CDL Air Brakes Practice Test 2026 — Free

Your air tanks have manual drain valves, not automatic drains. How often should you drain the water and oil from the tanks?

• Every month
• Before starting the engine each morning
• At the end of each day of driving

Correct answer: At the end of each day of driving

Compressed air carries water and compressor oil. Both settle in the bottom of the air tanks. If they build up, the brakes may not work correctly. In cold weather the trapped water can freeze and cause brake failure. With manual drain valves you open each valve yourself. Drain the tanks completely at the end of each day of driving. The tanks then sit empty overnight instead of holding water that can freeze. Once a month is far too rare. Water and oil collect every day the compressor runs. Draining in the morning is too late. The water has already stood in the tanks all night.

What does the air compressor governor control?

• The amount of fuel the engine uses
• The temperature of the engine
• When air is pumped into the air tanks

Correct answer: When air is pumped into the air tanks

The governor is the pressure switch for the air supply. It reads tank pressure and tells the compressor when to pump, so the brakes have enough air. The compressor makes the air; the governor sets the timing. In {state}, it stops the compressor at the cut-out level, about 125 psi. It restarts pumping at the cut-in level, about 100 psi. Check both readings against the manufacturer's specifications in your air brake check. A faulty governor can leave too little air for safe driving. Engine temperature is tempting, because the engine cooling system may cool the compressor. That cools the compressor, not the engine, and the governor does not control it.

If you pull hard on a slack adjuster and it moves more than ___ where the push rod attaches to it, it needs to be adjusted.

• 5/8 inch
• 1 inch
• 2 inches

Correct answer: 1 inch

The slack adjuster links the brake chamber push rod to the brake camshaft. The camshaft turns the S-cam, which forces the brake shoes against the drum. Free play in this linkage does no braking, so the shoes reach the drum late. Chock the wheels, release the parking brakes, and pull hard on the adjuster by hand. If it moves more than about 1 inch where the push rod attaches, get the brake adjusted. Too much slack makes a vehicle very hard to stop. Roadside inspections find brakes out of adjustment more than any other problem. Two inches is not a looser limit. It is far past the 1-inch point.

During the air brake check, the air pressure builds up too slowly. What can happen?

• The air will leak out of the tanks faster than normal.
• You may keep driving until the pressure builds back up.
• Air pressure may drop to a low point while driving, requiring an emergency stop.

Correct answer: Air pressure may drop to a low point while driving, requiring an emergency stop.

Air brakes work on stored air, so the compressor must replace every pound you use. With the engine at operating rpm, a dual air system must build from 85 to 100 psi within 45 seconds. Slower than that means the compressor cannot keep up with normal brake use. On the road the tank pressure then falls, the low air pressure warning comes on, and you are forced into an emergency stop. Waiting for the pressure to recover while you drive does not work, because each brake application spends more air than the compressor puts back. Fix the problem before you drive.

Air pressure can leak away over time, so parking and emergency brakes need mechanical force to hold them on. What supplies that mechanical force?

• Spring pressure
• Magnetic force
• Electric current

Correct answer: Spring pressure

Air pressure leaks away over time, so air alone cannot hold a brake on. Every truck, truck tractor and bus must have emergency and parking brakes held on by mechanical force. Powerful springs supply that force. While you drive, air pressure holds the springs back. When you pull the parking brake knob, air leaves the spring brakes and the springs apply them. A leak that empties the system applies them too, so the vehicle fails safe. Magnetic force and electric current both stop the instant power is lost, so neither could hold a parked truck.

Using your brakes too much makes them very hot. What can this excessive heat cause?

• Engine oil to overheat
• Tire pressure to decrease
• Brakes to fade or fail

Correct answer: Brakes to fade or fail

Brakes slow you by rubbing shoes or pads against the drums, and that rubbing makes heat. Too much heat changes the lining so it grips less, and it makes the drums expand. The shoes must then move farther and press with less force. That is brake fade. Keep overusing the brakes and the vehicle cannot be slowed down or stopped. On a long downgrade, let the engine braking effect hold you back and use the brakes only to help. This heat attacks your stopping power, not the engine oil or the tires.

At about what pressure, in psi, does the governor stop the air compressor from pumping air?

• 100 psi
• 150 psi
• 125 psi

Correct answer: 125 psi

The air compressor does not run all the time. A governor switches it on and off. In {state}, the governor reaches its cut-out level at about 125 psi and stops the compressor from pumping. When tank pressure falls to about 100 psi, the governor cuts in and pumping starts again. This cycle keeps enough air stored to apply the brakes many times. Check cut-in and cut-out during your final air brake check. A governor that fails may not keep enough air pressure for safe driving. Do not confuse cut-out with the safety valve, which opens at 150 psi. Do not confuse it with cut-in at 100 psi, where pumping starts instead of stopping.

