Practice questions written from the official CDL manual. Correct answers and explanations are shown below each question. (Turn on JavaScript for the interactive practice and exam-simulation modes.)
1. What is the main job of the governor in an air brake system?
- AIt warns the driver when air pressure gets dangerously low
- BIt controls when the air compressor pumps air into the tanksCorrect
- CIt drains water and oil out of the air storage tanks
- DIt applies the brakes evenly to all wheels at the same time
Why: The governor tells the air compressor when to start pumping (cut-in) and when to stop pumping (cut-out). This keeps the air pressure in the tanks within a safe operating range instead of building up without limit.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Air Compressor Governor)
2. At about what pressure does the governor usually stop the compressor from pumping more air (the cut-out level)?
- AAbout 55 psi
- BAbout 90 psi
- CAbout 125 psiCorrect
- DAbout 200 psi
Why: When tank pressure rises to the cut-out level, around 125 psi, the governor stops the compressor from pumping air. When pressure drops to the cut-in level, the governor lets the compressor start pumping again.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Air Compressor Governor)
3. Where does the air compressor pump the air it produces?
- AInto the air storage tanks (reservoirs)Correct
- BDirectly into the brake chambers at each wheel
- CInto the low-air-pressure warning device
- DInto the alcohol evaporator to keep it full
Why: The compressor pumps air into the air storage tanks, also called reservoirs. The tanks hold enough compressed air to make several stops even if the compressor stops working.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Air Storage Tanks)
4. Why is it important to drain the air storage tanks?
- ATo lower the air pressure before parking overnight
- BTo let the compressor rest and cool down
- CTo reset the low-air-pressure warning signal
- DTo remove water and compressor oil that collect insideCorrect
Why: Compressed air always contains some water and compressor oil, which collect in the tanks. Water can freeze in cold weather and cause brake failure, so the tanks must be drained. Tanks with manual drains should be drained at the end of each working day.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Air Tank Drains)
5. What is the purpose of the alcohol evaporator found in some air brake systems?
- ATo increase the pressure the compressor can build
- BTo reduce the risk of ice forming in the air brake system in cold weatherCorrect
- CTo warn the driver that the brakes are overheating
- DTo automatically adjust the slack adjusters
Why: The alcohol evaporator puts alcohol into the air system. This helps reduce the risk of ice forming in the valves and other parts during cold weather, which could keep the brakes from working.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Alcohol Evaporator)
6. The safety valve in the first (supply) air tank is usually set to open at what pressure?
- A55 psi
- B125 psi
- C150 psiCorrect
- D300 psi
Why: The safety valve protects the tank and the system from having too much pressure. It is usually set to open at 150 psi. If the safety valve releases air, something is wrong and the system needs to be checked by a mechanic.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Safety Valve)
7. What is the most common type of foundation brake found on heavy vehicles?
- AThe S-cam drum brakeCorrect
- BThe wedge disc brake
- CThe hydraulic disc brake
- DThe band brake
Why: Foundation brakes are used at each wheel. The most common type is the S-cam drum brake. Understanding how it works helps you know why proper adjustment and cool brakes matter.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Foundation Brakes)
8. In an S-cam drum brake, what happens when you push the brake pedal?
- AAir escapes from the tanks to slow the wheels using suction
- BHydraulic fluid is forced into the wheel cylinders
- CThe parking spring is released so the drum can spin freely
- DAir pressure moves the slack adjuster and twists the S-cam to press the shoes against the drumCorrect
Why: Pushing the brake pedal sends air pressure into the brake chamber, pushing a rod that moves the slack adjuster and twists the brake camshaft. This turns the S-cam, which forces the brake shoes against the inside of the drum to slow the wheel.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (S-cam Brakes)
9. When checking manual slack adjusters, about how far should the pushrod move when you pry against it?
- AAs far as it will go, with no limit
- BNo more than about one inchCorrect
- CExactly three inches
- DIt should not move at all
Why: Brakes that are out of adjustment are a leading cause of poor braking. When you pry against the pushrod, it should not move more than about one inch. If it moves more, the brakes may need adjustment and the vehicle can be unsafe.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Slack Adjusters)
10. What does the supply pressure gauge on the dash tell the driver?
- AHow hard the driver is pressing the brake pedal
- BHow hot the brake drums are getting
- CHow much air pressure is in the air storage tanksCorrect
- DHow many miles until the next required drain
Why: All air-braked vehicles have a supply pressure gauge that shows how much pressure is in the air tanks. This is different from the application pressure gauge, which shows how much air you are sending to the brakes when you press the pedal.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Supply Pressure Gauges)
11. You are going down a hill and must keep pushing the brake pedal harder and harder to keep the same braking power. What is most likely happening?
