South Carolina Air Brakes CDL Practice Test

This is a free 20-question practice test for the Air Brakes portion of the South Carolina Commercial Driver's License knowledge exam. Questions are pulled from a pool of 71 drawn from the AAMVA CDL Manual, which is the source document the South Carolina Department of Motor Vehicles uses to write its actual exam.

How it works: Click an answer. The correct choice highlights in green, and you'll see a short explanation. Aim for 85% or better before you sit for the real test in South Carolina.

Question 1 of 20

The "brake-system warning device" must come on no later than:

10 psi
30 psi
60 psi
100 psi

Correct. A federal-rule low-air-pressure warning device must activate at or before 60 psi.

Question 2 of 20

Slack adjusters need to be checked because:

They wear and require adjustment to maintain braking force
They never need attention
They are decorative
They affect only the parking brake

Correct. Slack adjusters control how far the pushrod must travel to apply the brake. Out-of-adjustment slack adjusters are the leading mechanical cause of out-of-service violations.

Question 3 of 20

Stopping distance for a vehicle with air brakes is the sum of:

Perception distance + reaction distance + brake-lag distance + braking distance
Brake-lag only
Reaction time only
Braking distance only

Correct. Air-brake stopping distance is perception + reaction + brake lag + braking — the brake lag is the unique addition.

Question 4 of 20

When should you NOT use the parking brakes?

When the vehicle is parked
When the brakes are very hot, such as after a long downgrade — they could damage if applied
At any time
When the engine is off

Correct. Hot brakes can be damaged by the spring brakes contracting against hot drums. Let the brakes cool before parking.

Question 5 of 20

When releasing brakes after a hard stop, you should:

Continue holding the brakes
Release smoothly to allow air to refill the tanks
Pump the brakes
Apply the parking brake immediately

Correct. Release the brakes smoothly. Air tanks refill while you drive; allow the compressor to recharge the system before the next braking event.

Question 6 of 20

The parts of an air-brake system include:

Air compressor, governor, air storage tanks, foot valve, and brake chambers
Master cylinder, brake fluid, and calipers
Hydraulic pump, lines, and disc rotors
Vacuum booster and reservoir

Correct. Air-brake systems use a compressor, governor, storage tanks, foot valve, and brake chambers — distinct from hydraulic systems used in cars.

Question 7 of 20

For a single vehicle (not a combination) with engine off and brakes released, the maximum allowable air-loss rate is:

1 psi/min
2 psi/min
3 psi/min
4 psi/min

Correct. A single vehicle is allowed no more than 2 psi/min of air loss with engine off and brakes released.

Question 8 of 20

A low-air-pressure warning device must come on at or before:

20 psi
40 psi
60 psi
80 psi

Correct. A federal-mandated low-air-pressure warning (light, buzzer, or wig-wag) must activate at or before 60 psi.

Question 9 of 20

You hear a steady "ssss" sound while parked with the engine off. This is most likely:

Normal venting
An air leak that needs immediate attention
The compressor running
Trailer brakes releasing

Correct. A steady hiss with the engine off indicates an air leak — find and repair before driving.

Question 10 of 20

When you make a normal stop with air brakes, you should:

Push the brake pedal down hard
Apply firm steady pressure to slow you smoothly
Pump the brakes
Use only the trailer brake

Correct. For normal stops, apply steady firm pressure on the foot valve and adjust to maintain a smooth stop.

Question 11 of 20

You should perform a static air-leakage test by:

Driving and watching the gauge
With the engine off, brakes released, watching air pressure for one minute
With the engine running at high RPM
By tapping the air tank with a wrench

Correct. Engine off, brakes released, fully charged system: watch for air loss over one minute. Then apply brakes and check again.

Question 12 of 20

Brake fade is the result of:

Cold brakes
Worn linings only
Heat from continuous use, which reduces braking effectiveness
Air leaks

Correct. Brake fade comes from heat. Manage downgrade speed by gear selection plus intermittent firm brake use, not continuous light pressure.

Question 13 of 20

When the spring brakes apply automatically as air pressure drops, this typically happens between:

100 and 120 psi
60 and 80 psi
20 and 45 psi
5 and 10 psi

Correct. Spring brakes will fully apply somewhere between 20 and 45 psi as system pressure drops, depending on the vehicle.

Question 14 of 20

The parking-brake control:

Is a yellow diamond-shaped knob
Is a red round knob
Is a green knob
Does not exist on modern trucks

Correct. The parking-brake control is a yellow diamond-shaped knob, distinct from the red round trailer-supply knob.

Question 15 of 20

Air tanks must be drained because:

They get heavy
Compressor oil and water can collect inside, contaminating the brake system
They expire
It is required after every trip

Correct. Water and compressor oil collect at the bottom of air tanks. They must be drained to keep the system clean and prevent winter freeze-ups.

Question 16 of 20

The application pressure gauge shows:

The pressure in the storage tanks
How much pressure is being applied to the brakes when you press the foot valve
Tire pressure
Engine oil pressure

Correct. The application gauge (when present) shows brake-application pressure — useful for spotting brake-system problems on long downgrades.

Question 17 of 20

In a dual air-brake system, you should let the air pressure build to at least what level before driving?

60 psi
80 psi
100 psi
125 psi

Correct. Wait until the system pressure is at least 100 psi before driving in a dual air-brake system.

Question 18 of 20

Air brakes are actually three brake systems combined:

Service, parking, and emergency
Engine, parking, and trailer
Drum, disc, and engine
None of the above

Correct. A complete air-brake system has the service brake (foot valve), parking brake (spring/yellow knob), and emergency brake (which is the parking brake working off the spring brake when air drops).

Question 19 of 20

You are driving a fully loaded combination vehicle on a steep downgrade. Your safe speed is 30 mph and you have already shifted to a lower gear. The proper braking technique is to:

Hold the brakes lightly the whole way
Apply firmly to slow to 25 mph (5 mph below safe speed), release, and let speed build back to 30, then repeat
Stay at 30 mph
Apply only the trailer brake

Correct. The "snub-and-release" method: brake firmly to drop 5 mph below safe speed, release until safe speed returns, repeat. Holding the brakes overheats them.

Question 20 of 20

Modern dual-circuit air-brake systems are required so that:

Two drivers can operate the truck
If one part of the system fails, the other still provides braking
The truck looks newer
Air pressure builds faster

Correct. Dual systems separate primary and secondary brake circuits so that a failure in one (rear or trailer) still leaves brakes available on the other axle group.

About the South Carolina Air Brakes exam

States typically administer 25 Air Brakes questions and require 80% to pass. Questions cover air-brake system parts, dual systems, supply pressure, brake-system warning, slack adjusters, the parking brake, the spring brake, the pre-trip air-system check, and the proper way to perform a leakage-rate test.

The South Carolina Department of Motor Vehicles follows the federal CDL standards established by FMCSA. To earn the Air Brakes credential, you must answer at least 80% of the questions correctly. Many candidates score lower the first time because the test pulls from a large pool — refreshing this page will give you a different mix of questions, drawn from the same authoritative source.

Want more practice? Try the full Air Brakes question bank or browse all South Carolina CDL practice tests.