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Key Facts: SC CDL Air Brakes Exam

$2.00

Fee per knowledge test

SCDMV Commercial Fees

80%

Minimum passing score

49 CFR 383.135

Written/oral

Official knowledge-exam delivery described in the manual

South Carolina Commercial Driver License Manual

< 60 psi

Low air pressure warning threshold

49 CFR 393.51

SCDMV administers an Air Brakes knowledge examination and requires at least 80% to pass; the agency does not publish a question count or time limit. Passing the knowledge test and demonstrating air-brake inspection and driving skills in a full-air-brake vehicle avoids or removes the L restriction on a South Carolina CDL.

Sample SC CDL Air Brakes Practice Questions

Try these sample questions to review concepts for the SC CDL Air Brakes exam. Each question includes a detailed explanation. Start the interactive quiz above for the full 208+ question experience with AI tutoring.

1In the air-brake system described by Section 5 of the South Carolina CDL Manual, what does the engine-driven compressor do?
A.To pump compressed air into the air storage tanks (reservoirs)
B.To cool the engine radiator fluid during heavy braking
C.To inject alcohol directly into the foundation brake drums
D.To circulate hydraulic fluid to the power steering gearbox
Explanation: Under Section 5 of the South Carolina CDL Manual, the air compressor is driven by the vehicle engine to compress atmospheric air and pump it into the primary and secondary air storage tanks.
2What controls when the air compressor pumps air into the air storage tanks?
A.The air compressor governor
B.The foot valve (brake pedal)
C.The safety relief valve on the supply tank
D.The tractor protection control valve
Explanation: The governor controls when the compressor pumps air. When reservoir pressure reaches the cut-out level (around 120-140 psi), the governor stops compressor pumping; when pressure drops to cut-in (around 100 psi), it starts pumping again.
3At what air pressure should the air compressor governor cut out and stop the compressor from pumping air?
A.Around 120 to 140 pounds per square inch (psi)
B.Around 60 to 75 psi
C.Around 175 to 200 psi
D.Around 85 to 100 psi
Explanation: Under FMCSA standards and the South Carolina CDL Manual, the governor cut-out pressure is typically set between 120 and 140 psi (frequently around 125 psi).
4At what air pressure should the air compressor governor cut in and cause the compressor to resume pumping?
A.Around 100 psi (typically between 90 and 105 psi)
B.Around 55 psi
C.Around 135 psi
D.Around 150 psi
Explanation: When reservoir pressure falls to approximately 100 psi (typically 90 to 105 psi), the governor cut-in activates the compressor to replenish the air storage tanks.
5Why does the South Carolina CDL Manual direct a driver to drain tanks equipped with manual drain valves at the end of each driving day?
A.To remove condensed water and compressor oil that can freeze and cause brake failure
B.To reduce the weight of the commercial vehicle for scale inspection
C.To reset the digital speedometer calibration
D.To flush road dust out of the brake shoe linings
Explanation: Compressing air creates heat and moisture. Water and compressor oil accumulate in the bottom of tanks; if not drained, oil damages rubber seals and water freezes in cold weather, causing total brake failure.
6How often should manual air tank drains be opened to purge water and oil?
A.At the end of each day's driving
B.Once a month during routine oil changes
C.Only when the low air pressure warning light comes on
D.Once a year during the annual commercial safety inspection
Explanation: Section 5 of the South Carolina CDL Manual states that drivers should drain manual air tanks at the end of each operating day to prevent moisture buildup and line freezing.
7During cold-weather operation, what role does an alcohol evaporator play in some air-brake systems?
A.To put alcohol into the air lines to prevent ice formation in cold weather
B.To cool the compressor crankcase oil during summer operations
C.To clean road grime off the external slack adjusters
D.To lubricate the foundation S-cam rollers
Explanation: Some air systems have an alcohol evaporator that introduces alcohol vapor into the system to lower the freezing point of moisture and prevent ice formation in valves and brake lines during winter.
8Does having an alcohol evaporator eliminate the need to drain the air storage tanks daily?
A.No; tanks must still be drained daily to remove accumulated water and oil
B.Yes; the alcohol completely consumes and evaporates all moisture and oil
C.Yes; but only if the ambient temperature is above 32 degrees Fahrenheit
D.Yes; alcohol evaporators convert moisture into pressurized air
Explanation: Having an alcohol evaporator does NOT eliminate the need to drain moisture. The alcohol merely prevents the water from freezing; the water and oil mixture must still be purged daily.
9At what pressure is the safety relief valve set to open on the supply (wet) reservoir?
A.150 psi
B.100 psi
C.120 psi
D.200 psi
Explanation: A mechanical safety relief valve is installed in the first (supply) tank to protect the system from excessive pressure; it is calibrated to open and vent air at 150 psi if the governor malfunctions.
10Section 5 of the South Carolina CDL Manual identifies which design as the most common foundation brake?
A.S-cam drum brakes
B.Wedge-actuated caliper disc brakes
C.Electromagnetic retarder drum brakes
D.Hydraulic power-boosted drum brakes
Explanation: Section 5 of the South Carolina CDL Manual notes that S-cam drum brakes are the most common type of foundation brake used on commercial heavy vehicle axles.

