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Key Facts: DC CDL Air Brakes Exam

80%

Minimum passing score on each CDL knowledge test

49 CFR 383.135

60 psi

Low air pressure warning threshold

DC CDL Manual Section 5.1

20-45 psi

Range where spring brakes come fully on

DC CDL Manual Section 5.1

100-125 psi

Approximate governor cut-in and cut-out pressures

DC CDL Manual Section 5.1

$10

Fee per CDL knowledge test

DC DMV Driver License Fees

The DC Air Brakes knowledge test covers Section 5 of the DC CDL Manual and is given in English only; DC DMV publishes no question count or separate time limit, and federal rules require 80% to pass. Key numbers from the DC manual: governor cut-in about 100 psi and cut-out about 125 psi, safety valve about 150 psi, low air warning before 60 psi, spring brakes fully on at 20 to 45 psi, and leakage limits of 2 psi (static) and 3 psi (applied) per minute for single vehicles.

Sample DC CDL Air Brakes Practice Questions

Try these sample questions to review concepts for the DC CDL Air Brakes exam. Each question includes a detailed explanation. Start the interactive quiz above for the full 213+ question experience with AI tutoring.

1According to Section 5 of the DC CDL Manual, air brakes are really which three braking systems?
A.Foot brake, retarder, and spring brake
B.Service brake, engine brake, and trailer hand brake
C.Front brake, rear brake, and parking brake
D.Service brake, parking brake, and emergency brake
Explanation: Section 5 says air brakes are really three different braking systems: the service brake, the parking brake, and the emergency brake. The emergency brake system uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.
2Your service brakes fail. According to the DC manual, what does the emergency brake system rely on?
A.A separate set of brake drums on the drive axle
B.The engine retarder and the transmission
C.Parts of the service and parking brake systems
D.A hydraulic backup pump
Explanation: Section 5 says the emergency brake system uses parts of the service and parking brake systems to stop the vehicle in a brake system failure.
3The air compressor on your vehicle has its own oil supply. What does Section 5 tell you to do?
A.Drain the compressor oil at the end of each day
B.Add alcohol to the compressor oil in cold weather
C.Nothing; compressor oil is checked only by a mechanic
D.Check the compressor oil level before driving
Explanation: Section 5.1 says the air compressor is connected to the engine through gears or a v-belt, may be air cooled or cooled by the engine cooling system, and may have its own oil supply or use engine oil. If the compressor has its own oil supply, check the oil level before driving.
4After start-up, your air pressure gauge keeps climbing. At about what reading should the governor stop the compressor?
A.Around 125 psi
B.Around 60 psi
C.Around 150 psi
D.Around 100 psi
Explanation: Section 5.1 says that when air tank pressure rises to the cut-out level, around 125 psi, the governor stops the compressor from pumping air. When pressure falls to the cut-in level, around 100 psi, the compressor starts pumping again.
5You fan the brakes and tank pressure drops. At about what reading should the compressor start pumping again?
A.Around 60 psi
B.Around 125 psi
C.Around 85 psi
D.Around 100 psi
Explanation: Section 5.1 says that when the tank pressure falls to the cut-in pressure, around 100 psi, the governor allows the compressor to start pumping again.
6What does Section 5 say the air storage tanks hold enough air to do?
A.Allow several brake applications even if the compressor stops
B.Release the spring brakes after the engine stops so you can move
C.Run the air horn and air seats for at least an hour
D.Inflate all the tires on the road if one of them goes flat
Explanation: Section 5.1 says the number and size of air tanks varies, and that the tanks will hold enough air to allow the brakes to be used several times even if the compressor stops working.
7Why must air tanks be drained, according to Section 5?
A.Draining lowers tank pressure so the governor cuts in and refreshes the air
B.Water and oil collect in them, and the water can freeze and cause brake failure
C.Draining removes road dust that would otherwise clog the brake valves
D.Federal rules require a daily pressure reset to zero
Explanation: Section 5.1 says compressed air usually has some water and some compressor oil in it, which is bad for the air brake system. The water can freeze in cold weather and cause brake failure, and the water and oil collect in the bottom of the tank, so drain the tanks completely.
8How do you open a manual drain valve on an air tank, according to the DC manual?
A.By pressing the brake pedal 10 times with the engine off
B.By removing the safety valve from the first tank
C.By a quarter turn or by pulling a cable
D.By loosening the hose at the brake chamber
Explanation: Section 5.1 says manual drains are operated by turning a quarter turn or by pulling a cable, and that you must drain the tanks yourself at the end of each day of driving. Automatic drains expel water and oil on their own.
9Some automatic air tank drains have electric heating devices. What does Section 5 say they are for?
A.To dry the brake linings after rain
B.To warm the compressed air before it reaches the brake chambers
C.To keep the alcohol evaporator from freezing
D.To help prevent freezing of the automatic drain in cold weather
Explanation: Section 5.1 says automatic air tanks are available with electric heating devices, which help prevent freezing of the automatic drain in cold weather. These tanks may also be equipped for manual draining.
10Your vehicle has an alcohol evaporator. What does Section 5 say you should do in cold weather?
A.Check the alcohol container and fill it as needed every day
B.Skip daily tank draining, since the alcohol removes water
C.Add alcohol only when the low air warning comes on
D.Pour alcohol directly into the brake chambers
Explanation: Section 5.1 says the alcohol evaporator puts alcohol into the air system to reduce the risk of ice in valves and other parts. Check the alcohol container and fill it as necessary every day during cold weather; daily tank drainage is still needed unless the system has automatic drain valves.

