CDL air brakes practice test
38 practice questions written from the AAMVA model Commercial Driver's License Manual (March 2025 edition). The mock test asks 25 questions and needs 20 correct, the federal 80% pass share.
Practice questions written from the AAMVA model CDL manual; not affiliated with FMCSA, AAMVA or any state licensing agency.
examprepare.help wrote this study aid. AAMVA, FMCSA and the state agencies did not write or review it, and your real test may differ.
Practice shows the right answer and the manual page after every question.The mock test asks 25 questions and needs 20 correct to pass.
The real air brakes test
Federal rules require at least 80% correct on every CDL knowledge test (49 CFR 383.135(b)(1)). On a 25-question test that is 20 correct.
Test sizes vary by state. Twenty-five questions is the common size of the air brakes test. Each state sets its own test, so your test may ask more or fewer. The 80% rule is the same in every state. Check your state licensing agency before your test.
Rule source: 49 CFR 383.135(b)(1) (80 percent to pass a CDL knowledge test).
Who takes this test
You take the air brakes test if your vehicle has air brakes, including air over hydraulic brakes. If you skip or fail it, your license carries restriction L: no vehicle with air brakes.
Manual section 1.1.1, Knowledge Tests, page 1-5. Restriction L: Section 1, page 1-3. Our questions come from Section 5 of the manual.
The CDL manual
Commercial Driver's License Manual, Modernized Testing System
AAMVA writes this model manual with support from the Federal Motor Carrier Safety Administration (FMCSA). Most states use it as their CDL manual. Our copy is the PDF that Service Oklahoma publishes.
Read the manual (PDF, Service Oklahoma) or the CDL manual page on the FMCSA site.
We read this edition on 2026-10-05. Your state may print its own edition. Check that your copy is the current one.
Key numbers from the manual
| Rule | Value | PDF page |
|---|---|---|
| Air compressor governor cut-in and cut-out | Cut-in around 100 psi, cut-out around 125 psi | 72 |
| Pressure the safety valve opens at | 150 psi | 73 |
| Low air pressure warning threshold | Must come on before pressure falls below 55 psi | 74 |
| Pressure at which spring brakes come fully on | 20 to 45 psi | 74 |
| Slack adjuster movement that needs adjustment | More than about one inch where the push rod attaches | 77 |
| Maximum air leakage rate without braking | Less than 2 psi per minute single, less than 3 psi per minute combination | 77 |
| Maximum air drop with the foot brake held one minute | 3 psi single, 4 psi combination | 78 |
| Dual air system pressure buildup rate | 85 to 100 psi within 45 seconds | 78 |
| Brake lag distance at 55 mph | About 32 feet | 79 |
| Total stopping distance with air brakes at 55 mph | Over 450 feet | 79 |
| Proper air brake technique on a downgrade | Apply to feel a slowdown, release about five mph below the safe speed | 80 |
Questions about the air brakes test
How many questions are on the CDL air brakes test?
Twenty-five questions is the common size of the air brakes test. Each state sets its own test, so your test may ask more or fewer. Our mock test asks 25.
What score do I need to pass the CDL air brakes test?
Federal rules require at least 80% correct on every CDL knowledge test (49 CFR 383.135(b)(1)). On a 25-question test that is 20 correct. Our mock test needs 20 of 25.
Who has to take the air brakes test?
You take the air brakes test if your vehicle has air brakes, including air over hydraulic brakes. If you skip or fail it, your license carries restriction L: no vehicle with air brakes. Manual section 1.1.1, Knowledge Tests, page 1-5. Restriction L: Section 1, page 1-3.
Are these the real CDL test questions?
No. examprepare.help wrote these 38 practice questions from the AAMVA model Commercial Driver's License Manual (March 2025 edition). They are not the real test questions. Practice questions written from the AAMVA model CDL manual; not affiliated with FMCSA, AAMVA or any state licensing agency. Check your state licensing agency before you take the real test.
Which manual are the questions from?
The AAMVA model Commercial Driver's License Manual (March 2025 edition). AAMVA writes this model manual with support from FMCSA, and most states use it as their CDL manual. Every answer cites the manual section and the printed page.
All 38 questions and answers
Read the bank before a mock test. Each answer names the manual section and the printed page.
