Question 1 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.
Wisconsin Air Brakes CDL Practice Test
This is a free 20-question practice test for the Air Brakes portion of the Wisconsin 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 Wisconsin Division 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 Wisconsin.
Question 2 of 20
Stopping distance for a vehicle with air brakes is the sum of:
Correct. Air-brake stopping distance is perception + reaction + brake lag + braking — the brake lag is the unique addition.
Question 3 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 4 of 20
Manual slack adjusters must be adjusted:
Correct. Drivers may adjust manual slack adjusters in compliance with manufacturer specifications. Automatic slack adjusters are now standard but still require periodic checking.
Question 5 of 20
The air compressor is driven by:
Correct. The air compressor is engine-driven, typically through gears at the front of the engine or by a v-belt.
Question 6 of 20
The trailer air-supply control:
Correct. The trailer air-supply (tractor protection) is a red eight-sided knob. Push in to supply air to the trailer; pull out to shut off air.
Question 7 of 20
In a dual air-brake system, you should let the air pressure build to at least what level before driving?
Correct. Wait until the system pressure is at least 100 psi before driving in a dual air-brake system.
Question 8 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.
Question 9 of 20
When the spring brakes apply automatically as air pressure drops, this typically happens between:
Correct. Spring brakes will fully apply somewhere between 20 and 45 psi as system pressure drops, depending on the vehicle.
Question 10 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.
Question 11 of 20
Modern dual-circuit air-brake systems are required so that:
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.
Question 12 of 20
In a dual air-brake system, the time required for air pressure to build from 85 to 100 psi should be no more than:
Correct. In dual air-brake systems, air should build from 85 to 100 psi within 45 seconds at engine governed RPM.
Question 13 of 20
If a trailer breaks away, the emergency brakes on the trailer will be applied because:
Correct. When the air line breaks (or is disconnected), trailer air pressure drops and the trailer spring brakes apply, stopping the trailer.
Question 14 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 15 of 20
A long downgrade requires:
Correct. Pick a low gear before starting down. Use the brakes in firm, intermittent applications: brake to 5 mph below safe speed, release, repeat.
Question 16 of 20
You should perform a static air-leakage test by:
Correct. Engine off, brakes released, fully charged system: watch for air loss over one minute. Then apply brakes and check again.
Question 17 of 20
Continuous use of the brakes on a long downgrade can cause:
Correct. Holding the brakes overheats them and causes brake fade — drums expand and brake compounds lose effectiveness, reducing braking force.
Question 18 of 20
In an emergency stop on a non-ABS vehicle, you should:
Correct. Stab braking applies brakes hard until the wheels lock, then releases when you feel skid — this slows the vehicle while keeping it straight.
Question 19 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 20 of 20
You hear a steady "ssss" sound while parked with the engine off. This is most likely:
Correct. A steady hiss with the engine off indicates an air leak — find and repair before driving.
About the Wisconsin 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 Wisconsin Division 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 Wisconsin CDL practice tests.