Maine Air Brakes CDL Practice Test

This is a free 20-question practice test for the Air Brakes portion of the Maine 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 Maine Bureau 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 Maine.

Question 1 of 20

Air storage tanks are used to:

Carry coolant
Hold compressed air for use by the brakes
Store fuel
House the alternator

Correct. Air storage (or "supply") tanks hold compressed air ready for brake application. Trucks have multiple tanks for redundancy.

Question 2 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 3 of 20

Continuous use of the brakes on a long downgrade can cause:

Improved braking
Brake fade — loss of braking effectiveness due to overheating
No effect
Faster wear of the air compressor

Correct. Holding the brakes overheats them and causes brake fade — drums expand and brake compounds lose effectiveness, reducing braking force.

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

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 6 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 7 of 20

The "tractor protection valve" closes (cuts off air to the trailer) automatically when:

You apply the brakes
Air pressure in the tractor drops to a dangerously low level (typically 20 to 45 psi)
You hit a bump
You shift gears

Correct. The tractor protection valve protects the tractor from a runaway air loss in case of a trailer break-away or air leak — it closes between 20 and 45 psi.

Question 8 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 9 of 20

A yellow ABS malfunction lamp on the cab dash means:

Everything is fine
The ABS on the tractor is malfunctioning
The trailer ABS is malfunctioning
Air pressure is low

Correct. A yellow ABS lamp on the dash indicates a tractor-ABS malfunction. The base brakes still work normally, but ABS is unavailable until repaired.

Question 10 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 11 of 20

You should test the parking brake by:

Pushing on it
Setting it, releasing service brakes, and gently trying to move the vehicle in low gear — it should hold
Driving without it
Banging on it with a hammer

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 12 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.

Question 13 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 14 of 20

You are driving a tractor-trailer with the air-pressure gauge dropping rapidly. You should:

Continue to your destination
Pull over safely as soon as possible
Apply the brakes hard
Speed up to reach a safe place

Correct. A rapid drop in air pressure indicates a serious leak. Pull over before the pressure drops to the point where the spring brakes apply and the vehicle stops on the road.

Question 15 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 16 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 17 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 18 of 20

Brake drums or discs that have cracks larger than ___ the width of the friction area should be replaced:

One-quarter
One-half
One-third
Three-quarters

Correct. Cracks longer than one-half the width of the friction area indicate the drum is unsafe and must be replaced.

Question 19 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 20 of 20

On a vehicle with ABS, in an emergency stop you should:

Pump the brakes
Apply the brakes the way you always would and steer where you need to go
Apply the parking brake
Coast

Correct. ABS lets you brake hard and continue steering. Brake firmly and avoid pumping — pumping defeats the system.

About the Maine 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 Maine Bureau 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 Maine CDL practice tests.