About the SC CDL Air Brakes Exam

The South Carolina CDL Air Brakes knowledge test is administered by the SCDMV for drivers operating commercial motor vehicles equipped with air brake systems. It covers Section 5 of the South Carolina Commercial Driver License Manual, including air compressors, governor cut-in/cut-out pressures, reservoir tanks, safety relief valves, foundation brakes (S-cam, wedge, disc), dual air systems, low pressure warning devices, emergency spring brakes, static and applied leakage tests, and proper downgrade braking techniques (snub braking). OpenExamPrep provides independent English-language four-option practice for these topics, informed by the manual and federal safety regulations. This bank is not an official translation, format simulation, or substitute for hands-on vehicle training.

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

Assessment

An SCDMV knowledge examination, available in written or oral form, covering Section 5 (Air Brakes) of the South Carolina Commercial Driver License Manual. Air Brakes is a knowledge qualification and restriction-removal test rather than an endorsement. To avoid or remove an L restriction, an applicant must also demonstrate the required hands-on air-brake inspection and complete the applicable skills testing in a vehicle with full air brakes. Testing in a vehicle with an air-over-hydraulic system results in a Z restriction.

Time Limit

Not published

Passing Score

80%

Exam / Certification Fees

$2.00 per knowledge test attempt. Depending on the transaction, SCDMV also charges a $15.00 CDL application fee, $2.50 for a CLP, and $25.00 for an eight-year CDL; a repeat skills test after a failed attempt is $25.00.

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.

Core Section 5 area

Air Brake System Components

Air compressor, governor cut-in (around 100 psi) and cut-out (around 120-140 psi), air storage tanks, manual and automatic tank drains, alcohol evaporators, and safety relief valves set at 150 psi.

Core Section 5 area

Brake Controls and Foundation Brakes

Brake pedal (foot valve), S-cam drum brakes, disc brakes, wedge brakes, slack adjusters, brake pushrods, and brake lining/drum wear tolerances.

Core Section 5 area

Dual Air Systems and Warning Devices

Primary and secondary air systems, supply and application pressure gauges, low air pressure warnings activating before pressure drops below 60 psi, and emergency spring-brake operation.

Core Section 5 area

Air Brake Inspection and Leakage Tests

Four-step air brake check: static leakage test (< 2 psi/min single, < 3 psi/min combination), applied leakage test (< 3 psi/min single, < 4 psi/min combination), low-pressure warning activation, and emergency spring brake pop-out (20-45 psi).

Core Section 5 area

Using Air Brakes and Emergency Procedures

Normal stops, brake lag distance, controlled braking vs stab braking, steep downgrade speed control (snub braking), brake fade, and parking brake application.

Preparing for the SC CDL Air Brakes Exam

What You Need to Know

  • Passing score: 80%
  • Assessment: An SCDMV knowledge examination, available in written or oral form, covering Section 5 (Air Brakes) of the South Carolina Commercial Driver License Manual. Air Brakes is a knowledge qualification and restriction-removal test rather than an endorsement. To avoid or remove an L restriction, an applicant must also demonstrate the required hands-on air-brake inspection and complete the applicable skills testing in a vehicle with full air brakes. Testing in a vehicle with an air-over-hydraulic system results in a Z restriction.
  • Time limit: Not published
  • Exam / certification fees: $2.00 per knowledge test attempt. Depending on the transaction, SCDMV also charges a $15.00 CDL application fee, $2.50 for a CLP, and $25.00 for an eight-year CDL; a repeat skills test after a failed attempt is $25.00. Official sources

Using Our Practice Resources

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SC CDL Air Brakes: Suggested Study Strategy

1Carefully study Section 5 (Air Brakes) of the South Carolina Commercial Driver License Manual.
2Memorize the critical pressure numbers: governor cut-in (around 100 psi), governor cut-out (around 120-140 psi), safety relief valve (150 psi), low-air warning (before pressure falls below 60 psi), and spring-brake control pop-out range (20-45 psi).
3Learn the air brake inspection steps in exact sequence: static leakage, applied leakage, low pressure warning, spring brake pop-out, and compressor build-up rate (85 to 100 psi within 45 seconds at operating RPM).

Frequently Asked Questions

How many questions are on the South Carolina CDL Air Brakes test?

SCDMV and the current South Carolina CDL Manual do not publish a question count or time limit. The manual requires a score of at least 80% on each required knowledge examination.

Is Air Brakes an endorsement on a South Carolina CDL?

No. Air Brakes is not an endorsement. It is a knowledge test and skills inspection requirement. If an applicant does not pass the test or tests in a vehicle without full air brakes, an L restriction is placed on their CDL, restricting them from operating air-brake equipped commercial vehicles.

At what air pressure must the low air warning activate on a commercial vehicle?

The South Carolina CDL Manual says the low-air warning must activate before pressure drops below 60 psi in either tank of a dual-air system. The warning may be a light, buzzer, or another visible signal on some older buses.

What is the allowable air leakage rate during the applied brake leakage test?

With the engine off, parking brakes released, and service brakes fully depressed for one minute, maximum allowable leakage is 3 psi per minute for a single vehicle and 4 psi per minute for a combination vehicle.

What is the proper technique for braking on long, steep downgrades?

Drivers should use 'snub braking': select a safe low gear, apply brakes firmly to slow the vehicle to approximately 5 mph below the safe speed, release the brakes, and repeat as speed builds back up. This gives brakes time to cool and prevents brake fade.