In an air brake system, which system applies and releases the brakes when you press the brake pedal during normal driving?

• Parking
• Service
• Emergency

Correct answer: Service

The brake pedal is the control for the service brake system. When you push it, air flows into each brake chamber and applies the brakes. Push harder for more air pressure; let up and the brakes release. That is how you make every normal stop and slowdown. Do not press and release the pedal needlessly: air goes out faster than the compressor can replace it, and if pressure gets too low the brakes will not work. The parking brake system is applied with the parking brake control, not the pedal. The emergency brake system uses parts of the other two and works only in a brake system failure.

A safety relief valve is usually set to open and let air out when tank pressure reaches ___ psi.

• 100
• 150
• 50

Correct answer: 150

The air compressor keeps pushing air into the first tank, so the pressure there can rise too high. The safety relief valve is in that tank. It opens at about 150 psi and lets the extra air out. This stops the pressure in the tank and the whole system from getting too high. A burst tank or air line drains your air, and with too little pressure the brakes stop working. 100 psi is the minimum you build in both systems before driving, not the relief setting. Air escaping from this valve means the system has a fault. Get a mechanic to repair it before you drive.

What happens if your vehicle's brake drums overheat?

• They expand.
• They crack and leak fluid.
• They shrink and grip the brake shoes harder.

Correct answer: They expand.

Braking presses the shoes and linings against the drums. That friction slows the vehicle and also creates heat. Too much heat makes the metal drums expand. The shoes must then move farther to reach the drums, so the force of that contact drops. This loss of stopping power is brake fade. Keep overusing the brakes and the vehicle may not slow down or stop at all. On a long downgrade, let the engine do most of the braking and save the service brakes. The drums cannot leak brake fluid — an air brake system works on compressed air.

Why is it important to remove water from the compressed air tanks?

• Water can build up pressure and force the safety valve open.
• Water can wash the oil off the brake linings and make them slip.
• Water can freeze in cold weather and cause brake failure.

Correct answer: Water can freeze in cold weather and cause brake failure.

Compressed air always carries some water and compressor oil, and both settle in the bottom of the air tank. In cold weather that water freezes. Ice inside the system can stop the brakes from working, so you lose braking at the moment you need it most. Drain each tank completely at the end of every driving day. Even with automatic drains, open the valves yourself to be sure. Water in the tank does not force the safety valve open; that valve opens only when air pressure climbs too high. Brake linings do not carry oil for water to wash off. You drain the tanks to keep your brakes working.

You are going down a long, steep hill at your safe speed of 40 mph. You apply the brakes until your speed drops to _____ mph, then release them.

• 30
• 35
• 25

Correct answer: 35

On a long, steep downgrade the engine controls your speed, not the brakes. You use the brakes only in short, firm applications. Brake until your speed is about 5 mph below your safe speed, then release them. Your safe speed here is 40 mph, so you slow to 35 mph and let the brakes cool. When speed climbs back to 40 mph, you brake again. Braking down to 30 or 25 mph keeps the brakes on too long. The extra heat causes brake fade. You then press harder and harder, and you may not be able to slow down or stop at all.

Using the service brakes too much makes them overheat. This heat can cause ___

• Expansion of the brake drums
• Increased friction in the brake linings
• Reduced tire pressure

Correct answer: Expansion of the brake drums

Brake drums are metal, and metal expands when it gets hot. Heavy use of the service brakes overheats the drums, so the drums grow. The shoes and linings must then move farther to reach the drums, and the force of that contact drops. That is brake fade: you press harder and harder for the same stopping power. Keep overusing the brakes and the vehicle may not slow down or stop at all. So control your speed with the engine and shift to a low gear before a downgrade. Brake heat makes the linings grip less, not more, and heat raises tire pressure instead of lowering it.

Your truck has a dual air brake system and minimum-size air tanks. The engine is at operating rpm. At most, how long should the air pressure take to build from 85 to 100 psi?

• 60 seconds
• 45 seconds
• 30 seconds

Correct answer: 45 seconds

The compressor must refill the tanks faster than you use air. With the engine at operating rpm and minimum-size tanks, a healthy dual air system builds from 85 to 100 psi in 45 seconds or less. Slower buildup means the air supply cannot keep up. Pressure can then drop too low while you drive and force an emergency stop, so do not drive until the problem is fixed. A 60-second buildup fails this check. More time is allowed only when the vehicle has larger than minimum air tanks.

