CDL Air Brakes Practice Test — 40 Free Questions (Unofficial)
An unofficial practice set for the CDL air brakes knowledge test. 40 original multiple-choice questions on required systems, reservoirs and warnings, emergency braking, adjusters and ABS, each with a short explanation and the federal rule it is based on.
- Questions: 40 multiple choice, each with an explanation.
- Based on: Federal Motor Carrier Safety Regulations, 49 CFR Parts 393 and 396 (public-domain U.S. government text, read on eCFR).
- Affiliation: Unofficial. Not affiliated with any exam board, agency or state.
- Last reviewed: 2026-11-04.
Topics covered
- Required Systems and Parking Brakes
- All-Wheel Brakes and Controls
- Reservoirs, Gauges, and Warnings
- Breakaway and Emergency Braking
- Chambers, Linings, Hoses, and Adjusters
- ABS, Performance, and Inspection
All 40 questions with answers
Try each question first, then open the answer. To practice them as a timed quiz with a score and a mistake list, use the practice test.
Required Systems and Parking Brakes
1. What three brake-system categories must a commercial motor vehicle meet under the federal required-brake rule?
Show answer
Answer: B. Service, parking, and emergency brake system requirements
Section 393.40 requires each CMV to meet applicable service, parking, and emergency brake system requirements.
2. For buses, trucks, and truck-tractors with air brakes manufactured on or after March 1, 1975, what minimum standard must the service brake system meet?
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Answer: D. FMVSS No. 121 in effect on the date of manufacture
Air-braked buses, trucks, and truck-tractors made on or after March 1, 1975 must have a service brake system meeting FMVSS No. 121 as of manufacture.
3. May the emergency brake control, service brake control, and parking brake control all be combined into one control on a covered post-July 1, 1973 vehicle?
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Answer: B. No; all three controls may not be combined
Section 393.40(e)(2) allows the emergency control to be combined with either the service or the parking control, but all three may not be combined.
4. An air-braked truck tractor manufactured after March 1, 1975 must have a parking brake system capable of what holding performance on a public road free of ice and snow?
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Answer: A. Holding the vehicle or combination stationary under any condition of loading in which it is found
Section 393.41 requires the parking brake to hold the vehicle or combination stationary under any loading condition found on a public road free of ice and snow.
5. For vehicles not subject to FMVSS 105/121 parking-brake requirements at manufacture, how must the parking brake be held in the applied position?
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Answer: B. By energy other than fluid pressure, air pressure, or electric energy
Section 393.41(c)(3) requires the parking brake to be held by energy other than fluid pressure, air pressure, or electric energy.
6. An agricultural commodity trailer, heavy hauler, or pulpwood trailer that is excepted from the FMVSS 121 parking-brake equipment rule must carry what instead?
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Answer: C. Sufficient chocking blocks to prevent movement when parked
Section 393.41(b) requires those excepted trailer types to carry sufficient chocking blocks to prevent movement when parked.
7. If the required brake systems are interconnected, what must still be true after failure of any part of the operating mechanism of one or more systems (except the service brake pedal or valve)?
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Answer: D. The motor vehicle will have operative brakes
Section 393.40(f) requires interconnected systems to be designed so that after such a failure the motor vehicle still has operative brakes.
All-Wheel Brakes and Controls
8. As a general rule, on which wheels must a commercial motor vehicle have brakes?
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Answer: B. All wheels
Section 393.42(a) states every CMV shall be equipped with brakes acting on all wheels, subject to listed exceptions.
9. A three-or-more-axle truck manufactured before July 25, 1980 lacks front-wheel brakes. What performance section must it still meet?
Show answer
Answer: C. Section 393.52 brake performance requirements
The front-brake exception in 393.42(b)(1) still requires those older multi-axle trucks to meet section 393.52.
10. Except for listed exceptions, what capability must all brakes with which a motor vehicle is equipped have?
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Answer: D. They must be capable of operating at all times
Section 393.48(a) requires that, except as provided, all equipped brakes must at all times be capable of operating.
11. When a lift axle is raised, must its brakes still be capable of operating?
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Answer: D. No; brakes on lift axles need not operate while the lift axle is raised, but must be capable of application when lowered and tires contact the roadway
Section 393.48(c)(6) allows raised lift-axle brakes to be inoperative while raised, but they must be capable of application when lowered onto the roadway.
12. An automatic device that reduces front-wheel braking force on an air-braked CMV must not reduce that force by more than what share of the force available when the device is disconnected?
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Answer: B. 50 percent
Section 393.48(b)(2) caps automatic front-wheel braking-force reduction at 50 percent of the force available when disconnected.
13. Except for the driveaway-towaway and qualifying surge-brake exceptions, for a motor vehicle manufactured after June 30, 1953 with power brakes, what must one application valve do when activated?
