Question 1 of 20
A low-air-pressure warning device must come on at or before:
Correct. A federal-mandated low-air-pressure warning (light, buzzer, or wig-wag) must activate at or before 60 psi.
Mississippi Air Brakes CDL Practice Test
This is a free 20-question practice test for the Air Brakes portion of the Mississippi 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 Mississippi Department of Public Safety Driver Service Bureau 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 Mississippi.
Question 2 of 20
When brake drums or shoes get very hot, you should:
Correct. Park where you can let the brakes cool. Do not apply the parking (spring) brake on overheated brakes — it can damage them or cause warpage.
Question 3 of 20
You should test the parking brake by:
Correct. After applying the parking brake and releasing the service brakes, gently try to move forward in low gear. If the vehicle moves, the parking brake is not holding.
Question 4 of 20
Air tanks must be drained because:
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 5 of 20
The S-cam:
Correct. When the foot valve is pressed, air pushes the brake-chamber pushrod, which moves the slack adjuster, which rotates the S-cam shaft, forcing the shoes against the drum.
Question 6 of 20
The parking-brake control:
Correct. The parking-brake control is a yellow diamond-shaped knob, distinct from the red round trailer-supply knob.
Question 7 of 20
Modulating valves on the trailer:
Correct. The trailer service brakes are modulated — pressing harder on the foot valve produces more brake-chamber pressure at the trailer.
Question 8 of 20
Total stopping distance with air brakes equals:
Correct. Add brake-lag distance to the standard perception + reaction + braking distance for any vehicle with air brakes.
Question 9 of 20
The parts of an air-brake system include:
Correct. Air-brake systems use a compressor, governor, storage tanks, foot valve, and brake chambers — distinct from hydraulic systems used in cars.
Question 10 of 20
The application pressure gauge shows:
Correct. The application gauge (when present) shows brake-application pressure — useful for spotting brake-system problems on long downgrades.
Question 11 of 20
The service brakes are operated by:
Correct. Service brakes are applied by the foot valve / brake pedal. The yellow and red knobs control parking and trailer-supply functions.
Question 12 of 20
The governor controls:
Correct. The governor cycles the compressor: it cuts the compressor in around 100 psi and cuts it out around 125 psi.
Question 13 of 20
The supply pressure gauges show:
Correct. The supply (primary and secondary) pressure gauges show air pressure available for braking — a critical reading before driving and during operation.
Question 14 of 20
Air-brake adjustment is critical because out-of-adjustment brakes:
Correct. Out-of-adjustment slack adjusters reduce braking force at the wheel, increasing stopping distance and risking brake failure on long downgrades.
Question 15 of 20
Air brakes take longer to work than hydraulic brakes because:
Correct. Brake-lag in air systems comes from the time air takes to travel through lines and reach all the brake chambers — typically 0.4 seconds added to perception/reaction time.
Question 16 of 20
When the air-pressure protection valve closes during driving (because pressure dropped too low), the trailer:
Correct. When tractor air pressure falls into the 20-45 psi range, the protection valve closes, cutting trailer air, which causes the trailer spring brakes to apply.
Question 17 of 20
When should you use the trailer hand valve to slow the rig?
Correct. Using the trailer hand valve alone applies brakes only at the trailer wheels and can cause the trailer to lock up and skid. Always use the foot valve to brake.
Question 18 of 20
Air loss from the brake system on a moving combination vehicle, with brakes applied, should not exceed:
Correct. Combination vehicle, brakes applied, engine off: no more than 4 psi/min air loss. Engine off, brakes released: no more than 3 psi/min.
Question 19 of 20
If you must drive a vehicle with manual front-wheel-brake limiting valve, you should keep it in the "normal" position:
Correct. Keep the front-wheel limiting valve in the "normal" position. Modern trucks rarely have this valve, but if equipped, leaving it in "slippery" reduces front braking and lengthens stopping distance.
Question 20 of 20
For a single vehicle with engine off and brakes applied, the maximum allowable air-loss rate is:
Correct. A single vehicle is allowed no more than 3 psi/min with brakes fully applied.
About the Mississippi 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 Mississippi Department of Public Safety Driver Service Bureau 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 Mississippi CDL practice tests.