About the DC CDL Air Brakes Exam

The DC CDL Air Brakes Knowledge Test is the DC DMV test for drivers of trucks and buses with air brakes, including air-over-hydraulic systems. It draws on Section 5 of the DC CDL Manual: air brake system parts, gauges and warnings, spring and parking brakes, ABS, dual air systems, the air brake inspection, and safe braking. OpenExamPrep offers this free, independent practice bank for those topics and the federal L and Z restrictions.

Exam sponsor: District of Columbia Department of Motor Vehicles (DC DMV). The requirements and fees below concern the certification or admission exam, separate from our free practice resources.

Assessment

Air Brakes is a knowledge test, not an endorsement. DC DMV lists it with General Knowledge for Class A and Class B, and the DC CDL Manual requires it whenever the vehicle has air brakes, including air-over-hydraulic brakes. Under 49 CFR 383.95, failing it or taking the skills test without air brakes brings restriction L (no air brake equipped CMV), and testing in an air-over-hydraulic vehicle brings Z (no full air brake equipped CMV). Tests are given in English at the Deanwood Road Test/CDL Center. This bank is independent English-language multiple-choice practice by OpenExamPrep, not an official DC DMV test copy or simulation.

Time Limit

not-published

Passing Score

80% (federal minimum, 49 CFR 383.135)

Exam / Certification Fees

$10 per knowledge test; $78 CLP; $117 CDL

Exam sponsor website

Fees, eligibility, and exam policies can change. Confirm them with the exam sponsor before applying or paying.

Official sources

Our practice resources: topics covered

We aim to reflect publicly available exam outlines and topic information in our study resources. Coverage, format, and difficulty may differ from the actual exam, and we cannot guarantee that every detail is accurate or current. Confirm exam requirements, fees, and policies with the official exam sponsor.

Manual Section 5.1

Air Brake System Parts

Compressor, governor, tanks and drains, alcohol evaporator, safety valve, brake pedal, and foundation brakes.

Manual Sections 5.1 and 5.2

Gauges, Warnings, and Dual Systems

Gauges, low air warnings, stop light switch, front brake limiting valves, and dual air systems.

Manual Section 5.1

Spring Brakes, Parking Brakes, and ABS

Spring brake pressures, parking controls, modulating and dual parking control valves, and ABS rules.