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Answer: A. Cause all of the service brakes on the motor vehicle or combination to operate
Section 393.49 requires one application valve to cause all service brakes on the vehicle or combination to operate (with listed exceptions). Scope now expressly excludes the exceptions in 393.49(b) and (c); answer A is unchanged.
14. Does the single application-valve rule prohibit an additional valve used to operate trailer brakes?
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Answer: B. No; an additional valve to operate trailer brakes is not prohibited
Section 393.49 expressly must not be construed to prohibit an additional valve to operate trailer brakes (or as required for buses in section 393.44).
Reservoirs, Gauges, and Warnings
15. For older air-braked vehicles not subject to FMVSS No. 121 at manufacture, a full service brake application with the engine stopped must not deplete pressure below what share of the gauge reading immediately before the application?
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Answer: B. 70 percent
Section 393.50(b) requires reserve capacity so a full application with the engine stopped does not drop below 70 percent of the pre-application gauge reading.
16. How must each service reservoir system be protected against loss of air pressure caused by failure between the service reservoir and the air-pressure source?
Show answer
Answer: D. By check valves or equivalent devices whose proper functioning can be checked without disconnecting any air line or fitting
Section 393.50(c) requires check valves or equivalent devices that can be checked without disconnecting lines or fittings.
17. Each air-brake reservoir must have what kind of condensate drain valve?
Show answer
Answer: A. A drain valve that can be manually operated (automatic valves are allowed if they can be operated manually or a manual drain means is retained)
Section 393.50(d) requires a manually operable condensate drain valve; automatic valves may be used if manually operable or a manual drain means is retained.
18. Under 49 CFR 393.51(c), excluding the older, smaller property-carrying vehicles exempted by paragraph (f), a CMV with air service brakes must be equipped with which of the following driver warnings and indicators?
Show answer
Answer: D. A pressure gauge and a warning signal
Section 393.51(c) requires both a pressure gauge and a warning signal for air-braked CMVs (and units towing air-braked vehicles). The general requirement in 393.51(c) is subject to 393.51(f): certain property-carrying vehicles with fewer than three axles, manufactured before July 1, 1973, and GVWR below 10,001 lb. The question now excludes this exception; answer D is unchanged.
19. For power units to which FMVSS No. 121 was not applicable at manufacture, the continuous low-air warning must activate whenever service-reservoir pressure is at or below 55 psi, or what alternative threshold?
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Answer: A. One-half of the compressor governor cutout pressure, whichever is less
Section 393.51(c)(2) sets the continuous warning at 55 psi and below, or one-half of compressor governor cutout pressure, whichever is less.
20. Where must the air-pressure gauge be visible?
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Answer: B. To a person seated in the normal driving position
Section 393.51(c)(1) requires a pressure gauge visible to a person seated in the normal driving position.
21. The low-air warning signal for covered older power units must be audible or visible to a person in the normal driving position and must provide what kind of warning?
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Answer: A. A continuous warning while pressure remains at or below the threshold
Section 393.51(c)(2) requires a continuous warning whenever pressure is at or below the stated threshold.
Breakaway and Emergency Braking
22. For air-braked towing units, within what pressure band must the tractor protection valve or similar device operate automatically?
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Answer: D. Between 20 psi and 45 psi
Section 393.43(a) requires the tractor protection valve or similar device to operate automatically between 20 psi and 45 psi (138-310 kPa).
23. An air-braked truck tractor towing an air-braked trailer must have how many means of activating the emergency features of the trailer brakes?
Show answer
Answer: A. Two means: one automatic and one manual from the driving seat
Section 393.43(b) requires two means: automatic on supply reduction to a fixed pressure in the 20-45 psi band, and a manual control operable from the driving seat.
24. May the manual emergency means be arranged so that it can prevent operation of the automatic means?
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Answer: B. No; the manual means may not be arranged to prevent automatic operation
Section 393.43(b) states that in no instance may the manual means be arranged to permit its use to prevent operation of the automatic means.
25. Upon breakaway from the towing vehicle, how quickly must required trailer brakes apply, and how long must they remain applied?
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Answer: D. Automatically and immediately; remain applied for at least 15 minutes
Section 393.43(d) requires automatic immediate application on breakaway, remaining applied for at least 15 minutes.
26. Air brake systems on towed vehicles must use no-bleed-back relay emergency valves or equivalent devices for what purpose?
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Answer: C. To safeguard the supply reservoir against backflow of air to the towing vehicle when towing-vehicle air pressure is reduced
Section 393.43(e) requires design that safeguards the towed-vehicle supply reservoir against backflow to the towing vehicle on pressure reduction.
