Air Brakes CDL Practice Test

The Air Brakes test covers the dual air-brake system used on virtually every modern commercial vehicle. If you skip this test or fail it, your CDL will carry an "L" restriction that prevents you from operating any vehicle with air brakes — which rules out almost every truck on the road. The air-brakes exam is the most-failed CDL knowledge test, so plan extra study time.

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What's on the Air Brakes exam

States typically administer 25 Air Brakes questions and require 80% to pass. Questions cover air-brake system parts, dual systems, supply pressure, brake-system warning, slack adjusters, the parking brake, the spring brake, the pre-trip air-system check, and the proper way to perform a leakage-rate test.

Topics covered

Who needs this endorsement

Anyone who plans to drive a vehicle equipped with air brakes — which is essentially every commercial truck and bus over 26,001 lbs GVWR. The skills test must be taken in a vehicle with full air brakes to remove the air-brake restriction.

All 71 Air Brakes Practice Questions

Click any answer to reveal the correct one and the explanation. Take a state-specific 20-question randomized round on any of the state pages.

Question 1 of 71 · #98
The parts of an air-brake system include:
Correct. Air-brake systems use a compressor, governor, storage tanks, foot valve, and brake chambers — distinct from hydraulic systems used in cars.
Question 2 of 71 · #99
The governor controls:
Correct. The governor cycles the compressor: it cuts the compressor in around 100 psi and cuts it out around 125 psi.
Question 3 of 71 · #100
Air storage tanks are used to:
Correct. Air storage (or "supply") tanks hold compressed air ready for brake application. Trucks have multiple tanks for redundancy.
Question 4 of 71 · #101
Air tanks must be drained because:
Correct. Water and compressor oil collect at the bottom of air tanks. They must be drained to keep the system clean and prevent winter freeze-ups.
Question 5 of 71 · #102
The S-cam:
Correct. When the foot valve is pressed, air pushes the brake-chamber pushrod, which moves the slack adjuster, which rotates the S-cam shaft, forcing the shoes against the drum.
Question 6 of 71 · #103
Slack adjusters need to be checked because:
Correct. Slack adjusters control how far the pushrod must travel to apply the brake. Out-of-adjustment slack adjusters are the leading mechanical cause of out-of-service violations.
Question 7 of 71 · #104
Manual slack adjusters must be adjusted:
Correct. Drivers may adjust manual slack adjusters in compliance with manufacturer specifications. Automatic slack adjusters are now standard but still require periodic checking.
Question 8 of 71 · #105
The supply pressure gauges show:
Correct. The supply (primary and secondary) pressure gauges show air pressure available for braking — a critical reading before driving and during operation.
Question 9 of 71 · #106
The application pressure gauge shows:
Correct. The application gauge (when present) shows brake-application pressure — useful for spotting brake-system problems on long downgrades.
Question 10 of 71 · #107
A low-air-pressure warning device must come on at or before:
Correct. A federal-mandated low-air-pressure warning (light, buzzer, or wig-wag) must activate at or before 60 psi.
Question 11 of 71 · #108
In a dual air-brake system, the time required for air pressure to build from 85 to 100 psi should be no more than:
Correct. In dual air-brake systems, air should build from 85 to 100 psi within 45 seconds at engine governed RPM.
Question 12 of 71 · #109
The spring brakes:
Correct. Spring brakes use heavy springs that apply the brake when air pressure is released or drops below 20-45 psi — they serve as both parking and emergency brake.
Question 13 of 71 · #110
The parking-brake control:
Correct. The parking-brake control is a yellow diamond-shaped knob, distinct from the red round trailer-supply knob.
Question 14 of 71 · #111
The trailer air-supply control:
Correct. The trailer air-supply (tractor protection) is a red eight-sided knob. Push in to supply air to the trailer; pull out to shut off air.
Question 15 of 71 · #112
When should you NOT use the parking brakes?
Correct. Hot brakes can be damaged by the spring brakes contracting against hot drums. Let the brakes cool before parking.
