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Key Facts: OK CDL Air Brakes Exam

25

Questions on official Oklahoma Air Brakes test

Service Oklahoma CDL Manual

80%

Minimum passing score (20 correct)

49 CFR 383.135

55 psi

Pressure the low-air warning must activate before

Service Oklahoma CDL Manual Section 5

20-45 psi

Tractor protection / spring brake pop-out range

Service Oklahoma CDL Manual Section 5

Service Oklahoma requires an 80% passing score (20 of 25 correct) on the CDL Air Brakes knowledge test. Passing the knowledge test and demonstrating the air brake pre-trip inspection and operational checks during the skills test is required to operate air-brake equipped commercial vehicles without an L restriction.

Sample OK CDL Air Brakes Practice Questions

Try these sample questions to review concepts for the OK CDL Air Brakes exam. Each question includes a detailed explanation. Start the interactive quiz above for the full 220+ question experience with AI tutoring.

1What is the primary function of the air compressor in a commercial motor vehicle air brake system?
A.To pump compressed air directly into the storage reservoirs
B.To cool the engine radiator fluid using compressed air jets
C.To lubricate the transmission gears with air pressure
D.To filter particulate soot from the exhaust tailpipe
Explanation: The air compressor is driven by the vehicle engine (via gears or belts) and pumps filtered atmospheric air under pressure into the air storage reservoirs for brake operation.
2How is the air compressor in most heavy commercial motor vehicles lubricated and cooled?
A.It has its own separate oil reservoir and is cooled by dry ice
B.It is lubricated by engine oil and cooled by engine coolant or air cooling fins
C.It is lubricated by brake fluid and cooled by fuel recirculation
D.It requires no lubrication because the internal pistons are plastic
Explanation: Most commercial vehicle air compressors receive pressurized lubricating oil directly from the engine oil system and are plumbed into the engine cooling system for liquid cooling.
3What is the purpose of the air compressor governor in an air brake system?
A.It controls when the air compressor pumps air into the storage tanks and when it stops pumping
B.It limits the maximum road speed of the commercial vehicle to 65 mph
C.It measures the temperature of the rear brake drums
D.It adjusts the fuel injection volume based on turbo boost
Explanation: The governor monitors air tank pressure and controls the compressor unloader valves, cutting out (stopping pumping) at maximum pressure and cutting in (resuming pumping) when pressure drops.
4During the Oklahoma CDL air-brake inspection, which governor cut-out reading is within the accepted range?
A.Between 60 and 80 psi
B.Between 175 and 200 psi
C.Between 90 and 100 psi
D.Between 120 and 140 pounds per square inch (psi)
Explanation: For the inspection check, the Oklahoma CDL Manual says the governor must cut out between 120 and 140 psi. Its system-description section describes normal cut-out as around 125 psi.
5At approximately what pressure does the air compressor governor cut in and command the compressor to resume pumping?
A.Around 100 psi (or approximately 20 to 25 psi below the cut-out pressure)
B.At exactly 40 psi
C.At 140 psi
D.Only when the reservoir pressure drops to zero
Explanation: The governor cuts in when tank pressure drops roughly 20 to 25 psi below cut-out, typically around 100 psi (and not less than 85 psi), resuming air compression.
6Why must compressed air storage tanks be drained regularly if equipped with manual drain valves?
A.To remove accumulated water and compressor oil, which corrode valves and can freeze in winter causing total brake failure
B.To prevent air tanks from becoming too heavy for vehicle suspension brackets
C.To allow fresh nitrogen gas to enter the brake system
D.To equalize tire pressure across dual rear wheels
Explanation: Compressing air condenses water vapor and atomizes compressor oil. If not drained daily, moisture rusts valves, degrades rubber diaphragms, and freezes in cold weather, causing brake lockup or failure.
7How often should manual air tank drain valves be operated by the commercial driver?
A.At the end of each day of driving (or operating shift)
B.Once every six months during oil changes
C.Only when the low air warning buzzer sounds
D.Once a year during the annual state inspection
Explanation: Manual drain valves should be opened at the end of each day's driving to purge water and sludge accumulated during that operating shift.
8What is the function of the safety relief valve installed in an air brake storage tank?
A.To protect the air tanks and system from excessive pressure if the governor malfunctions, usually opening at 150 psi
B.To release air whenever the vehicle exceeds the speed limit
C.To supply air to the passenger door opening cylinders
D.To purge tire pressure if tires become overheated
Explanation: The safety valve is an emergency mechanical pressure relief device calibrated to exhaust air if tank pressure reaches 150 psi, preventing tank rupture if the governor fails to cut out.
9What device can be installed in an air brake system to help prevent moisture from freezing in cold winter weather?
A.An alcohol evaporator
B.A kerosene burner under the air tank
C.A freon refrigerant compressor
D.An open flame glow plug inside the primary reservoir
Explanation: An alcohol evaporator puts alcohol vapor into the air system to lower the freezing point of water and reduce ice formation in cold weather. Drivers must check the alcohol level daily.
10What happens when you depress the foot brake pedal (treadle valve) in an air brake vehicle?
A.Air pressure is pushed against mechanical brake chambers, forcing pushrods outward to apply brake shoes against drums
B.Hydraulic fluid is squirted onto the brake lining surfaces
C.Engine oil is diverted into the transmission retarder
D.The air compressor stops pumping and vents all tank air to the atmosphere
Explanation: Pressing the foot pedal meters air pressure from reservoirs into brake chambers. The air pushes against a flexible diaphragm, moving the pushrod and slack adjuster to apply the brakes.