- 1.Air brakes are really how many different braking systems?
5.1 – The Parts of an Air Brake System
- Three: service, parking, and emergency
The manual says air brakes are really three different braking systems: service brake, parking brake, and emergency brake. Manual: 5.1 – The Parts of an Air Brake System, page 5-1.
- 2.What does the air compressor do in an air brake system?
5.1.1 – Air Compressor
- It pumps air into the air storage tanks
The air compressor pumps air into the air storage tanks, also called reservoirs. Manual: 5.1.1 – Air Compressor, page 5-1.
- 3.At about what pressure does the governor allow the air compressor to start pumping again?
5.1.2 – Air Compressor Governor
- About 100 psi
When tank pressure falls to the cut-in pressure of around 100 psi, the governor allows the compressor to start pumping again. Manual: 5.1.2 – Air Compressor Governor, page 5-1.
- 4.Why do air storage tanks hold enough air for several brake applications?
5.1.3 – Air Storage Tanks
- So the brakes can still be used several times if the compressor stops working
The tanks hold enough air to allow the brakes to be used several times even if the compressor stops working. Manual: 5.1.3 – Air Storage Tanks, page 5-1.
- 5.How often must you drain air tanks that have manual drain valves?
5.1.4 – Air Tank Drains
- At the end of each day of driving
With manually operated drain valves you must drain the tanks yourself at the end of each day of driving so water and oil do not build up. Manual: 5.1.4 – Air Tank Drains, page 5-1.
- 6.Why is it important to drain water from air tanks?
5.1.4 – Air Tank Drains
- Water can freeze in cold weather and cause brake failure
Compressed air carries water and compressor oil that can freeze in cold weather and cause brake failure, so the tanks should be drained. Manual: 5.1.4 – Air Tank Drains, page 5-2.
- 7.What is an alcohol evaporator used for in an air brake system?
5.1.5 – Alcohol Evaporator
- To reduce the risk of ice forming in air brake valves in cold weather
An alcohol evaporator puts alcohol into the air system to reduce the risk of ice in air brake valves and other parts during cold weather. Manual: 5.1.5 – Alcohol Evaporator, page 5-2.
- 8.At what pressure is the safety valve usually set to open?
5.1.6 – Safety Valve
- 150 psi
The safety relief valve is usually set to open at 150 psi, and if it releases air something is wrong and must be fixed by a mechanic. Manual: 5.1.6 – Safety Valve, page 5-2.
- 9.How does the most common foundation brake, the s-cam brake, work?
5.1.8 – Foundation Brakes
- Air pushes the rod out, turning the s-cam so it forces the brake shoes against the drum
When the brake pedal is pushed, the s-cam forces the brake shoes away from one another and against the inside of the brake drum. Manual: 5.1.8 – Foundation Brakes, page 5-2.
- 10.A visible low air pressure warning signal must come on before the air pressure falls below what level?
5.1.11 – Low Air Pressure Warning
- 55 psi
A warning signal you can see must come on before the air pressure in the tanks falls below 55 psi, usually as a red light with a buzzer. Manual: 5.1.11 – Low Air Pressure Warning, page 5-3.
- 11.What position should the front brake limiting valve control be in for normal stopping power?
5.1.13 – Front Brake Limiting Valve
- Normal
The limiting valve cuts normal air pressure to the front brakes in half when set to slippery, so keep the control in the normal position for normal stopping power. Manual: 5.1.13 – Front Brake Limiting Valve, page 5-3.
- 12.At what air pressure do tractor and straight truck spring brakes come fully on?
5.1.14 – Spring Brakes
- 20 to 45 psi
Spring brakes come fully on when air pressure drops into the range of 20 to 45 psi, typically 20 to 30 psi. Manual: 5.1.14 – Spring Brakes, page 5-3.
- 13.Why should you never push the brake pedal down when the spring brakes are on?
5.1.14 – Spring Brakes
- The brakes could be damaged by the combined forces of the springs and air pressure
Never push the brake pedal down when the spring brakes are on, because the brakes could be damaged by the combined forces of the springs and the air pressure. Manual: 5.1.14 – Spring Brakes, page 5-3.
- 14.How do you apply the parking brakes on a newer vehicle with air brakes?