Manual Section 5.3

Inspecting Air Brakes

Slack adjusters, drums and linings, warning and spring brake tests, buildup rate, leakage, governor, and brake tests.

Manual Section 5.4

Using Air Brakes

Stops, emergency braking, brake lag, fade, downgrade technique, low air pressure, and parking brakes.

49 CFR 383; DC DMV pages

Restrictions and DC Rules

L and Z restrictions and removal, the 80% standard, DC class test lists, and retests.

Preparing for the DC CDL Air Brakes Exam

What You Need to Know

  • Passing score: 80% (federal minimum, 49 CFR 383.135)
  • Assessment: Air Brakes is a knowledge test, not an endorsement. DC DMV lists it with General Knowledge for Class A and Class B, and the DC CDL Manual requires it whenever the vehicle has air brakes, including air-over-hydraulic brakes. Under 49 CFR 383.95, failing it or taking the skills test without air brakes brings restriction L (no air brake equipped CMV), and testing in an air-over-hydraulic vehicle brings Z (no full air brake equipped CMV). Tests are given in English at the Deanwood Road Test/CDL Center. This bank is independent English-language multiple-choice practice by OpenExamPrep, not an official DC DMV test copy or simulation.
  • Time limit: not-published
  • Exam / certification fees: $10 per knowledge test; $78 CLP; $117 CDL Official sources

Using Our Practice Resources

  • Work through all 213 available questions
  • Review every answer and explanation
  • Track weak areas and revisit them
  • Use our AI tutor for tough concepts

DC CDL Air Brakes: Suggested Study Strategy

1Study Section 5 of the DC CDL Manual (2005 CDL Testing System, July 2014 version); its low air warning figure is 60 psi.
2Memorize the pressures: cut-in about 100 psi, cut-out about 125 psi, safety valve about 150 psi, low air warning before 60 psi, and spring brakes fully on at 20 to 45 psi.
3Know the air brake check limits: about one inch of slack adjuster movement, 85 to 100 psi within 45 seconds, and leakage under 2 psi (single) or 3 psi (combination) per minute with brakes released.
4Practice the downgrade method: brake until you are about 5 mph below your safe speed (about three seconds), release, and repeat.
5Know the restrictions: L means no air brake equipped CMV and Z means no full air brake equipped CMV.

Frequently Asked Questions

How many questions are on the DC Air Brakes test, and how long do I have?

DC DMV does not publish the number of questions or a separate time limit for the Air Brakes test. It lists 90 minutes for General Knowledge and 60 minutes for endorsement tests, and federal rule 49 CFR 383.135 requires at least 80% correct on every CDL knowledge test.

Is Air Brakes an endorsement?

No. It is a knowledge test that removes a restriction. Under 49 CFR 383.95, failing it or taking the skills test in a vehicle without air brakes puts restriction L (no air brake equipped CMV) on your license, and testing in an air-over-hydraulic vehicle puts restriction Z (no full air brake equipped CMV).

Who needs the DC Air Brakes test?

The DC CDL Manual says you must take it if your vehicle has air brakes, including air-over-hydraulic brakes. DC DMV lists Airbrakes with General Commercial as required knowledge tests for Class A and Class B.

How do I remove an L or Z restriction later?

Under 49 CFR 383.135(b)(7), you must pass the air brake knowledge test, successfully perform the air brake pre-trip inspection, and pass a skills test, which the state may give in modified form, showing you can safely operate the vehicle's air brakes.

What happens if I fail?

You must wait three calendar days and may retest on the fourth day. If you fail six times within 12 months, you cannot take a seventh test until 12 months after the first failure. Each attempt costs $10, and all DC CDL knowledge tests are in English only.

Is this an official DC DMV practice test?

No. This is independent practice by OpenExamPrep, written in English multiple-choice format from the DC CDL Manual, DC DMV pages, and 49 CFR Part 383. It is a study aid, not an official DC DMV test copy or simulation, and it does not replace the air brake pre-trip or skills test.