27. On an air-braked bus, if a brake line to a front wheel is broken, what must the braking system still allow?
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Answer: B. Application of the brakes on the rear wheels despite that breakage
Section 393.44 requires that if any front-wheel brake line is broken, the driver can still apply the rear-wheel brakes.
28. The automatic emergency-activation pressure for trailer emergency features must not be set lower than 20 psi nor higher than what upper limit?
Show answer
Answer: B. 45 psi
Section 393.43(b) requires the fixed automatic pressure to be not lower than 20 psi nor higher than 45 psi.
Chambers, Linings, Hoses, and Adjusters
29. Brake tubing and hose installation must meet which of these conditions?
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Answer: A. Be long and flexible enough for normal motions, secured against chafing or kinking, and kept from contacting the exhaust or other high-temperature sources
Section 393.45(b) requires adequate length/flexibility, securing against mechanical damage, and protection from exhaust and high temperatures.
30. On each end of an axle, how must service brake chambers and spring brake chambers compare in size?
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Answer: B. They must be the same size
Section 393.47(b) requires service and spring brake chambers on each end of an axle to be the same size.
31. What must be true of the effective length of the slack adjuster on each end of an axle?
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Answer: C. It must be the same on each end
Section 393.47(c) requires the effective slack-adjuster length on each end of an axle to be the same.
32. For non-steering axle air drum brakes (measured at the shoe center), what is the minimum lining/pad thickness before the vehicle must not be operated (unless a wear indicator is reached first)?
Show answer
Answer: B. 1/4 inch (6.4 mm)
Section 393.47(d)(2) sets a 1/4 inch (6.4 mm) minimum for air-braked non-steering drum linings/pads at the shoe center, or to the wear indicator if marked.
33. For steering-axle air drum brakes with a continuous strip of lining, what is the minimum thickness at the shoe center?
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Answer: D. 3/16 inch (4.8 mm)
Section 393.47(d)(1) sets 3/16 inch (4.8 mm) at the shoe center for a continuous-strip steering-axle air drum lining.
34. For a standard-stroke Type 30 clamp-type brake chamber, what is the brake readjustment limit for pushrod stroke?
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Answer: B. 2 inches (50.8 mm)
The table in section 393.47(e) lists a 2-inch (50.8 mm) standard-stroke readjustment limit for Type 30 clamp chambers.
35. Each commercial motor vehicle manufactured on or after October 20, 1994 and equipped with an air brake system must meet which automatic adjustment requirement?
Show answer
Answer: D. Automatic brake adjustment system requirements of FMVSS No. 121 applicable at manufacture
Section 393.53(b) requires air-braked CMVs manufactured on or after October 20, 1994 to meet FMVSS No. 121 automatic brake adjustment requirements applicable at manufacture.
ABS, Performance, and Inspection
36. Beginning on which manufacture date must truck tractors (except driveaway-towaway operations) be equipped with an antilock brake system meeting FMVSS No. 121?
Show answer
Answer: C. March 1, 1997
Section 393.55(c)(1) requires ABS on truck tractors manufactured on or after March 1, 1997 (except driveaway-towaway).
37. Each trailer manufactured on or after March 1, 1998 and subject to the air-brake ABS rule must be equipped with what exterior indicator?
Show answer
Answer: A. An ABS malfunction indicator lamp meeting FMVSS No. 121
Section 393.55(e) requires an exterior ABS malfunction indicator lamp on covered trailers manufactured on or after March 1, 1998.
38. Upon service-brake application, a motor vehicle must be capable of stopping from 20 mph within the distance in the performance table. How must that stopping-distance test surface be conditioned?
Show answer
Answer: D. On a hard surface that is substantially level, dry, smooth, and free of loose material
Section 393.52(c) requires tests on a hard, substantially level, dry, smooth surface free of loose material, staying in a 12-foot lane.
39. Before driving a motor vehicle, what must the driver be satisfied about regarding vehicle condition?
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Answer: C. That the motor vehicle is in safe operating condition
Section 396.13(a) requires the driver to be satisfied that the motor vehicle is in safe operating condition before driving.
40. Under the parts-and-accessories maintenance rule, parts specified in part 393 (including brakes) must be in what condition?
Show answer
Answer: B. Safe and proper operating condition at all times
Section 396.3(a)(1) requires parts and accessories, including those in part 393, to be in safe and proper operating condition at all times.
Frequently asked questions
Is this an official exam?
No. These are original, unofficial practice questions written by PocketLearn. They are not released exam items. Your state CDL manual and the state knowledge test are the final word. Some state tests ask about inspection steps that are not in the federal equipment rules.
How many questions are there?
40 multiple-choice questions in 6 topics. Each question has an explanation.
Is it free?
Yes. You can open the practice set without an account; signing in lets you save progress and review mistakes.
Can I use it on my phone?
It opens in a mobile browser. A laptop or tablet is easier for longer sets.