CDL air brakes practice test
The air brakes test is the one that keeps the air brake restriction off your licence. It covers the parts of the system, how to inspect it, the pressure numbers you must know by heart, and how to brake safely on long grades. These sample questions are drawn from Section 5 of the FMCSA CDL manual.
- 25questions on the real test
- 80%pass mark, 20 correct answers
- 60questions in the PassRig bank
- S5of the FMCSA CDL manual
Free air brakes practice quiz
Ten questions from the PassRig bank. Tap an answer to lock it in and read why it is right, with the manual section it comes from. The score keeps count; 80% is the pass mark. The full bank, timed mock exams and mistake review live in the app.
0 of 10 right0 of 10 answered
An air brake system is really three braking systems combined. They are the:
Answer: A. Service, parking and emergency brakesThe service brake system applies and releases the brakes when you use the brake pedal in normal driving. The parking brake system applies and releases the parking brakes with the parking brake control. The emergency brake system uses parts of both to stop the vehicle if the brake system fails.
Source: FMCSA CDL Manual, Section 5 (Air Brakes: Introduction)
Brake lag in an air brake system is the time it takes for air to reach the brakes. At 55 mph on dry pavement this adds about how much to your stopping distance?
Answer: B. About 32 feetWith air brakes it takes a little time, half a second or more, for the air to flow through the lines to the brakes. At 55 mph on dry pavement that brake lag adds about 32 feet on top of perception, reaction and braking distance.
Source: FMCSA CDL Manual, Section 5.4.4 (Stopping Distance)
Brake fade is caused by:
Answer: B. Excessive heat from using the brakes too muchBraking creates heat. Using the service brakes too much, instead of relying on the engine braking effect, overheats them. The excess heat causes chemical changes in the lining that reduce friction and makes the drums expand, so the shoes have to move farther and press with less force. Brakes out of adjustment make fade worse.
Source: FMCSA CDL Manual, Section 5.4.5 (Brake Fading or Failure)
If the air compressor is belt driven, you should check the belt during your inspection for:
Answer: A. Proper tightness and conditionA loose or worn belt can slip or break, and then the compressor stops supplying air. The compressor may also be gear driven, and it needs oil either from the engine or its own supply.
Source: FMCSA CDL Manual, Section 5.1.1 (Air Compressor)
When you release the brake pedal, what happens to the air that was applying the brakes?
Answer: D. It is let out of the brake chambers, so tank pressure drops and the compressor must replace itEach brake application uses up stored air. Pressing and releasing the pedal repeatedly can use air faster than the compressor replaces it, which is why fanning the brakes is dangerous.
Source: FMCSA CDL Manual, Section 5.1.7 (The Brake Pedal)
A 'wig wag' low-pressure warning device:
Answer: D. Drops a mechanical arm into your view when air pressure in the system falls too lowThe low air warning is usually a red light, and a buzzer may also come on. A wig wag is another type: it drops a mechanical arm into your view when system pressure gets low. An automatic wig wag rises out of view when pressure is restored. A manual reset type must be put back out of view by hand, and will not stay there until pressure is above 55 psi.
Source: FMCSA CDL Manual, Section 5.1.11 (Low Air Pressure Warning)
The modulating control valve found on some vehicles lets you:
Answer: D. Apply the spring brakes gradually with a hand lever, for example if the service brakes failThe further you move the lever, the harder the spring brakes apply. It is spring-loaded so you can feel the braking, and it can be locked in place to hold the vehicle parked.
Source: FMCSA CDL Manual, Section 5.1.15 (Parking Brake Controls: Modulating Control Valves)
In a dual air brake system, the primary system usually operates:
Answer: C. The brakes on the rear axle or axlesMost newer heavy vehicles have two separate air systems sharing one compressor. The primary system runs the rear brakes and the secondary the front, so a leak in one leaves the other to stop you.
Source: FMCSA CDL Manual, Section 5.2 (Dual Air Brake Systems)
In a dual air system, pressure should build from 85 to 100 psi within:
Answer: A. 45 secondsWith the engine running at normal operating idle, typically 600 to 900 rpm, watch the air gauge. In a dual air system pressure should build from about 85 to 100 psi within 45 seconds. Slow build-up means the vehicle does not meet the standard, and you may run low on air while driving.
Source: FMCSA CDL Manual, Section 5.3.3 (Check Rate of Air Pressure Build-up)
When stab braking, you release the brakes as soon as the wheels lock. After release, the wheels may take how long to start rolling again?
Answer: C. Up to one secondStab braking means applying full brakes, releasing when the wheels lock, and reapplying as soon as the wheels roll. If you reapply before they are rolling, the vehicle will not straighten out.
Source: FMCSA CDL Manual, Section 5.4.3 (Emergency Stops: Stab Braking)
What the air brakes test covers
- Parts of an air brake system: compressor, governor, tanks, valves
- Dual air brake systems and spring brakes
- Pressure numbers: cut-in, cut-out, low air warning, spring brake pop-out
- Inspecting air brakes: the applied leakage test and the governor test
- Brake lag and total stopping distance with air brakes
- Braking on downgrades and brake fade
- Emergency and parking brake use
Who needs it
Anyone who wants to drive a vehicle with air brakes. Skip it and your CDL carries an L or Z restriction.
The test has 25 multiple-choice questions and the passing score is 80%, so you can miss 5. Your state DMV or DOT administers it; the federal rules are on the FMCSA CDL page. Study Section 5 of the CDL manual alongside these questions.
More CDL practice tests
Guides for this test
Air Brakes test questions
How many questions are on the CDL air brakes test?
25 questions. You need 20 right, the 80% pass mark.
What happens if I skip the air brakes test?
Your CDL gets an air brake restriction, so you cannot drive a vehicle with full air brakes. Most trucking jobs need it, so almost every driver takes the test.
Is the air brakes test an endorsement?
No. It removes a restriction instead of adding an endorsement, but it is a separate written knowledge test with its own 80% pass mark.
Which numbers do I have to memorise for the air brakes test?
The ones the test loves: low air warning before 55 psi (60 in older manuals), spring brakes pop out between 20 and 45 psi, governor cut-out around 125 psi, cut-in around 100 psi, and the leak-down limits in the applied leakage test.