5.1.15 – Parking Brake Controls
- Pull out the diamond-shaped yellow push-pull control knob
On newer vehicles you pull the diamond-shaped yellow control knob out to put on the parking brakes and push it in to release them. Manual: 5.1.15 – Parking Brake Controls, page 5-3.
- 15.What is a dual air brake system?
5.2 – Dual Air Brake
- Two separate air brake systems that use a single set of brake controls
A dual air brake system has two separate air brake systems that use a single set of brake controls and each have their own air tanks, hoses, and lines. Manual: 5.2 – Dual Air Brake, page 5-5.
- 16.In a typical dual air brake system, what do the two systems operate?
5.2 – Dual Air Brake
- One operates the rear axle brakes and the other the front axle brakes
One system typically operates the regular brakes on the rear axle or axles, and the other operates the regular brakes on the front axle and possibly one rear axle. Manual: 5.2 – Dual Air Brake, page 5-5.
- 17.Before driving a vehicle with a dual air system, how much pressure should build up in both systems?
5.2 – Dual Air Brake
- A minimum of 100 psi
Before driving, allow time for the air compressor to build a minimum of 100 psi pressure in both the primary and secondary systems. Manual: 5.2 – Dual Air Brake, page 5-5.
- 18.What should you do if the low air pressure warning comes on in either system of a dual air system while driving?
5.2 – Dual Air Brake
- Stop right away and safely park the vehicle
If the warning comes on while driving, stop right away and safely park the vehicle, because either the front or the rear brakes will not fully operate. Manual: 5.2 – Dual Air Brake, page 5-5.
- 19.When checking slack adjusters, how much movement means they probably need adjustment?
5.3.2 – During Step 5 Walk-around Inspection
- More than about one inch where the push rod attaches
If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjustment. Manual: 5.3.2 – During Step 5 Walk-around Inspection, page 5-6.
- 20.What is the most common problem found in roadside inspections?
5.3.2 – During Step 5 Walk-around Inspection
- Out-of-adjustment brakes
The manual says out-of-adjustment brakes are the most common problem found in roadside inspections, and vehicles with too much brake slack can be very hard to stop. Manual: 5.3.2 – During Step 5 Walk-around Inspection, page 5-6.
- 21.All vehicles built since what year have automatic slack adjusters?
5.3.2 – During Step 5 Walk-around Inspection
- 1994
All vehicles built since 1994 have automatic slack adjusters, though they must still be checked because they can be damaged or out of adjustment. Manual: 5.3.2 – During Step 5 Walk-around Inspection, page 5-6.
- 22.How long may a crack in a brake drum be?
5.3.2 – During Step 5 Walk-around Inspection
- No longer than one half the width of the friction area
Brake drums or discs must not have cracks longer than one half the width of the friction area. Manual: 5.3.2 – During Step 5 Walk-around Inspection, page 5-6.
- 23.With a fully charged air system and the engine off, what is the maximum air leakage rate for a single vehicle?
5.3.3 – Step 7 Final Air Brake Check
- Less than two psi in one minute
The loss rate should be less than two psi in one minute for single vehicles and less than three psi in one minute for combination vehicles. Manual: 5.3.3 – Step 7 Final Air Brake Check, page 5-6.
- 24.With the engine off and the foot brake fully applied for one minute, how much may the air pressure drop on a combination vehicle?
5.3.3 – Step 7 Final Air Brake Check
- No more than four pounds in one minute
With the foot brake held for one minute, the pressure may drop no more than three pounds on a single vehicle or four pounds on a combination vehicle. Manual: 5.3.3 – Step 7 Final Air Brake Check, page 5-7.
- 25.When testing the low pressure warning signal, before what pressure must the signal come on?
5.3.3 – Step 7 Final Air Brake Check
- Before the pressure drops to less than 55 psi
The low air pressure warning signal must come on before the pressure drops to less than 55 psi in the air tank with the lowest pressure. Manual: 5.3.3 – Step 7 Final Air Brake Check, page 5-7.
- 26.How can you check that the spring brakes come on automatically?
5.3.3 – Step 7 Final Air Brake Check
- Fan off the air pressure and watch for the tractor protection and parking brake valves to pop out at 20 to 45 psi
Continue to fan off the air pressure; the tractor protection valve and parking brake valve should close or pop out at 20 to 45 psi, causing the spring brakes to come on. Manual: 5.3.3 – Step 7 Final Air Brake Check, page 5-7.
