CDL brakes practice questions
20 free practice questions from 49 CFR Part 393 Subpart C. Each answer is explained and linked to the rule it comes from.
CDL Brakes
20 questions across 8 topics. After each answer you see why it is right, with a link to the official source.
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Required brake systems
1. What does 393.40(e)(2) say about combining the emergency brake control with the other brake controls?
- A. It must be a separate control that is never combined with any other control
- B. It must be combined with both the service brake control and the parking brake control
- C. It may be combined with either the service brake control or the parking brake control, but all three controls may not be combined
- D. It may be combined only with a trailer hand valve
Show answer
Answer: C. It may be combined with either the service brake control or the parking brake control, but all three controls may not be combined
"The emergency brake control may be combined with either the service brake control or the parking brake control. However, all three controls may not be combined."
Source: eCFR: 49 CFR 393.40, Required brake systems (as of Nov 3, 2026)
2. What does 393.44 say the braking system of a bus with air brakes must be constructed to allow if any brake line to a front wheel is broken?
- A. The driver can apply the brakes on the rear wheels despite the breakage
- B. The bus must stop on its own within 10 seconds
- C. The front brakes must keep working through a backup hose
- D. The passengers must be able to apply an emergency brake from the rear door
Show answer
Answer: A. The driver can apply the brakes on the rear wheels despite the breakage
The braking system must be built so "in the event any brake line to any of the front wheels is broken, the driver can apply the brakes on the rear wheels despite such breakage." The section also says "Every bus shall meet this requirement or comply with the regulations in effect at the time of its manufacture."
Source: eCFR: 49 CFR 393.44, Front brake lines, protection (as of Nov 3, 2026)
3. Under 393.49(a), what must one application valve do on a power-braked vehicle manufactured after June 30, 1953 (outside the section's exceptions)?
- A. Apply only the trailer brakes
- B. Release the parking brakes on every axle
- C. Operate only the front-axle service brakes
- D. Cause all of the service brakes on the vehicle or combination to operate
Show answer
Answer: D. Cause all of the service brakes on the vehicle or combination to operate
The braking system must be arranged so that "one application valve must when activated cause all of the service brakes on the motor vehicle or combination motor vehicle to operate."
Source: eCFR: 49 CFR 393.49, Control valves for brakes (as of Nov 3, 2026)
Brakes on all wheels and exceptions
4. Which vehicles does 393.42(b)(1) excuse from having brakes on the front wheels?
- A. Any two-axle truck manufactured before July 25, 1980
- B. Trucks or truck tractors having three or more axles and manufactured before July 25, 1980
- C. Any bus with three or more axles, regardless of age
- D. Any truck tractor operated only intrastate
Show answer
Answer: B. Trucks or truck tractors having three or more axles and manufactured before July 25, 1980
"Trucks or truck tractors having three or more axles and manufactured before July 25, 1980, are not required to have brakes on the front wheels." They must still meet 393.52.
Source: eCFR: 49 CFR 393.42, Brakes required on all wheels (as of Nov 3, 2026)
5. For a semitrailer with a gross weight of 3,000 pounds or less to be excused from brakes under 393.42(b)(3), what limit does the rule set on its axle weight as a share of the sum of the towing vehicle's axle weights?
- A. 75 percent
- B. 40 percent
- C. 60 percent
- D. 50 percent
Show answer
Answer: B. 40 percent
The exception applies "if the axle weight of the towed vehicle does not exceed 40 percent of the sum of the axle weights of the towing vehicle."
Source: eCFR: 49 CFR 393.42, Brakes required on all wheels (as of Nov 3, 2026)
6. According to 393.48(c)(6), when must brakes on a lift axle be capable of being applied?
- A. Only when the vehicle is fully loaded
- B. Only when the parking brake is set
- C. Only at speeds above 20 mph
- D. Whenever the lift axle is lowered and the tires contact the roadway
Show answer
Answer: D. Whenever the lift axle is lowered and the tires contact the roadway
Lift-axle brakes need not work while the axle is raised, but they "must be capable of being applied whenever the lift axle is lowered and the tires contact the roadway."
Source: eCFR: 49 CFR 393.48, Brakes to be operative (as of Nov 3, 2026)
7. What is the most that 393.48(b)(2) lets an automatic device reduce front-wheel braking force, compared with the force available when the device is disconnected?
- A. 50 percent
- B. 75 percent
- C. 90 percent
- D. 100 percent
Show answer
Answer: A. 50 percent
"Automatic devices must not reduce the front-wheel braking force by more than 50 percent of the braking force available when the automatic device is disconnected."