Question 16 of 71 · #113
When should you use the trailer hand valve to slow the rig?
Correct. Using the trailer hand valve alone applies brakes only at the trailer wheels and can cause the trailer to lock up and skid. Always use the foot valve to brake.
Question 17 of 71 · #114
Pre-trip air-brake check (Step 1) is to:
Correct. In the seven-step air-brake check the first step is testing the parking brake — release service brakes, set parking brake, gently try to move the vehicle in low gear.
Question 18 of 71 · #115
For a single vehicle (not a combination) with engine off and brakes released, the maximum allowable air-loss rate is:
Correct. A single vehicle is allowed no more than 2 psi/min of air loss with engine off and brakes released.
Question 19 of 71 · #116
For a combination vehicle (tractor and trailer) with engine off and brakes released, the maximum allowable air-loss rate is:
Correct. A combination vehicle is allowed no more than 3 psi/min with brakes released, or 4 psi/min with brakes applied.
Question 20 of 71 · #117
For a single vehicle with engine off and brakes applied, the maximum allowable air-loss rate is:
Correct. A single vehicle is allowed no more than 3 psi/min with brakes fully applied.
Question 21 of 71 · #118
In a dual air-brake system, you should let the air pressure build to at least what level before driving?
Correct. Wait until the system pressure is at least 100 psi before driving in a dual air-brake system.
Question 22 of 71 · #119
You hear a steady "ssss" sound while parked with the engine off. This is most likely:
Correct. A steady hiss with the engine off indicates an air leak — find and repair before driving.
Question 23 of 71 · #120
When you make a normal stop with air brakes, you should:
Correct. For normal stops, apply steady firm pressure on the foot valve and adjust to maintain a smooth stop.
Question 24 of 71 · #121
A long downgrade requires:
Correct. Pick a low gear before starting down. Use the brakes in firm, intermittent applications: brake to 5 mph below safe speed, release, repeat.
Question 25 of 71 · #122
Continuous use of the brakes on a long downgrade can cause:
Correct. Holding the brakes overheats them and causes brake fade — drums expand and brake compounds lose effectiveness, reducing braking force.
Question 26 of 71 · #123
Brake fade is the result of:
Correct. Brake fade comes from heat. Manage downgrade speed by gear selection plus intermittent firm brake use, not continuous light pressure.
Question 27 of 71 · #124
You should perform a static air-leakage test by:
Correct. Engine off, brakes released, fully charged system: watch for air loss over one minute. Then apply brakes and check again.
Question 28 of 71 · #125
When the spring brakes apply automatically as air pressure drops, this typically happens between:
Correct. Spring brakes will fully apply somewhere between 20 and 45 psi as system pressure drops, depending on the vehicle.
Question 29 of 71 · #126
In an emergency stop on a non-ABS vehicle, you should:
Correct. Stab braking applies brakes hard until the wheels lock, then releases when you feel skid — this slows the vehicle while keeping it straight.
Question 30 of 71 · #127
On a vehicle with ABS, in an emergency stop you should:
Correct. ABS lets you brake hard and continue steering. Brake firmly and avoid pumping — pumping defeats the system.
Question 31 of 71 · #128
Air brakes take longer to work than hydraulic brakes because:
Correct. Brake-lag in air systems comes from the time air takes to travel through lines and reach all the brake chambers — typically 0.4 seconds added to perception/reaction time.
Question 32 of 71 · #129
Stopping distance for a vehicle with air brakes is the sum of:
Correct. Air-brake stopping distance is perception + reaction + brake lag + braking — the brake lag is the unique addition.
Question 33 of 71 · #130
When brake drums or shoes get very hot, you should:
Correct. Park where you can let the brakes cool. Do not apply the parking (spring) brake on overheated brakes — it can damage them or cause warpage.
Question 34 of 71 · #131
Wet brakes:
Correct. After fording water, brakes may be wet. Drive slowly while applying light brake pressure to dry them.
Question 35 of 71 · #132
A condition called "brake fade" is most likely on:
Correct. Brake fade comes from heat from extended use — most commonly on long, steep downgrades where the brakes are working continuously.
Question 36 of 71 · #133
Which of these is the safest speed for descending a long, steep grade?
Correct. Choose a speed and gear that lets the brakes work intermittently without continuous pressure. Posted truck-grade signs help.
Question 37 of 71 · #134
The "brake-system warning device" must come on no later than:
Correct. A federal-rule low-air-pressure warning device must activate at or before 60 psi.
Question 38 of 71 · #135
Modulating valves on the trailer:
Correct. The trailer service brakes are modulated — pressing harder on the foot valve produces more brake-chamber pressure at the trailer.
Question 39 of 71 · #136
After connecting the trailer air lines, you should:
Correct. Charge the trailer system, then test the trailer brakes by pulling out the trailer-supply knob to make sure they hold the trailer.
Question 40 of 71 · #137
The "tractor protection valve" closes (cuts off air to the trailer) automatically when:
Correct. The tractor protection valve protects the tractor from a runaway air loss in case of a trailer break-away or air leak — it closes between 20 and 45 psi.
Question 41 of 71 · #138
If a trailer breaks away, the emergency brakes on the trailer will be applied because:
Correct. When the air line breaks (or is disconnected), trailer air pressure drops and the trailer spring brakes apply, stopping the trailer.
Question 42 of 71 · #139
You should never leave the vehicle unattended without:
Correct. Set the parking brake. On grades or where unintended movement could occur, chock the wheels in addition.
Question 43 of 71 · #140
Air brakes are actually three brake systems combined:
Correct. A complete air-brake system has the service brake (foot valve), parking brake (spring/yellow knob), and emergency brake (which is the parking brake working off the spring brake when air drops).
Question 44 of 71 · #141
Pushing the brake pedal harder makes:
Correct. The foot valve modulates air pressure to the brake chambers in proportion to how hard you press the pedal.
Question 45 of 71 · #142
If you must drive a vehicle with manual front-wheel-brake limiting valve, you should keep it in the "normal" position:
Correct. Keep the front-wheel limiting valve in the "normal" position. Modern trucks rarely have this valve, but if equipped, leaving it in "slippery" reduces front braking and lengthens stopping distance.
Question 46 of 71 · #143
Antilock braking systems (ABS):
Correct. ABS prevents wheel lockup during hard braking, allowing you to steer to safety. ABS does not necessarily shorten stopping distance.
Question 47 of 71 · #144
A yellow ABS malfunction lamp on the cab dash means:
Correct. A yellow ABS lamp on the dash indicates a tractor-ABS malfunction. The base brakes still work normally, but ABS is unavailable until repaired.
Question 48 of 71 · #145
A yellow ABS malfunction lamp on the trailer (typically left side) means:
Correct. A trailer-mounted yellow ABS lamp indicates the trailer's ABS is malfunctioning. Service brakes still work; have the system repaired.
Question 49 of 71 · #146
You should test the parking brake by:
Correct. After applying the parking brake and releasing the service brakes, gently try to move forward in low gear. If the vehicle moves, the parking brake is not holding.
Question 50 of 71 · #147
The service brakes are operated by:
Correct. Service brakes are applied by the foot valve / brake pedal. The yellow and red knobs control parking and trailer-supply functions.
Question 51 of 71 · #148
Air-brake disc brakes use:
Correct. Disc brakes use a brake chamber and slack adjuster like S-cam drum brakes, but the force closes a caliper on a rotor instead of forcing shoes against a drum.
Question 52 of 71 · #149
When checking the brakes on a downgrade, you should:
Correct. Watch the air-pressure gauge. If pressure drops faster than the compressor can recover, brakes are being used too aggressively for the gear selection.
Question 53 of 71 · #150
A "wet tank" is:
Correct. The first storage tank — sometimes called the wet tank or supply tank — is where water and oil from the compressor collect. Drain it daily.
Question 54 of 71 · #151
You should drain the air tanks:
Correct. Drain all air tanks at the end of each working day to remove water and compressor oil that has condensed in the tanks.
Question 55 of 71 · #152
Brake drums or discs that have cracks larger than ___ the width of the friction area should be replaced:
Correct. Cracks longer than one-half the width of the friction area indicate the drum is unsafe and must be replaced.
Question 56 of 71 · #153
Total stopping distance with air brakes equals:
Correct. Add brake-lag distance to the standard perception + reaction + braking distance for any vehicle with air brakes.
Question 57 of 71 · #154
In a fully charged dual air-brake system at idle, you should test the low-pressure warning by:
Correct. Engine off, fan the brake pedal to bleed pressure. The low-pressure warning device should activate before pressure drops below 60 psi.
Question 58 of 71 · #155
When the air-pressure protection valve closes during driving (because pressure dropped too low), the trailer:
Correct. When tractor air pressure falls into the 20-45 psi range, the protection valve closes, cutting trailer air, which causes the trailer spring brakes to apply.
Question 59 of 71 · #156
You are driving a fully loaded combination vehicle on a steep downgrade. Your safe speed is 30 mph and you have already shifted to a lower gear. The proper braking technique is to:
Correct. The "snub-and-release" method: brake firmly to drop 5 mph below safe speed, release until safe speed returns, repeat. Holding the brakes overheats them.
Question 60 of 71 · #157
When releasing brakes after a hard stop, you should:
Correct. Release the brakes smoothly. Air tanks refill while you drive; allow the compressor to recharge the system before the next braking event.
Question 61 of 71 · #158
The air compressor is driven by:
Correct. The air compressor is engine-driven, typically through gears at the front of the engine or by a v-belt.
Question 62 of 71 · #159
Modern trucks have ABS that is required on:
Correct. Federal rule: tractors built after March 1, 1997, and trailers and single-unit air-braked vehicles after March 1, 1998, must have ABS.
Question 63 of 71 · #160
You are driving a tractor-trailer with the air-pressure gauge dropping rapidly. You should:
Correct. A rapid drop in air pressure indicates a serious leak. Pull over before the pressure drops to the point where the spring brakes apply and the vehicle stops on the road.
Question 64 of 71 · #161
The brake-system warning light or buzzer is designed to alert you when:
Correct. The brake-system warning device alerts you to dangerous low air pressure — typically activating at or before 60 psi.
Question 65 of 71 · #162
During a brake check, the brake pedal should not move more than ___ when you push it down hard with the engine off and air pressure built up:
Correct. After full charge, holding the foot valve down should produce a firm pedal that moves only a few inches. Pedal sinking to the floor indicates a leak.
Question 66 of 71 · #163
Modern dual-circuit air-brake systems are required so that:
Correct. Dual systems separate primary and secondary brake circuits so that a failure in one (rear or trailer) still leaves brakes available on the other axle group.
Question 67 of 71 · #164
To check air-brake adjustment quickly, you can:
Correct. Pushrod travel beyond about 1 inch (varies by chamber size) indicates an adjustment issue. Use a marked stroke check or have a mechanic verify.
Question 68 of 71 · #165
Air-brake adjustment is critical because out-of-adjustment brakes:
Correct. Out-of-adjustment slack adjusters reduce braking force at the wheel, increasing stopping distance and risking brake failure on long downgrades.
Question 69 of 71 · #166
Air loss from the brake system on a moving combination vehicle, with brakes applied, should not exceed:
Correct. Combination vehicle, brakes applied, engine off: no more than 4 psi/min air loss. Engine off, brakes released: no more than 3 psi/min.
Question 70 of 71 · #167
You should never use the parking brake when the brakes are very hot because:
Correct. Hot brakes cooled with parking brake applied can warp the drum or rotor, or cause the spring brake to fail to release.
Question 71 of 71 · #168
You should check the air-system pressure build-up time during pre-trip:
Correct. In dual air-brake systems, the air should build from 85 to 100 psi within 45 seconds with the engine at governed RPM.