The supply (or emergency) line keeps the trailer charged. If it disconnects or loses pressure, trailer spring brakes apply automatically as an emergency response.
Question 45

When coupling, you should connect the air lines:
- A Service to supply
- B In any order
- C Service to service (yellow), supply to supply (red) Correct answer
- D Without paying attention to color
Show explanation

Yellow service connects to yellow service; red supply connects to red supply. Reverse-coupling causes brake failure.
Question 46

Glad hands are:
- A A type of brake shoe
- B The connection points for tractor-trailer air lines Correct answer
- C A type of mirror
- D A trailer's landing gear
Show explanation

Glad hands are the snap-together couplings on the air lines between tractor and trailer. They have rubber seals that wear out and must be inspected.
Question 47

When ABS detects wheel lockup, it:
- A Reduces brake pressure briefly to allow the wheel to roll Correct answer
- B Increases brake pressure
- C Locks the wheel further
- D Disables steering
Show explanation

ABS rapidly modulates pressure to keep the wheel just below lockup, preserving steering and reducing stopping distance.
Question 48

The brake light circuit:
- A Has no air-brake connection
- B Activates whenever the brake pedal is pressed Correct answer
- C Activates only with parking brake
- D Activates with the horn
Show explanation

A pressure-sensitive switch in the brake circuit activates the brake lights whenever the brake pedal is pressed.
Question 49

A "compressor governor" stuck open will cause:
- A No air pressure to build
- B Brake failure immediately
- C The compressor to run continuously and pressure to remain high Correct answer
- D Tire failure
Show explanation

A stuck-open governor allows continuous compressor operation, leading to overpressure or premature wear. Pop-off valves prevent dangerous overpressure.
Question 50

A persistent hissing from a brake chamber when brakes are released indicates:
- A Tire leak
- B Engine starvation
- C A leaking brake chamber diaphragm — repair before driving Correct answer
- D Normal operation
Show explanation

A leaking diaphragm reduces braking force and signals imminent chamber failure. Do not drive until repaired.

Mississippi · Air Brakes CDL Practice Test

About the Air Brakes exam in Mississippi

How to schedule the written test in Mississippi

The air compressor is connected to the engine:

The air compressor governor controls:

Air storage tanks are used to:

Why must you drain the air tanks?

The supply pressure gauge shows:

The low-pressure warning device must activate at:

A "wig-wag" is:

Spring brakes activate automatically when air pressure drops to:

The parking brake control on most trucks is:

The trailer air supply control is colored and shaped:

The maximum allowable air loss for a single vehicle with the engine off and brakes fully applied is:

For combination vehicles, maximum air loss is:

When the brake pedal is pressed, air pressure is sent to:

Brake fade is caused by:

Riding the brakes downhill:

On a vehicle without ABS, the proper emergency braking technique is:

On a vehicle WITH ABS, the proper emergency braking technique is:

Before driving with air brakes, you should wait for the air pressure to build to at least:

A leak in the air brake system can be detected by:

Slack adjusters with too much travel indicate:

ABS is required on all air-braked vehicles built after:

If ABS fails on a tractor:

Most heavy trucks have:

On a dual air brake system, both systems should build to operating pressure within:

When the parking brake is "set," the spring brakes:

You should never use the trailer parking brake to:

Compared to hydraulic brakes, air brakes have:

Total stopping distance for an air-braked vehicle is approximately:

How do you check the slack adjuster?

During the air brake check, the air-pressure-build test verifies that:

The modulator valve in an air brake system:

In cold weather, alcohol evaporators in the air system:

In freezing weather, you should drain the air tanks:

The application pressure gauge shows:

The most common type of foundation brake on heavy trucks is:

Air disc brakes:

The brake chamber:

The trailer hand valve (also called the trolley valve) should be used only:

Brake lag for air brakes is approximately:

A typical loaded tractor-trailer at 55 mph stops in approximately:

Spring brakes provide:

If the air pressure warning light comes on while driving:

The trailer service air line is the air line that:

The trailer supply (emergency) line:

When coupling, you should connect the air lines:

Glad hands are:

When ABS detects wheel lockup, it:

The brake light circuit:

A "compressor governor" stuck open will cause:

A persistent hissing from a brake chamber when brakes are released indicates: