Required brake systems, definitions and stopping distances 27
A manually operated device that reduces front-wheel braking effort may be used only on vehicles built before what date?49 CFR 393.48(b)(1)
A semitrailer weighs 3,000 pounds or less. It needs no brakes if its axle weight is no more than what share of the towing vehicle's axle weights?49 CFR 393.42(b)(3)
A truck has a lift axle. What does section 393.48 say about the brakes on it?49 CFR 393.48(c)(6)
Air-braked buses, trucks and truck tractors built on or after what date must have service brakes that meet FMVSS No. 121?49 CFR 393.40(b)(2)
An air-braked truck was built before FMVSS No. 121 applied to it. Which rules govern its service brakes?49 CFR 393.40(b)(2)
An older truck has a manual front-wheel brake limiting valve. When may the driver use it?49 CFR 393.48(b)(1)
By how much may an automatic device reduce front-wheel braking force?49 CFR 393.48(b)(2)
Does the one-valve rule in section 393.49 prohibit an additional valve that works only the trailer brakes?49 CFR 393.49(a)
For air-braked trailers, which manufacture date brings in FMVSS No. 121 under section 393.40?49 CFR 393.40(b)(2)
From what point is stopping distance measured in section 393.52?49 CFR 393.52(a)(3)
Show all 168 questions (158 more)
Required brake systems, definitions and stopping distances, continued
From what speed are the stopping distances in the section 393.52 brake performance table measured?49 CFR 393.52(a)
On a vehicle with power brakes built after June 30, 1953, what must one application valve do?49 CFR 393.49(a)
On an air-braked bus, a brake line to a front wheel breaks. What must the driver still be able to do?49 CFR 393.44
On an air-braked vehicle, an automatic front brake limiting device must not be operable above what brake application pressure?49 CFR 393.48(b)(2)
On which wheels must a commercial motor vehicle have brakes, as a general rule?49 CFR 393.42(a)
Using only the emergency brake system, within what distance must a single-unit truck over 10,000 pounds GVWR stop from 20 mph?49 CFR 393.52(d)
Using only the emergency brake system, within what distance must a tractor-semitrailer stop from 20 mph?49 CFR 393.52(d)
What does an air-over-hydraulic brake subsystem do, as section 393.5 defines it?49 CFR 393.5
What is the general rule in section 393.48 about the brakes a vehicle is equipped with?49 CFR 393.48(a)
What kind of failure is an emergency brake system designed to cover, under the definition in section 393.5?49 CFR 393.5
What must the brakes on every commercial motor vehicle be adequate to do?49 CFR 393.40(a)
Which system does section 393.5 call the primary brake system used for slowing and stopping a vehicle?49 CFR 393.5
Which system falls outside the definition of an air brake system in section 393.5?49 CFR 393.5
Which trucks and truck tractors are not required to have brakes on the front wheels?49 CFR 393.42(b)(1)
Within what distance must the service brakes stop a full-size bus from 20 mph?49 CFR 393.52(d)
Within what distance must the service brakes stop a single-unit truck over 10,000 pounds GVWR from 20 mph?49 CFR 393.52(d)
Within what distance must the service brakes stop a tractor-semitrailer combination from 20 mph?49 CFR 393.52(d)
Parking and emergency brakes 14
A pulpwood trailer is excepted from the FMVSS No. 121 parking brake rule. What must it carry?49 CFR 393.41(b)
A truck is designed to tow an air-braked trailer. Which parking brakes must its parking brake control work?49 CFR 571.121, S5.6.4
A vehicle's service, parking and emergency brake systems are interconnected. What must be true if part of one system's operating mechanism fails?49 CFR 393.40(f)(1)
All the air in a truck's brake system has bled down to zero. What must keep the parking brakes applied?49 CFR 571.121, S5.6.3.2
At a periodic inspection the parking brake control is actuated and no brakes on the vehicle or combination apply. What is the result?49 CFR Part 396, Appendix A, 1.b
From where must the driver be able to work the emergency brake control required by section 393.40(e)?49 CFR 393.40(e)(2)
How does FMVSS No. 121 relate the parking brake control to the service brake control on trucks and buses?49 CFR 571.121, S5.6.4
How must the parking brake control be identified, under FMVSS No. 121?49 CFR 571.121, S5.6.4
On a vehicle not covered by FMVSS No. 105 or 121, how must the parking brake be capable of being applied at all times?49 CFR 393.41(c)(2)
Through which control must the FMVSS No. 121 emergency brake system be applied, released and modulated?49 CFR 571.121, S5.7.2
Under section 393.41(c)(3), which kinds of energy may NOT be what holds a parking brake in the applied position?49 CFR 393.41(c)(3)
What must the parking brake on an air-braked truck be capable of, under section 393.41(b)?49 CFR 393.41(b)
When must a parking brake system not be capable of being released, under section 393.41(c)(3)?49 CFR 393.41(c)(3)
Which combination of brake controls does section 393.40(e)(2) rule out?49 CFR 393.40(e)(2)
Tractor protection and trailer breakaway 13
A trailer that is required to have brakes breaks away from the towing vehicle. What must its brakes do?49 CFR 393.43(d)
After a breakaway, how long must the trailer's brakes remain applied?49 CFR 393.43(d)
An air-braked tractor tows air-braked trailers. How many means of activating the trailer's emergency brake features must it have?49 CFR 393.43(b)
At a periodic inspection, the tractor protection valve on a power unit is missing. What is the result?49 CFR Part 396, Appendix A, 1.g
The automatic means of applying the trailer's emergency brakes works at a fixed towing vehicle pressure. Within what limits must that pressure be set?49 CFR 393.43(b)
How may the manual trailer emergency control relate to the automatic one?49 CFR 393.43(b)
On an air-braked towing unit, the tractor protection valve must operate automatically when the towing vehicle's air pressure is in what range?49 CFR 393.43(a)
On which trailers may fewer than all of the required brakes apply on breakaway?49 CFR 393.43(d)
What do no-bleed-back relay emergency valves protect on a towed vehicle?49 CFR 393.43(e)
What does section 393.43(b) require of the manual device that activates the trailer's emergency brakes?49 CFR 393.43(b)
What is the towing vehicle protection system in FMVSS No. 121 required to protect?49 CFR 571.121, S5.1.3
What is the towing vehicle protection system in section 393.43(a) meant to secure if the trailer breaks away?49 CFR 393.43(a)
Which operations are excepted from the trailer emergency and breakaway braking rules in section 393.43(b), (c) and (d)?49 CFR 393.43(f)
Compressor, governor, reservoirs and drains 17
A truck was built after March 1, 1975. Under section 570.57, how long may its compressor take to restore the pressure used by one full brake application?49 CFR 570.57(a)(2)
An air-braked vehicle predates FMVSS No. 121. After one full service brake application with the engine stopped, how much of the gauge pressure must remain?49 CFR 393.50(b)
Besides service reservoirs, FMVSS No. 121 requires trucks and buses to have one of two things. Which pair?49 CFR 571.121, S5.1.2
Does Part 570 by itself impose inspection duties on a driver or a motor carrier?49 CFR 570.53
The federal inspection standard in section 570.57 sets a floor for governor cut-in pressure unless the manufacturer recommends another value. What is it?49 CFR 570.57(a)(4)
How must it be possible to check that a reservoir's protective check valve is working?49 CFR 393.50(c)
In the section 570.57 inspection procedure, are trailers coupled while air pressure buildup is timed?49 CFR 570.57(a)
May an air reservoir use an automatic condensate drain valve?49 CFR 393.50(d)
Under the inspection standard in section 570.57, one full brake application with the engine stopped may lower reservoir pressure by no more than what?49 CFR 570.57(a)(1)
Under the same inspection standard, what is the ceiling for governor cut-out pressure, absent a manufacturer's value?49 CFR 570.57(a)(4)
What counts as a full service brake application in section 393.50(b)?49 CFR 393.50(b)
What does the inspection standard in section 570.57 say about the compressor drive belt?49 CFR 570.57(a)(7)
What is the lowest governor cut-in pressure FMVSS No. 121 allows on a bus?49 CFR 571.121, S5.1.1.1
What is the lowest governor cut-in pressure FMVSS No. 121 allows on a truck?49 CFR 571.121, S5.1.1.1
What kind of drain valve must each air reservoir have?49 CFR 393.50(d)
What protects a service reservoir system if a leak opens between it and the source of air pressure?49 CFR 393.50(c)
Which air compressor condition is a listed defect at a periodic inspection?49 CFR Part 396, Appendix A, 1.h
Gauges, low air warnings and leak rates 15
A truck built in 1972 has a governor that cuts out at 100 psi. By what pressure must its low air warning be on?49 CFR 393.51(c)(2)
At a periodic inspection the low pressure warning device is found inoperative. What is the result?49 CFR Part 396, Appendix A, 1.f
Engine stopped, reservoirs fully charged, service brakes fully applied. How much may pressure drop in 1 minute on a combination vehicle under section 570.57?49 CFR 570.57(a)(6)
Engine stopped, reservoirs fully charged, service brakes fully applied. How much may pressure drop in 1 minute on a single vehicle under section 570.57?49 CFR 570.57(a)(6)
Engine stopped, service brakes released. Under section 570.57, how much may air pressure drop in 1 minute on a combination vehicle?49 CFR 570.57(a)(5)
Engine stopped, service brakes released. Under section 570.57, how much may air pressure drop in 1 minute on a single vehicle?49 CFR 570.57(a)(5)
How much extra pressure drop per minute does section 570.57 allow for each additional towed vehicle?49 CFR 570.57(a)(5)
May the air pressure gauge itself serve as the low air pressure warning signal under FMVSS No. 121?49 CFR 571.121, S5.1.5
On a power unit that predates FMVSS No. 121, at what reservoir pressure must the low air warning be giving a continuous signal?49 CFR 393.51(c)(2)
On a truck built to FMVSS No. 121, the low air warning must be on whenever the ignition is on and service reservoir pressure is below what?49 CFR 571.121, S5.1.5
Under the inspection standard in section 570.57, a vehicle that was not built to FMVSS No. 121 must activate its low pressure warning at a level not less than what?49 CFR 570.57(a)(3)
What must every bus, truck and truck tractor have to tell the driver of a failure in the service brake system?49 CFR 393.51(a)
Where must the air pressure gauge required by FMVSS No. 121 be, and what must it show?49 CFR 571.121, S5.1.4
Which low air warning signal meets S5.1.5 of FMVSS No. 121?49 CFR 571.121, S5.1.5
Which two devices must an air-braked commercial motor vehicle have under section 393.51(c), whatever its date of manufacture?49 CFR 393.51(c)
Chambers, slack adjusters, stroke and linings 27
A brake actuator is of a type not listed in the section 393.47 tables. What limits its pushrod stroke?49 CFR 393.47(e)(2)
A brake lining carries a wear indicator. How does section 393.47 treat it?49 CFR 393.47(d)
A brake with an automatic adjuster is out of adjustment at a periodic inspection. What does Appendix A to Part 396 say about adjusting it by hand?49 CFR Part 396, Appendix A, 1.m(4)
A steering axle has air drum brakes and shoes with a continuous strip of lining. What is the least lining thickness allowed at the shoe center?49 CFR 393.47(d)(1)
A steering axle has air drum brakes and shoes with two pads. What is the least lining thickness allowed at the shoe center?49 CFR 393.47(d)(1)
Air-braked commercial motor vehicles built on or after what date must have automatic brake adjusters?49 CFR 393.53(b)
