Montana Air Brakes CDL Practice Test

This is a free 20-question practice test for the Air Brakes portion of the Montana 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 Montana Motor Vehicle Division 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 Montana.

Question 1 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 2 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 3 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 4 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 5 of 20

Which of these is the safest speed for descending a long, steep grade?

The posted speed limit
A speed slow enough that the brakes can hold without overheating, typically below the speed limit
Whatever speed you went up the hill
Maximum gear-down speed

Correct. Choose a speed and gear that lets the brakes work intermittently without continuous pressure. Posted truck-grade signs help.

Question 6 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 7 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 8 of 20

A yellow ABS malfunction lamp on the trailer (typically left side) means:

The trailer brakes have failed
The trailer ABS is malfunctioning
The trailer is overweight
Tire pressure is low

Correct. A trailer-mounted yellow ABS lamp indicates the trailer's ABS is malfunctioning. Service brakes still work; have the system repaired.

Question 9 of 20

When checking the brakes on a downgrade, you should:

Use the air-pressure gauge to monitor brake-system pressure for excessive use
Pump the pedal
Drive faster
Ignore the gauge

Correct. Watch the air-pressure gauge. If pressure drops faster than the compressor can recover, brakes are being used too aggressively for the gear selection.

Question 10 of 20

In a fully charged dual air-brake system at idle, you should test the low-pressure warning by:

Letting the air bleed down with the engine off and brakes off, fanning the brakes if needed; warning should activate at or before 60 psi
Holding the foot valve down for one minute
Removing the alternator belt
Disconnecting the compressor

Correct. Engine off, fan the brake pedal to bleed pressure. The low-pressure warning device should activate before pressure drops below 60 psi.

Question 11 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 12 of 20

The spring brakes:

Are held off by air pressure and apply automatically when pressure drops
Are activated only by the foot valve
Are activated only by the trailer hand valve
Are not used in modern trucks

Correct. Spring brakes use heavy springs that apply the brake when air pressure is released or drops below 20-45 psi — they serve as both parking and emergency brake.

Question 13 of 20

Wet brakes:

Improve braking
Cause reduced power, brake pulling, or wheel lockup
Have no effect
Are normal in winter

Correct. After fording water, brakes may be wet. Drive slowly while applying light brake pressure to dry them.

Question 14 of 20

When brake drums or shoes get very hot, you should:

Park immediately
Continue driving
Pull over and let them cool, but do not apply the parking brake until they have cooled
Throw water on them

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 15 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 16 of 20

The air compressor is driven by:

The transmission
The engine, usually by gears or belt
An electric motor
A separate engine

Correct. The air compressor is engine-driven, typically through gears at the front of the engine or by a v-belt.

Question 17 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 18 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.

Question 19 of 20

In an emergency stop on a non-ABS vehicle, you should:

Apply the brakes lightly
Use stab braking — full brake until wheels lock, release, repeat
Pump the brakes rapidly
Apply only the trailer hand valve

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 20 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.

About the Montana 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 Montana Motor Vehicle Division 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 Montana CDL practice tests.