Air brakes are really three braking systems. What are they?
Right! The three systems are the service brake, parking brake and emergency brake.
Get ready for your free CDL practice tests.
Practice every commercial driver's license knowledge test from your state's own CDL manual, one test at a time.
- Class A, B, C
- Air Brakes
- 80% to Pass
Try a CDL question
1 of 2Someone else loaded and secured your trailer. Who is responsible for checking the cargo?
Right! You are responsible for the cargo whether or not you loaded it.
Choose an answer to check it against the explanation
Or select it on the map
Choose the state that issues your CDL
Where are you starting from?
A CDL isn’t one test. Which knowledge tests you take depends on the vehicle you’ll drive and what it carries, and a driver adding a letter needs far less than someone starting out. Pick the start that fits you, and the planner below lists your tests.
- Most learnersMy first CDLYou hold a regular license and want to drive a truck or bus for work. The planner starts you on Class A with air brakes, the setup a tractor-trailer needs; a Class A license also lets you drive Class B and C vehicles, given the endorsements they call for.Show my tests
- Already licensedAdding an endorsementYou have a CDL and need one more letter on it: Hazmat, Tank Vehicles, Doubles and Triples, Passenger or School Bus. Each one is a knowledge test of its own, so you study only that test.Show my tests
- Moving upFrom Class B to Class AYou drive a straight truck or bus and want to pull a heavy trailer. Class A adds the Combination Vehicles test, plus Doubles and Triples if you’ll pull more than one trailer.Show my tests
Which CDL tests does each class need?
Three things decide your tests: the class of vehicle, whether it has air brakes, and any special load or passengers it carries. Pick a class, tick what applies, and your plan fills in. Each knowledge test is graded by itself, so take them one at a time: General Knowledge first, as every other test builds on it. A few states add a test of their own, and your state’s CDL page lists every one.
| Test | Class A | Class B | Class C |
|---|---|---|---|
| Every class | |||
| General KnowledgeInspection, safe driving, cargo and emergencies: the base of every other test | ✓ | ✓ | ✓ |
| For your vehicle | |||
| Combination VehiclesCoupling, trailers, rollovers and the air lines between tractor and trailer | ✓ | – | – |
| with air brakes | with air brakes | with air brakes | |
| For what you carry or pull (endorsements) | |||
| if you pull them | – | – | |
| – | if you carry them | if you carry them | |
| – | if you drive one | if you drive one | |
| if you haul it | if you haul it | if you haul it | |
| if you haul it | if you haul it | if you haul it | |
From learner’s permit to CDL
Federal rules set the same five steps in every state. What changes from state to state is the paperwork: the forms, fees and offices, which your state’s CDL page covers.
The DOT medical exam
Before anything else, a medical examiner listed on FMCSA’s National Registry checks your vision, hearing and health. The result goes to your state electronically, and you keep a medical card current for as long as you drive.
The knowledge tests
At the licensing office you take the written tests your plan lists, starting with General Knowledge. Each is scored by itself, so a strong result on one never lifts another: get each one solid before you sit it.
Your commercial learner’s permit
Pass them and you get a CLP, which lets you drive a commercial vehicle with a licensed CDL driver beside you. You must hold it at least 14 days before the skills test.
Entry-level training
A first Class A or B license, a move from B to A, or a first Passenger, School Bus or Hazmat endorsement needs training from a provider on FMCSA’s Training Provider Registry. The provider records it in that registry, and the state checks it there before you test.
The skills test
Three parts in a vehicle of the class you want: the pre-trip inspection, basic control exercises and a drive on public roads. The vehicle matters: test in an automatic or without air brakes and your license carries that limit.
Every federal CDL rule, with its source Your state’s steps and forms
Air brakes and the vehicle inspection
Two parts of CDL study that come down to an order and a handful of numbers. Both show up on the knowledge tests, and the same checks are done again on the skills test.
The seven-step inspection
- Vehicle overview
- Check the engine compartment
- Start the engine and inspect inside the cab
- Turn off the engine and check the lights
- Walk around the vehicle
- Check the signal lights
- Start the engine and check the brakes
On the skills test you do the inspection in front of the examiner: point to each part as you check it and say what you are looking for.
Air brake numbers to know
| Low air pressure warning | Must come on before tank pressure falls below 55 psi |
|---|---|
| Air compressor | Starts pumping at about 100 psi and stops at about 125 psi |
| Leakage, brakes released | Under 2 psi a minute (single vehicle), 3 psi (combination) |
| Leakage, foot brake held | Under 3 psi a minute (single vehicle), 4 psi (combination) |
| Spring brakes | Come on by themselves at the maker’s setting, between 20 and 45 psi |
| Pressure build-up | 85 to 100 psi within 45 seconds, on a dual air system |
From the CDL manual most states use (sections 2.1.5, 5.1.11 and 5.3.3). A few states word these checks differently, so read your own state’s manual, and a vehicle maker’s figures come first for that vehicle.