About the OK CDL Air Brakes Exam

The Oklahoma CDL Air Brakes written knowledge test is administered by Service Oklahoma for commercial drivers who operate vehicles equipped with air brakes. It covers Section 5 of the Oklahoma Commercial Driver License Manual: air brake system components (compressor, governor, tanks, drain valves, safety relief valve, foundation brakes, slack adjusters), dual air brake system operation, step-by-step air brake safety checks, leakage rate thresholds, warning signals, and emergency braking techniques. OpenExamPrep provides independent English-language four-option practice questions based on the official manual and FMCSA regulations. They are not an official test simulation or a substitute for hands-on vehicle training.

Exam sponsor: Service Oklahoma. The requirements and fees below concern the certification or admission exam, separate from our free practice resources.

Assessment

A computerized multiple-choice knowledge examination administered by Service Oklahoma covering Section 5 (Air Brakes) of the Oklahoma Commercial Driver License Manual. Removing an L or Z air-brake restriction also requires the air-brake pre-trip inspection and the skills demonstration Service Oklahoma assigns for the applicant's restriction.

Time Limit

No time limit

Passing Score

80% (20 out of 25 questions correct)

Exam / Certification Fees

$25 for 1 exam, $30 for 2, $35 for 3, or $40 for 4 exams per session at CareerTech ServiceOK testing centers; license issuance and endorsement fees are separate.

Exam sponsor website

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

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.

Core Section 5 area

Air Brake System Components

Air compressor, governor cut-in/cut-out, storage tanks, drains, safety valve, brake chambers, slack adjusters, and S-cam/disc brakes.

Core Section 5 area

Dual Air Brake Systems

Primary and secondary circuits, pressure gauges, minimum operating pressure, build-up time, and low air pressure warning devices.

Core Section 5 area

Inspecting Air Brake Systems

Step-by-step air brake check, static leakage test, applied leakage test, warning buzzer test, valve pop-out test, and slack adjuster travel.

Core Section 5 area

Using Air Brakes and Emergency Braking

Normal stopping, brake lag distance, downgrade snub braking, brake fade, controlled vs stab braking, ABS, and parking brake use.

State/federal licensing area

Oklahoma CDL Licensing and Air Brake Restrictions

Service Oklahoma testing, L restriction removal, Z restriction, testing fees, and 80% passing standard.

Preparing for the OK CDL Air Brakes Exam

What You Need to Know

  • Passing score: 80% (20 out of 25 questions correct)
  • Assessment: A computerized multiple-choice knowledge examination administered by Service Oklahoma covering Section 5 (Air Brakes) of the Oklahoma Commercial Driver License Manual. Removing an L or Z air-brake restriction also requires the air-brake pre-trip inspection and the skills demonstration Service Oklahoma assigns for the applicant's restriction.
  • Time limit: No time limit
  • Exam / certification fees: $25 for 1 exam, $30 for 2, $35 for 3, or $40 for 4 exams per session at CareerTech ServiceOK testing centers; license issuance and endorsement fees are separate. Official sources

Using Our Practice Resources

  • Work through all 220 available questions
  • Review every answer and explanation
  • Track weak areas and revisit them
  • Use our AI tutor for tough concepts

OK CDL Air Brakes: Suggested Study Strategy

1Distinguish normal governor operation (about 125 psi cut-out and about 100 psi cut-in) from the 120-140 psi cut-out range used in the inspection procedure.
2Learn the key thresholds: the safety valve usually opens at 150 psi, the low-air warning must activate before 55 psi, and the parking brake valve pops out between 20 and 45 psi.
3Understand proper downgrade braking: select a low gear and use 'snub braking' (brake firmly until 5 mph below safe speed, release, and repeat) to prevent brake fade.

Frequently Asked Questions

How many questions are on the Oklahoma CDL Air Brakes knowledge test?

The Oklahoma CDL Air Brakes knowledge test consists of 25 multiple-choice questions. You must answer at least 20 questions correctly (80%) to pass.

What happens if I do not take or pass the Air Brakes test in Oklahoma?

If you do not pass the Air Brakes knowledge test, or if you take your skills test in a vehicle without air brakes, Service Oklahoma will place an L restriction on your CDL, which restricts you from operating any commercial vehicle equipped with full air brakes.

What are the acceptable air leakage rates during an air brake check?

For a single vehicle with the engine off and brakes applied, the maximum allowable leakage rate is 3 psi per minute (2 psi with brakes released). For a combination vehicle, the maximum is 4 psi per minute with brakes applied (3 psi released).

At what air pressure must the low air warning signal activate?

The Oklahoma CDL Manual requires the visible low-air warning to activate before tank pressure falls below 55 psi. Large buses commonly warn at 80 to 85 psi.