An automatic brake adjuster has been replaced with a manual adjuster. How does a periodic inspection treat it?49 CFR Part 396, Appendix A, 1.m(2)
An inspector hears air leaking at a brake chamber. How does Appendix A to Part 396 treat it?49 CFR Part 396, Appendix A, 1.a(4)
At a periodic inspection, under what conditions is pushrod stroke measured?49 CFR Part 396, Appendix A, 1.a(5)
From where must a brake adjustment indicator be discernible, under FMVSS No. 121?49 CFR 571.121, S5.1.8(b)
On a non-steering axle with air disc brakes, below what pad thickness may the vehicle not be operated?49 CFR 393.47(d)(2)
On a non-steering axle with air drum brakes, below what lining thickness may the vehicle not be operated?49 CFR 393.47(d)(2)
On a wedge brake, how far may the scribe mark on the lining move?49 CFR 393.47(f)
What does section 393.47 require of the service brake chambers on the two ends of one axle?49 CFR 393.47(b)
What is the least pad thickness allowed on a steering axle with air disc brakes?49 CFR 393.47(d)(1)
What is the long stroke readjustment limit for a Type 16 clamp-type chamber?49 CFR 393.47(e)
What is the pushrod stroke readjustment limit for a Type 12 clamp-type chamber with standard stroke?49 CFR 393.47(e)
What is the pushrod stroke readjustment limit for a Type 30 clamp-type chamber with long stroke?49 CFR 393.47(e)
What is the pushrod stroke readjustment limit for a Type 30 clamp-type chamber with standard stroke?49 CFR 393.47(e)
What must match between the slack adjusters on each end of an axle?49 CFR 393.47(c)
What must the construction, installation and maintenance of brake components prevent, under section 393.47(a)?49 CFR 393.47(a)
Which air brakes must display under-adjustment with a brake adjustment indicator?49 CFR 393.53(c)
Which brake drum condition is a listed defect at a periodic inspection?49 CFR Part 396, Appendix A, 1.c(1)
Which lining condition, apart from thickness, fails a periodic inspection?49 CFR Part 396, Appendix A, 1.a(6)
Which mismatch across a power unit's steering axle is a listed defect at a periodic inspection?49 CFR Part 396, Appendix A, 1.a(8)
Which three clamp-type chamber sizes share a standard stroke readjustment limit of 1 3/4 inches?49 CFR 393.47(e)
Who sets the minimum thickness for brake drums and rotors under section 393.47?49 CFR 393.47(g)
Antilock brake systems 15
The ABS malfunction indicator stays lit the whole time power is applied to the ABS controller. How does a periodic inspection treat that?49 CFR Part 396, Appendix A, 1.l(3)
An ABS malfunction exists when the ignition is switched off. What happens to the warning when the ignition is switched on again?49 CFR 571.121, S5.1.6.2(a)
An air-braked straight truck or bus must have ABS if it was built on or after what date?49 CFR 393.55(c)(2)
How many ABS malfunction indicators does a power unit built after March 1, 2001 have, and how many must work?49 CFR Part 396, Appendix A, 1.l, footnote 1
The ignition is turned to on and the ABS has no fault. What must the ABS malfunction lamp do?49 CFR 571.121, S5.1.6.2(a)
Power is first applied to the ABS controller and the ABS malfunction indicator does not light. How does a periodic inspection treat that?49 CFR Part 396, Appendix A, 1.l(2)
Tractors built on or after what date must have an in-cab lamp that shows an ABS malfunction on a towed vehicle?49 CFR 393.55(d)(2)
Truck tractors built on or after what date must have an antilock brake system?49 CFR 393.55(c)(1)
What color is the external ABS malfunction indicator lamp on a trailer?49 CFR 571.121, S5.2.3.3(b)(2)
What does an antilock brake system automatically control, as FMVSS No. 121 defines it?49 CFR 571.121, S4
What identifies the function of a trailer's external ABS malfunction lamp?49 CFR 571.121, S5.2.3.3(b)(3)
When does a trailer's external ABS lamp run its check of lamp function?49 CFR 571.121, S5.2.3.3(d)
Where must the ABS malfunction indicator lamp of a truck tractor be mounted?49 CFR 571.121, S5.1.6.2(a)