Federal rules every CDL driver follows
Your state issues the license, but federal rules fix what matters once you’re driving: who may drive where, which vehicle needs which class, how much alcohol is too much and how long you may drive. Training before your first license and the Drug & Alcohol Clearinghouse are federal too, the same in every state.
- 21years old
To drive across state lines
The federal minimum for interstate driving. Most states license drivers from 18 for trips that stay inside the state.
- 26,001lb
Where Class A and B begin
A single vehicle rated that heavy needs Class B. A combination that heavy, towing a trailer rated over 10,000 lb, needs Class A.
- 0.04BAC
The alcohol limit at the wheel of a CMV
Half the limit most car drivers know. Reaching it in a commercial vehicle is a major offense that costs a CDL for a year or more.
- 11hours
Of driving in a shift, hauling freight
Only after 10 hours off duty, and only inside a 14-hour window from the start of the shift. Bus drivers have their own limits.
A study guide for each CDL test
- General Knowledge
- Air Brakes
- Combination Vehicles
- Doubles and Triples
- Passenger
- School Bus
- Tank Vehicles
- Hazardous Materials
Guides for professional drivers
The road rules behind the General Knowledge test, told through everyday traffic.
- Seeing it from the cabThe blind spots, wide turns and long stops that car drivers forget about.
- Weather on a long haulFog, glare, hydroplaning and icy bridges, with what to do in each.
- Fit to driveFatigue, alcohol and phones at the wheel, before the tighter federal limits for commercial drivers.
- The full guide libraryRight-of-way, signals and markings, a study plan and more.
How many questions are on the CDL General Knowledge test?
50 questions in 49 of 50 states, and you need 40 right. Illinois asks 30, with 24 to pass. Every CDL knowledge test needs 80% in every state, under a federal rule, and each one is graded on its own. Every state has 1,900+ CDL questions here to practice from.
| Test | Questions | To pass | Can miss |
|---|---|---|---|
| General Knowledge | 50 | 40 | 10 |
| Air Brakes | 25 | 20 | 5 |
| Combination Vehicles | 20 | 16 | 4 |
| Hazardous Materials | 30 | 24 | 6 |
| Tank Vehicles | 20 | 16 | 4 |
| Doubles and Triples | 20 | 16 | 4 |
| Passenger | 20 | 16 | 4 |
| School Bus | 20 | 16 | 4 |
Why 80% in every stateThe federal testing standard behind every CDL pass mark, with the other rules all states share.
Our practice tests keep each test’s size and pass mark in your state. Take one again until a pass is routine, not lucky: with only a handful of misses allowed, a single bad section sinks a test.
Where our CDL questions come from
A practice test is only as good as the page it rests on. Here is the record for every CDL question on this site.
- Source
- Your state’s commercial driver’s manual, the book its CDL tests are written from. Every rule is put in our own words, and each answer explains why it’s right and names the manual’s section and printed page.
- Never from
- The federal rules on this page. They help you plan and keep your license, but a test question rests on the manual alone.
- Cost
- Nothing. General Knowledge, Air Brakes, Combination Vehicles and every endorsement test, with no sign-up.
- Checked
- Every week, each state’s CDL manual is compared with the copy on the state’s site. A new edition sends each changed rule back for review before its questions return.
CDL manuals checked · November 4, 2026
48of 50
state CDL manuals confirmed as the newest edition in that check.
Editorial policyWhat’s newCDL test questions, answered
What is a restriction on a CDL?
A letter on the license that takes something away, most often because of the vehicle you tested in. Skip or fail Air Brakes and you may not drive a vehicle with them; take the skills test in an automatic and a manual gearbox is off limits. Retest in the right vehicle and it comes off.
Which endorsements can I have on a learner’s permit?
Only Passenger, School Bus and Tank Vehicles, each with limits while you learn: no passengers aboard other than examiners, trainers or other learners, and a tank only when it is empty. Hazmat and Doubles and Triples wait until you hold the CDL itself.
How long is a commercial learner’s permit good for?
Up to one year. If it runs out before you pass the skills test, you take the knowledge tests again to get a new one.
What is the Drug & Alcohol Clearinghouse?
A federal database of drug and alcohol violations by commercial drivers. Your state looks you up in it before it issues, renews or upgrades a CLP or CDL; our federal CDL rules page has the details.
Are these the questions on the real CDL test?
No. States keep their test questions private, and this is a free study aid, not the official test. Ours are written from your state’s CDL manual, and each explanation names the section to read.
Your plan, your state
Each state’s CDL page lists its knowledge tests with their manual pages, builds your plan by class and keeps your scores as you pass them.
Which state issues your CDL?
For your state
Saved on this device, so the CDL pages open there next time.