Source: eCFR: 49 CFR 393.48, Brakes to be operative (as of Nov 3, 2026)
Parking brakes
8. Under 393.41(b), which trailers must carry sufficient chocking blocks to prevent movement when parked?
- A. Every air-braked trailer with three or more axles
- B. Every trailer manufactured before January 1, 1975
- C. An agricultural commodity trailer, heavy hauler or pulpwood trailer
- D. Every trailer parked on a grade
Show answer
Answer: C. An agricultural commodity trailer, heavy hauler or pulpwood trailer
"An agricultural commodity trailer, heavy hauler or pulpwood trailer shall carry sufficient chocking blocks to prevent movement when parked."
Source: eCFR: 49 CFR 393.41, Parking brake system (as of Nov 3, 2026)
9. Under 393.41(c)(3), the parking brake system must be held in the applied position by energy other than which kinds?
- A. Fluid pressure, air pressure, or electric energy
- B. Spring action or mechanical force
- C. The driver's muscular effort
- D. A ratchet or locking pawl
Show answer
Answer: A. Fluid pressure, air pressure, or electric energy
"The parking brake system shall be held in the applied position by energy other than fluid pressure, air pressure, or electric energy."
Source: eCFR: 49 CFR 393.41, Parking brake system (as of Nov 3, 2026)
Brake tubing, hoses and components
10. What does 393.45(b)(3) require about how brake tubing and hose are installed?
- A. They must be painted a color that shows wear
- B. They must be installed so they cannot contact the exhaust system or any other source of high temperatures
- C. They must be routed inside the frame rails only
- D. They must be replaced every 12 months
Show answer
Answer: B. They must be installed so they cannot contact the exhaust system or any other source of high temperatures
Brake tubing and hose must "Be installed in a manner that prevents it from contacting the vehicle's exhaust system or any other source of high temperatures."
Source: eCFR: 49 CFR 393.45, Brake tubing and hoses; hose assemblies and end fittings (as of Nov 3, 2026)
Breakaway and emergency braking
11. An air-braked truck tractor that tows air-braked trailers must have two means of activating the emergency features of the trailer brakes under 393.43(b). What does the rule say about how the manual means may be arranged?
- A. It must be mounted outside the cab, behind the driver's seat
- B. It may be arranged so the driver can use it to block the automatic means
- C. It must always be a device separate from the automatic means
- D. It may never be arranged so that its use prevents operation of the automatic means
Show answer
Answer: D. It may never be arranged so that its use prevents operation of the automatic means
The manual means must be "readily operable by a person seated in the driving seat", and "In no instance may the manual means be so arranged as to permit its use to prevent operation of the automatic means." The two means "may be, but are not required to be, separate."
Source: eCFR: 49 CFR 393.43, Breakaway and emergency braking (as of Nov 3, 2026)
12. What minimum time does 393.43(d) set for a trailer's brakes to remain in the applied position after breakaway?
- A. 1 minute
- B. 5 minutes
- C. 15 minutes
- D. 10 minutes
Show answer
Answer: C. 15 minutes
Trailer brakes must apply "automatically and immediately upon breakaway" and "must remain in the applied position for at least 15 minutes."
Source: eCFR: 49 CFR 393.43, Breakaway and emergency braking (as of Nov 3, 2026)
Automatic brake adjusters and ABS
13. On a CMV manufactured on or after October 20, 1994 with an air brake system that has an external automatic adjustment mechanism and an exposed pushrod, what must display the condition of service brake under-adjustment under 393.53(c)?
- A. A pressure gauge on the dashboard reading in psi
- B. A label showing the date of the last manual brake adjustment
- C. The ABS malfunction lamp in the cab
- D. A brake adjustment indicator conforming to FMVSS No. 121
Show answer
Answer: D. A brake adjustment indicator conforming to FMVSS No. 121
Under 393.53(c), "the condition of service brake under-adjustment must be displayed by a brake adjustment indicator conforming to the requirements of Federal Motor Vehicle Safety Standard No. 121".
Source: eCFR: 49 CFR 393.53, Automatic brake adjusters and brake adjustment indicators (as of Nov 3, 2026)
14. Under 393.55(e), which vehicles must be equipped with an exterior ABS malfunction indicator lamp?
- A. Each truck tractor manufactured on or after March 1, 1997
- B. Only trailers used in driveaway-towaway operations
- C. Each trailer (including a trailer converter dolly) manufactured on or after March 1, 1998 and subject to 393.55(c)(2)
- D. Every trailer, regardless of its date of manufacture
Show answer
Answer: C. Each trailer (including a trailer converter dolly) manufactured on or after March 1, 1998 and subject to 393.55(c)(2)
"Each trailer (including a trailer converter dolly) manufactured on or after March 1, 1998, and subject to the requirements of paragraph (c)(2) of this section, shall be equipped with an ABS malfunction indicator lamp which meets the requirements of FMVSS No. 121".