- 27.In a dual air system with the engine at operating rpm, how fast should pressure build from 85 to 100 psi?
5.3.3 – Step 7 Final Air Brake Check
- Within 45 seconds
In dual air systems the pressure should build from 85 to 100 psi within 45 seconds; older single systems build from 50 to 90 psi within 3 minutes. Manual: 5.3.3 – Step 7 Final Air Brake Check, page 5-7.
- 28.How should you test the parking brake?
5.3.3 – Step 7 Final Air Brake Check
- Stop the vehicle, apply the parking brake, and gently pull against it in a low gear
To test the parking brake, stop the vehicle, put the parking brake on, and gently pull against it in a low gear to make sure it holds. Manual: 5.3.3 – Step 7 Final Air Brake Check, page 5-7.
- 29.With a manual transmission, when should you push the clutch in during a normal stop with air brakes?
5.4.1 – Normal Stops
- When the engine rpm is down close to idle
Control the brake pressure for a smooth, safe stop, and with a manual transmission do not push the clutch in until the engine rpm is down close to idle. Manual: 5.4.1 – Normal Stops, page 5-8.
- 30.What does brake lag add to stopping distance at 55 mph on dry pavement?
5.4.4 – Stopping Distance
- About 32 feet
Brake lag is the time for air to flow through the lines to the brakes, and at 55 mph on dry pavement it adds about 32 feet. Manual: 5.4.4 – Stopping Distance, page 5-8.
- 31.At 55 mph, what is the total stopping distance for an air brake vehicle under good conditions?
5.4.4 – Stopping Distance
- Over 450 feet
At 55 mph for an average driver under good traction and brake conditions, the total stopping distance with air brakes is over 450 feet. Manual: 5.4.4 – Stopping Distance, page 5-8.
- 32.What causes brake fade?
5.4.5 – Brake Fading or Failure
- Excessive heat that chemically changes the lining and expands the drums
Brake fade comes from excessive heat causing chemical changes in the brake lining and expansion of the brake drums, which reduce friction. Manual: 5.4.5 – Brake Fading or Failure, page 5-9.
- 33.Why do brakes that are out of adjustment fade faster?
5.4.5 – Brake Fading or Failure
- The adjusted brakes overheat and fade after doing more than their share of the work
Out-of-adjustment brakes stop doing their share of the work, so the other brakes can overheat and fade until there is not enough braking to control the vehicle. Manual: 5.4.5 – Brake Fading or Failure, page 5-9.
- 34.On a long downgrade, what is the proper air brake technique?
5.4.6 – Proper Braking Technique
- Apply the brakes just hard enough to feel a slowdown and release about five mph below the safe speed
Apply the brakes just hard enough to feel a definite slowdown, release when about five mph below your safe speed, and repeat when the speed returns to it. Manual: 5.4.6 – Proper Braking Technique, page 5-9.
- 35.What should you do if the low air pressure warning comes on?
5.4.7 – Low Air Pressure
- Stop and safely park the vehicle as soon as possible
If the low air pressure warning comes on, stop and safely park as soon as possible, because controlled braking is possible only while enough air remains. Manual: 5.4.7 – Low Air Pressure, page 5-9.
- 36.When should you avoid using the parking brakes?
5.4.8 – Parking Brakes
- When the brakes are very hot from a steep grade or very wet in freezing temperatures
Do not use the parking brakes if the brakes are very hot or very wet in freezing temperatures, because they can be damaged by heat or freeze in place. Manual: 5.4.8 – Parking Brakes, page 5-9.
- 37.What should you do if you park a vehicle with wet brakes in freezing weather?
5.4.8 – Parking Brakes
- Use wheel chocks and dry the brakes by using them lightly in a low gear
If the brakes are wet, use wheel chocks to hold the vehicle and use the brakes lightly in a low gear to heat and dry them, rather than setting the parking brake. Manual: 5.4.8 – Parking Brakes, page 5-9.
- 38.If your air tanks do not have automatic drains, when should you drain them?
5.4.8 – Parking Brakes
- At the end of each working day
If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil, or the brakes could fail. Manual: 5.4.8 – Parking Brakes, page 5-9.