Where on a semitrailer is the external ABS malfunction lamp mounted?49 CFR 571.121, S5.2.3.3(c)(1)
Which truck tractors built on or after March 1, 1997 are excepted from the ABS rule in section 393.55?49 CFR 393.55(c)(1)
Hoses, inspection defects and brake inspectors 19
A brake hose bulges when air pressure is applied. How does a periodic inspection treat it?49 CFR Part 396, Appendix A, 1.d(2)
A driver picks up intermodal equipment. Which brake parts must the driver inspect under section 392.7(b)?49 CFR 392.7(b)
Coiled nonmetallic brake tubing connects a tractor and trailer. How long must the straight segment at each end be?49 CFR 393.45(c)(1)
For which people does a carrier not have to keep evidence of qualifications to inspect air brake systems?49 CFR 396.25(e)
How deep must damage to an air brake hose go before it fails a periodic inspection?49 CFR Part 396, Appendix A, 1.d(1)
How long and flexible must brake tubing and hose be?49 CFR 393.45(b)(1)
How long must a carrier keep evidence of a brake inspector's qualifications?49 CFR 396.25(e)
How much brake-related training or experience qualifies a person as a brake inspector, absent an approved program or certificate?49 CFR 396.25(d)(3)(ii)
On application of the service brakes, the shoes on one axle fail to move. How does a periodic inspection treat the vehicle?49 CFR Part 396, Appendix A, 1.a(1)
Two air hoses are joined by sliding their ends over a piece of tubing and clamping them. How does a periodic inspection treat the splice?49 CFR Part 396, Appendix A, 1.d(4)
What does passing the CDL air brake tests count toward under section 396.25?49 CFR 396.25(d)(3)(i)
What must brake tubing and hose be kept from contacting?49 CFR 393.45(b)(3)
What must every air brake connection be free of, under section 393.45(d)?49 CFR 393.45(d)
What size of air leak at a brake hose or brake tubing fails a periodic inspection?49 CFR Part 396, Appendix A, 1.d(3) and 1.e(1)
Which brake items are on the section 392.7 list of parts a driver must be satisfied are in good working order before driving?49 CFR 392.7(a)
Which brake items must a driver vehicle inspection report cover?49 CFR 396.11(a)(1)
Which brake tubing condition is a listed defect at a periodic inspection?49 CFR Part 396, Appendix A, 1.e(2)
Which federal standard must brake tubing, hoses, hose assemblies and end fittings meet?49 CFR 393.45(a)
Who is a brake inspector under section 396.25?49 CFR 396.25(b)
Air brake restrictions and the test topics 21
A driver wants the air brake restriction removed. Must the complete skills test be retaken?49 CFR 383.135(b)(7)
An applicant passes the air brake knowledge questions but takes the skills test in a truck without air brakes. What follows?49 CFR 383.95(a)(1)
An applicant takes the skills test in a truck with air over hydraulic brakes. What restriction follows?49 CFR 383.95(b)(1)
Besides a modified skills test, what must an applicant do to remove the air brake or full air brake restriction?49 CFR 383.135(b)(7)
Can the air brake restriction appear on a commercial learner's permit?49 CFR 383.95(a)(1)
For the skills test and the air brake restriction, what counts as air brakes?49 CFR 383.95(a)(2)
The full air brake restriction applies only to applicants who take skills tests on or after what date?49 CFR 383.153(g)
How many air brake knowledge areas does section 383.111(b) list?49 CFR 383.111(b)
The knowledge list names tests for the pressure at which two things should activate. Which two?49 CFR 383.111(b)(6)(iii)
On the skills test in an air-braked vehicle, what must the applicant do with the air brake controls and monitoring devices?49 CFR 383.113(a)(2)(i)
What counts as air over hydraulic brakes for the full air brake restriction?49 CFR 383.95(b)(2)
What must a skills test applicant determine about air pressure build up?49 CFR 383.113(a)(2)(v)