Source: eCFR: 49 CFR 393.55, Antilock brake systems (as of Nov 3, 2026)
Reservoirs, gauges and warning signals
15. For air-braked vehicles not subject to FMVSS No. 121 when made, and all vacuum-braked vehicles, what level does 393.50(b) set that a full service brake application with the engine stopped must not deplete the air pressure or vacuum below?
- A. 70 percent of the gauge reading just before the application
- B. 40 percent of the gauge reading just before the application
- C. 50 percent of the gauge reading just before the application
- D. 25 percent of the gauge reading just before the application
Show answer
Answer: A. 70 percent of the gauge reading just before the application
The reserve or reservoir must allow a full service application "without depleting the air pressure or vacuum below 70 percent of that indicated by the air or vacuum gauge immediately before the brake application is made."
Source: eCFR: 49 CFR 393.50, Reservoirs required (as of Nov 3, 2026)
16. What kind of drain valve does 393.50(d) require on each reservoir of an air-braked vehicle?
- A. An automatic drain valve that cannot be operated by hand
- B. A condensate drain valve that can be manually operated
- C. A pressure-relief valve instead of a drain
- D. No drain valve, so the reservoir stays sealed
Show answer
Answer: B. A condensate drain valve that can be manually operated
"Each reservoir must have a condensate drain valve that can be manually operated." Automatic drain valves are allowed only if they can be operated manually or a manual means of draining is retained.
Source: eCFR: 49 CFR 393.50, Reservoirs required (as of Nov 3, 2026)
17. Which vehicles does 393.51(f) exclude from the warning signal and gauge rules for air brakes, vacuum brakes and air- or vacuum-assisted hydraulic brakes in paragraphs (c), (d) and (e)?
- A. Property-carrying CMVs with less than three axles that were manufactured before July 1, 1973 and have a manufacturer's GVWR less than 10,001 pounds
- B. All buses manufactured before March 1, 1975
- C. Any vehicle used only in intrastate commerce
- D. Property-carrying CMVs with three or more axles manufactured before July 1, 1973
Show answer
Answer: A. Property-carrying CMVs with less than three axles that were manufactured before July 1, 1973 and have a manufacturer's GVWR less than 10,001 pounds
"The rules in paragraphs (c), (d) and (e) of this section do not apply to property carrying commercial motor vehicles which have less than three axles and" both "were manufactured before July 1, 1973" and "have a manufacturer's gross vehicle weight rating less than 4,536 kg (10,001 pounds)."
Source: eCFR: 49 CFR 393.51, Warning signals, air pressure and vacuum gauges (as of Nov 3, 2026)
18. On a vehicle with vacuum-activated service brakes, what vacuum level in the supply reservoir does 393.51(d)(2) set for the continuous warning signal?
- A. Less than 2 inches of mercury
- B. Less than 4 inches of mercury
- C. Less than 8 inches of mercury
- D. Less than 1 inch of mercury
Show answer
Answer: C. Less than 8 inches of mercury
The warning signal must give a continuous warning "whenever the vacuum in the vehicle's supply reservoir is less than 203 mm (8 inches) of mercury."
Source: eCFR: 49 CFR 393.51, Warning signals, air pressure and vacuum gauges (as of Nov 3, 2026)
Brake performance
19. For "all other property-carrying vehicles and combinations of property-carrying vehicles," what service brake application and braking distance from 20 mph does the 393.52(d) table set?
- A. 60 feet
- B. 75 feet
- C. 120 feet
- D. 40 feet
Show answer
Answer: D. 40 feet
The table row "(3) All other property-carrying vehicles and combinations of property-carrying vehicles" lists 43.5 percent, 14 feet per second per second, 40 feet (service brakes) and 90 feet (emergency brakes).
Source: eCFR: 49 CFR 393.52, Brake performance (as of Nov 3, 2026)
20. Under 393.52(c)(2), what lane must the vehicle be in when a stopping-distance test begins?
- A. The right shoulder of any public road
- B. The center of a 12-foot-wide lane
- C. The center of an 8-foot-wide lane
- D. Any lane at least 20 feet wide
Show answer
Answer: B. The center of a 12-foot-wide lane
"The vehicle must be in the center of a 12-foot-wide lane when the test begins and must not deviate from that lane during the test."
Source: eCFR: 49 CFR 393.52, Brake performance (as of Nov 3, 2026)