CDL Air Brakes practice test

Needed to drive any vehicle with air brakes, which covers most trucks and buses. The real test has 25 questions in most states.

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Study all 40 questions with answers

At about what pressure does the governor stop the air compressor from pumping (cut-out)?
125 psi
The governor stops the compressor at about 125 psi. It starts pumping again (cut-in) around 100 psi.
At about what pressure does the governor let the compressor start pumping again (cut-in)?
100 psi
Cut-in is about 100 psi. If it is much lower or higher than spec, the governor needs service.
The low air pressure warning must come on before pressure in the tanks drops below:
60 psi
The warning light, buzzer or wig wag must activate before supply pressure falls below 60 psi.
In most vehicles, spring brakes come on automatically when air pressure drops to:
Between 20 and 45 psi
Spring brakes are held off by air. When pressure drops into the 20-45 psi range, the springs apply the brakes.
With the engine off and brakes released, a single vehicle should lose no more than:
2 psi in one minute
The static leakage limit is 2 psi per minute for a single vehicle and 3 psi per minute for a combination.
With the engine off and the brake pedal held down, a single vehicle should lose no more than:
3 psi in one minute
Applied leakage should stay under 3 psi per minute for a single vehicle and 4 psi per minute for a combination.
In a dual air system, how long should it take to build pressure from 85 to 100 psi at fast idle?
Within 45 seconds
Build-up from 85 to 100 psi should take 45 seconds or less in a dual air system. Slower means a compressor or leak problem.
At what pressure does the safety relief valve usually open?
150 psi
The safety valve protects the tanks from too much pressure. It is usually set to open at 150 psi.
Why are air tanks drained?
To remove water and compressor oil that collect in them
Water and oil in the tanks can freeze or damage the system. Tanks without automatic drains should be drained at the end of each day.
What is the most common type of foundation brake on air-brake trucks?
S-cam drum brakes
S-cam brakes are the most common. The S-shaped cam spreads the brake shoes against the drum.
With the parking brake released and the engine off, how far should a manual slack adjuster move when pulled by hand?
No more than about 1 inch where the pushrod attaches
If the slack adjuster moves more than about an inch, the brakes likely need adjusting.
Brake drum cracks are a defect if they are:
Longer than half the width of the friction area
Drums must not have cracks longer than half the width of the friction area.
Why does an air brake system take longer to stop than hydraulic brakes?
Air takes time to flow through the lines to the brakes (brake lag)
Brake lag is about half a second. At 55 mph that adds roughly 32 feet to your stopping distance.
What color and shape is the parking brake control knob?
Yellow and diamond-shaped
The parking brake control is a yellow diamond knob. The red eight-sided knob is the trailer air supply on combinations.
When should you avoid using the parking brakes?
When the brakes are very hot or very cold and wet
Hot drums can be damaged when parking brakes apply, and wet brakes can freeze to the drums in cold weather. Use wheel chocks instead.
What is an alcohol evaporator for?
To reduce the risk of ice in brake valves and lines in cold weather
It puts alcohol into the air system so moisture is less likely to freeze and block the brakes.
The low air pressure warning comes on while you are driving. What should you do?
Stop and safely park as soon as possible
Low pressure means you could lose braking ability or the spring brakes could lock up. Stop safely while you still control where.
In a dual air system, the primary system usually operates the brakes on:
The rear axle or axles
Usually the primary system runs the rear brakes and the secondary system runs the front brakes. Both may serve the trailer.
How do you test that the spring brakes come on automatically?
Fan off air by pumping the brake pedal until the parking brake knob pops out, around 20 to 45 psi
Pumping the pedal drops pressure. The parking brake valve should pop out in the 20-45 psi range on most vehicles.
Air-braked trucks and buses built on or after March 1, 1998 must have:
Anti-lock brakes (ABS)
Federal rules require ABS on air-braked trucks and buses built on or after March 1, 1998. Tractors needed it a year earlier.
What powers the air compressor?
The engine, through gears or a belt
The compressor is driven by the engine through gears or a V-belt.
Why do air brake systems have storage tanks?
To hold enough air for several brake applications if the compressor stops
Tanks store compressed air so you can still brake several times if the compressor stops working.
What does the supply pressure gauge show?
How much air pressure is in the tanks
The supply pressure gauge shows tank pressure. In dual systems there's a needle or gauge for each half.
What does the application pressure gauge show?
How much air pressure you're applying to the brakes
It shows the pressure being applied. Needing more pressure to hold the same speed can mean brakes are fading.
What happens when you push the brake pedal down harder?
More air pressure is applied to the brakes
The foot valve controls air to the brakes. Pushing harder sends more air and more braking.
Why shouldn't you pump (fan) air brakes unnecessarily?
It wastes air and can drop pressure too low
Each application uses air. Fanning lets air out faster than the compressor can replace it.
What holds spring brakes off when you're driving?
Air pressure
Spring brakes are held back by air. Lose the air and the springs apply the brakes.
Why shouldn't you push the brake pedal while the spring brakes are on?
The combined force of the springs and air can damage the brakes
Pushing the pedal with spring brakes applied can overload brake parts.
Should front-wheel brakes be used on slippery roads in modern trucks?
Yes, front brakes are good in all conditions
Tests show front-wheel braking does not easily cause skids. Modern trucks have full front braking all the time.
What do automatic slack adjusters that are out of adjustment usually mean?
There's a mechanical problem that needs repair
Automatic slack adjusters shouldn't need manual adjustment. If they're out, the brakes need repair, not just adjusting.
How does stab braking work?
Brake all the way, release when wheels lock, reapply when they roll again
Stab braking is for vehicles without ABS. It can take up to a second for wheels to start rolling after you release.
What are the three systems in an air brake setup?
Service, parking and emergency brakes
Service brakes slow you in normal driving, parking brakes hold the vehicle, and the emergency system stops it if the service system fails.
Why can water in the air tanks be dangerous in winter?
It can freeze and cause brake failure
Water and oil collect in tanks. In cold weather the water can freeze in valves and lines.
How do you test the parking brake?
Set it, then gently pull against it in a low gear
With the parking brake on, the vehicle should not move when you gently try to pull forward.
How do you test the service brakes?
At about 5 mph, push the brake pedal firmly
Go about 5 mph and brake firmly. Pulling to one side or a delayed stop means trouble.
What happens if air pressure drops too low while driving?
The spring brakes can come on suddenly
Spring brakes apply when pressure drops low enough, which can stop the vehicle suddenly. Stop safely when you get a low-air warning.
Spring brakes are only as good as:
The brake adjustment
Poorly adjusted brakes won't hold well, whether you use service brakes or spring brakes.
What is the purpose of the air dryer or alcohol evaporator in cold climates?
To keep moisture from freezing in the air system
Both keep moisture from freezing in the brake system in cold weather.
Before driving, how many times should you check the air system's warning signal works?
Each time you do your pre-trip inspection
Checking the low-air warning is part of every pre-trip inspection.
In a dual air system, the secondary system usually operates the brakes on:
The front axle
Usually the primary system runs the rear brakes and the secondary runs the front brakes.

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