On a vehicle with air brakes, what is a "wig wag"?

• A light that flashes when the engine is overheating
• A mechanical arm that signals low air pressure
• A gauge that shows how much air pressure you are applying to the brakes

Correct answer: A mechanical arm that signals low air pressure

A wig wag is a second kind of low air warning. Instead of a light, it drops a mechanical arm down into your view when pressure in the system falls. In {state}, the arm drops when system pressure falls below 55 psi. An automatic wig wag rises back out of view once pressure climbs above that level. A manual reset type must be pushed back by hand, and it will not stay up until the pressure is restored. When the arm drops, stop right away while you can still control the brakes. If pressure keeps falling, the spring brakes come on by themselves at 20 to 45 psi, and the stop is abrupt and uncontrolled. The wig wag watches air pressure only. It is not an engine temperature light, and it is not a gauge that shows how much air pressure you apply to the brakes.

Which statement about the braking power of spring brakes is correct?

• It depends on the service brakes being in adjustment.
• It decreases as the vehicle's speed increases.
• It is most effective when the vehicle is fully loaded.

Correct answer: It depends on the service brakes being in adjustment.

Air pressure holds the powerful springs back while you drive. When air leaves the brake chamber, the springs put on the vehicle's own brakes. Those are the same brakes you apply with the brake pedal, so one adjustment sets the power of both. If the brakes are out of adjustment, neither the regular brakes nor the emergency and parking brakes work right. You lose your normal brakes and your backup at the same time. Spring force is mechanical and fixed. It does not drop as you speed up, and a heavier load only makes the same force stop you more slowly.

On a newer vehicle with air brakes, how do you apply the parking brakes?

• Push the parking brake control knob in.
• Press the brake pedal down and hold it.
• Pull the parking brake control knob out.

Correct answer: Pull the parking brake control knob out.

Powerful springs, not air, hold the parking brakes on. Air pressure holds those springs back while you drive. Pulling the yellow, diamond-shaped knob out lets that air escape. The springs then push the brakes on and hold the vehicle mechanically. Push the knob in to send air back and release them. That mechanical hold is the safety point: air can leak away while you are parked, but the springs cannot. Pushing the knob in releases the brakes, so it leaves the vehicle free to roll away. Holding the brake pedal down uses only the service brakes; they let go the moment you step off. Never push the pedal down while the spring brakes are on, because the springs and the air pressure together can damage the brakes.

On a straight truck, the spring brakes will be fully on by the time air pressure drops to ___ psi.

• 60
• 20
• 100

Correct answer: 20

Air pressure holds the spring brakes off. Powerful springs are caged by that air. If the air leaks away, the springs put the brakes on by themselves. On a tractor or straight truck they come fully on as pressure falls through the 20 to 45 psi range, so by 20 psi they are fully applied. Do not wait for that. Stop as soon as the low air pressure warning light and buzzer come on, while you can still control the brakes. A spring-brake stop is abrupt and uncontrolled. 60 psi is too high: in {state} that is at or near the warning level, not where the springs grab.

What does the stop light switch do on a vehicle with air brakes?

• It activates the turn signals when you apply the brakes.
• It makes the brake lights come on when you use the air brakes.
• It turns on a warning light when the air pressure gets too low.

Correct answer: It makes the brake lights come on when you use the air brakes.

The air brake system uses an electric switch that air pressure operates. When you press the brake pedal, air pressure closes that switch and your brake lights come on. This warns drivers behind you the moment you start slowing. A loaded truck needs a long distance to stop, so a following driver who gets no warning can hit you from behind. The switch does not control the turn signals. Those come on from the turn signal lever, not from the brake pedal.

On a vehicle with air brakes, which type of foundation brake is the most common?

• Wedge drum brake
• S-cam drum brake
• Hydraulic disc brake

Correct answer: S-cam drum brake

Air pressure enters the brake chamber and pushes the push rod out. The push rod moves the slack adjuster, which twists the S-cam. The S-cam spreads the brake shoes against the inside of the drum, and that friction slows the wheel. The S-cam drum brake is the most common foundation brake, so it is the layout you inspect. Pull each slack adjuster: more than about one inch of movement means the brakes need adjustment. Too much slack makes the vehicle very hard to stop. Wedge and disc brakes do the same job but are less common. The disc brake used with air brakes is air-operated, not hydraulic.