Michigan Air Brakes CDL Practice Test

This is a free 20-question practice test for the Air Brakes portion of the Michigan 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 Michigan Department of State 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 Michigan.

Question 1 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 2 of 20

The governor controls:

Engine RPM
When the air compressor pumps air into the air storage tanks
Brake-pedal travel
Trailer hand-valve operation

Correct. The governor cycles the compressor: it cuts the compressor in around 100 psi and cuts it out around 125 psi.

Question 3 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 4 of 20

Total stopping distance with air brakes equals:

Perception distance + reaction distance + brake-lag distance + effective braking distance
Reaction distance only
Brake-lag distance only
Perception distance + braking distance

Correct. Add brake-lag distance to the standard perception + reaction + braking distance for any vehicle with air brakes.

Question 5 of 20

Manual slack adjusters must be adjusted:

Only by a mechanic at the dealership
By the driver, when needed, following manufacturer specifications
Never
Only at the time of vehicle inspection

Correct. Drivers may adjust manual slack adjusters in compliance with manufacturer specifications. Automatic slack adjusters are now standard but still require periodic checking.

Question 6 of 20

Modulating valves on the trailer:

Don't exist
Apply increasing brake pressure as you push the foot valve harder
Are emergency-only
Open the parking brake

Correct. The trailer service brakes are modulated — pressing harder on the foot valve produces more brake-chamber pressure at the trailer.

Question 7 of 20

To check air-brake adjustment quickly, you can:

Measure the slack adjuster pushrod travel; if it has moved more than about 1 inch when brakes are applied at 90+ psi, adjustment is needed
Listen to the sound
Look at the tires
Check the dashboard light

Correct. Pushrod travel beyond about 1 inch (varies by chamber size) indicates an adjustment issue. Use a marked stroke check or have a mechanic verify.

Question 8 of 20

The supply pressure gauges show:

The brake-pedal pressure
The pressure in the air storage tanks
The amount of air being used by the brakes
The trailer brake pressure only

Correct. The supply (primary and secondary) pressure gauges show air pressure available for braking — a critical reading before driving and during operation.

Question 9 of 20

After connecting the trailer air lines, you should:

Drive away
Charge the trailer air system, check that the trailer brakes work, and inspect glad-hand connections
Lock the trailer brakes
Turn off the supply

Correct. Charge the trailer system, then test the trailer brakes by pulling out the trailer-supply knob to make sure they hold the trailer.

Question 10 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 11 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 12 of 20

If you must drive a vehicle with manual front-wheel-brake limiting valve, you should keep it in the "normal" position:

Always
Only on slick roads
In the slippery position only
Only when empty

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 13 of 20

When should you use the trailer hand valve to slow the rig?

Whenever you want trailer-only braking
Never; using the trailer hand valve alone can cause a trailer skid
Always when going downhill
Only on dry pavement

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 14 of 20

You should drain the air tanks:

Once a month
Once a year
At the end of every working day
Never

Correct. Drain all air tanks at the end of each working day to remove water and compressor oil that has condensed in the tanks.

Question 15 of 20

For a combination vehicle (tractor and trailer) with engine off and brakes released, the maximum allowable air-loss rate is:

2 psi/min
3 psi/min
4 psi/min
6 psi/min

Correct. A combination vehicle is allowed no more than 3 psi/min with brakes released, or 4 psi/min with brakes applied.

Question 16 of 20

Air brakes take longer to work than hydraulic brakes because:

Air takes longer to flow than fluid
Air-brake lines are longer than hydraulic lines and have more components
Drivers wait longer to brake
Air brakes are simpler

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 17 of 20

When the air-pressure protection valve closes during driving (because pressure dropped too low), the trailer:

Continues without effect
Spring brakes apply, and the trailer stops
Loses electrical power
Turns off its lights

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 18 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 19 of 20

Pre-trip air-brake check (Step 1) is to:

Test the parking brake
Test the air leakage rate
Time the air-pressure build-up
Check the slack adjusters

Correct. In the seven-step air-brake check the first step is testing the parking brake — release service brakes, set parking brake, gently try to move the vehicle in low gear.

Question 20 of 20

You should never use the parking brake when the brakes are very hot because:

It will not hold
It can damage the brakes or cause warpage
It will release on its own
It will use too much air

Correct. Hot brakes cooled with parking brake applied can warp the drum or rotor, or cause the spring brake to fail to release.

About the Michigan 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 Michigan Department of State 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 Michigan CDL practice tests.