Needed to drive any truck or bus with air brakes, or your CDL gets an air brake restriction. You need 80% correct to pass (49 CFR 383.135).
The CDL air brake knowledge test covers how compressed-air brakes are built, inspected and used. Anyone who will drive a vehicle with air brakes needs it. Fail it and your license carries an air brake restriction. A passing score is 80 percent correct under 49 CFR 383.135.
Last checked against the handbook and 49 CFR on .
Federal rules name seven topic areas: part names, dirty or wet air, severed air lines, low-pressure readings, pre-trip checks, en route and post-trip checks, and operating habits such as ABS, emergency stops and parking. 49 CFR 383.111
| Item | Value |
|---|---|
| Governor cut-out (compressor stops pumping) | About 125 psi |
| Governor cut-in (compressor starts pumping) | About 100 psi |
| Safety relief valve opens | About 150 psi |
| Minimum pressure before driving a dual system | 100 psi in both primary and secondary |
| Low air warning must activate before pressure falls below | 55 psi |
| Spring brakes apply automatically | Roughly 20 to 45 psi |
| Tractor protection valve and parking valve pop out | Roughly 20 to 45 psi |
| Pressure buildup rate, dual system, engine at idle | 85 to 100 psi in 45 seconds or less |
| Large bus warning devices commonly signal at | 80 to 85 psi |
| Vehicle | Static test (brakes released) | Applied test (foot brake held) |
|---|---|---|
| Single vehicle | 2 psi | 3 psi |
| Combination of 2 vehicles | 3 psi | 4 psi |
| Combination of 3 or more vehicles | 5 psi | 6 psi |
You must answer at least 80 percent of the questions correctly. Federal rule 49 CFR 383.135 sets that mark for every CDL knowledge test, and a lower score on this test gets you an air brake restriction instead of a pass. 49 CFR 383.135
It bars you from driving any commercial vehicle with air brakes. It is added if you fail the air brake knowledge test or take your skills test in a truck that has no air brakes. 49 CFR 383.95
The governor lets the compressor pump when pressure falls to about 100 psi (cut-in) and stops it at about 125 psi (cut-out). The handbook gives a typical cut-out range of 120 to 140 psi, depending on the manufacturer. CDL Handbook 5.1CDL Handbook 5.3
The warning must sound while pressure is still above 55 psi in each circuit. Many large buses signal earlier, commonly at 80 to 85 psi. If it activates while you drive, stop safely right away. CDL Handbook 5.2
Static test, one minute: 2 psi for a single vehicle, 3 psi for two vehicles, 5 psi for three or more. Applied test with the foot brake held: 3, 4 and 6 psi for the same three cases. CDL Handbook 5.3
New to the CDL? Read how to get a CDL, which endorsements you need and the pre-trip inspection checklist.
HighwayQuiz is not affiliated with any state DMV or the FMCSA. Your state’s current CDL manual always wins over this site.