What must a skills test applicant determine about the air connections between vehicles?49 CFR 383.113(a)(2)(ii)
Which air line topic is one of the seven air brake knowledge areas?49 CFR 383.111(b)(3)
Which air loss tests must a driver know for en route and post-trip inspections?49 CFR 383.111(b)(6)(ii)
Which code on a CDL stands for no full air brake equipped CMV?49 CFR 383.153(a)(10)(ii)
Which contaminants does the air brake knowledge list name as dangers to the air supply?49 CFR 383.111(b)(2)
Which four items fall under general operating practices in the air brake knowledge list?49 CFR 383.111(b)(7)
Which three kinds of device does the pre-trip inspection knowledge area for air brakes name?49 CFR 383.111(b)(5)
Why must a skills test applicant inspect the low pressure warning devices?49 CFR 383.113(a)(2)(iii)
With the engine running, what must a skills test applicant make sure of?49 CFR 383.113(a)(2)(iv)
How this list was compiled: Written from the Electronic Code of Federal Regulations, Title 49, sections 383.95, 383.111, 383.113, 383.135, 383.153, 392.7, 393.5, 393.40 to 393.55 (393.42 as amended February 19, 2026 and 393.45 as amended July 21, 2026), 396.11 (as amended February 19, 2026), 396.25, Appendix A to Part 396, 570.53, 570.57 and 571.121 (Federal Motor Vehicle Safety Standard No. 121), at https://www.ecfr.gov/current/title-49, text in force on 2026-10-01, checked 2026-10-03. Each question names its section. Licence: Own work. Questions, answer choices and explanations were written for this site from the regulation text; nothing is copied from a state CDL manual, the AAMVA test item pool or another practice test.
How it works
This unofficial CDL air brakes practice test asks 168 questions, most from 49 CFR Part 393, FMVSS No. 121 and Part 396 inspection defects, checked 3 October 2026. The in-cab check sequence and braking technique are left to state DMV manuals.
Read the full guide
Where the air brake numbers are written
49 CFR 383.111(b) lists seven knowledge areas for the CDL air brakes test. The figures sit in three places: the 20 to 45 psi tractor protection range in 393.43, the 60 psi warning in FMVSS No. 121 and the leak rates in 570.57, a standard for state inspection programs.
An air brake CDL practice test in rounds of ten
Each round asks ten questions. When a round ends, More options opens nine topics, from parking and emergency brakes to antilock brake systems. Exam mode draws 25 questions, shows no marks until the end and wants 20 right, the 80 percent in 49 CFR 383.135.
Standard stroke limits in 393.47(e): 1 3/4 inches for clamp-type Types 16, 20 and 24, 2 inches for Type 30.
Leak rates in 570.57 per minute, engine stopped: single vehicle 2 psi released, 3 applied; combination 3 and 4.
Wrong answers wait under Missed in More options until you answer them right.
Questions and answers
How many questions are on the air brakes CDL test?
Federal rules give no number. 49 CFR 383.133 says each test must contain a set number of questions, so check your state agency's CDL page. The air brakes test CDL applicants sit is passed at 80 percent. Exam mode's 25 questions are this site's choice.
Is there a CDL air brake test cheat sheet?
Not here. Print, with a tick on With answer key, puts a CDL air brakes practice test and answers on paper. A memorized list helps little: 49 CFR 383.133 makes a state use a different version when an applicant retakes a failed test.
What if I skip the air brakes CDL test?
Your license carries restriction L. Under 49 CFR 383.95 a state adds it when an applicant fails the air brake questions or takes the skills test in a vehicle without air brakes. Testing with air over hydraulic brakes brings restriction Z.
Does a general CDL practice test, air brakes included, cover the same ground?
They are separate knowledge areas. 49 CFR 383.111(a) lists 20 general areas and 383.111(b) lists seven for air brakes. This air brake practice test asks only the air brake side.