- AThe brakes are fading from overheatingCorrect
- BThe air tanks are overfilled with pressure
- CThe governor has reached cut-out
- DThe stop light switch has failed
Why: If you must apply more and more air pressure (shown on the application pressure gauge) to hold the same speed, the brakes are fading. Fade happens when brakes overheat, and it is a sign to slow down and use a lower gear or engine braking.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Application Pressure Gauge) / §5.4 — Brake Fading
12. By law, at what pressure must the low-air-pressure warning signal come on for most vehicles?
- ABefore pressure drops below 100 psi
- BBefore pressure drops below 80 psi
- COnly after the tanks are completely empty
- DBefore pressure drops below 55 psiCorrect
Why: Vehicles with air brakes must have a low-air-pressure warning signal, usually a red light and a buzzer. It must come on before the air pressure in the tanks falls below 55 psi, giving the driver time to stop safely.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Low Air Pressure Warning)
13. On some older vehicles, what is the 'wig wag' device?
- AA valve that drains water from the tanks automatically
- BA mechanical arm that drops into the driver's view when air pressure gets too lowCorrect
- CA lever that manually adjusts the front brakes
- DA gauge that shows how hot the brakes are
Why: A wig wag is a type of low-air-pressure warning found on older vehicles. It is a mechanical arm that drops down into the driver's view when pressure in the system drops below about 55 psi. It warns you the same way a warning light and buzzer do.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Low Air Pressure Warning)
14. What is the purpose of the stop light switch in an air brake system?
- AIt shuts off the compressor at cut-out pressure
- BIt releases the spring brakes when you park
- CIt turns on the brake lights when you push the brake pedalCorrect
- DIt opens the safety valve if pressure gets too high
Why: The stop light switch is an air-pressure electrical switch. When you push the brake pedal, air pressure closes the switch and turns on your brake lights, warning drivers behind you that you are slowing or stopping.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Stop Light Switch)
15. The front brake limiting valve found on some older trucks (made before 1975) was designed to do what when set to the 'slippery' position?
- AReduce the air pressure to the front brakes by about halfCorrect
- BIncrease braking power to all four wheels
- CShut off the rear brakes completely
- DTurn off the low-air-pressure warning buzzer
Why: Older trucks had a front brake limiting valve with 'normal' and 'slippery' settings. In the slippery position it cut front brake pressure by about half. Experts later learned front brakes actually help you stop and rarely cause skids, so newer vehicles do not have this valve.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Front Brake Limiting Valves)
16. On a tractor or straight truck, at about what pressure will the spring (parking) brakes come fully on?
- AWhen pressure rises above 125 psi
- BOnly when the engine is turned off
- CWhen pressure reaches 100 psi
- DWhen pressure drops into the range of about 20 to 45 psiCorrect
Why: Spring brakes use powerful springs held back by air pressure. As air leaks away, the springs push the brakes on. On a truck or tractor they come fully on when air pressure falls to about 20 to 45 psi (often 20 to 30 psi), so do not wait for that to happen while driving.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Spring Brakes)
17. What actually holds the spring (parking) brakes in the applied position?
- AConstant air pressure from the compressor
- BPowerful mechanical springsCorrect
- CHydraulic fluid under pressure
- DAn electric magnet in each wheel
Why: Spring brakes are held on by powerful springs, not by air. When you drive, air pressure holds the springs back so the brakes release. When air is released or leaks away, the springs apply the brakes. That is why they still work if the air system fails.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Parking Brake Controls)
18. On newer vehicles, what does the parking brake control usually look like?
- AA round green button
- BA silver toggle switch
- CA yellow, diamond-shaped knob you pull out to apply and push in to releaseCorrect
- DA red, octagon-shaped knob
Why: On newer vehicles the parking brake is set with a yellow, diamond-shaped knob. You pull it out to apply the brakes and push it in to release. The red, eight-sided knob is the separate trailer air supply control on combination vehicles.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Parking Brake Controls)
19. When should you avoid using the parking brake?
- AWhen the brakes are very hot from hard use, or wet in freezing weatherCorrect
- BWhenever you park on level ground
- CAny time the engine is still running
- DOnly when the trailer is loaded
Why: Do not set the parking brake if the brakes are very hot, such as right after a steep downgrade, because hot brakes can be damaged or later fail to release. Also avoid it if the brakes are wet and temperatures are below freezing, because they can freeze so the vehicle cannot move.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Parking Brake Controls)
20. What best describes a dual air brake system?
- AOne air system that powers two trailers at once
- BA system that uses air for the front brakes and hydraulic fluid for the rear
- CA backup electric brake that takes over when air runs low
- DTwo separate air brake systems that use a single set of brake controlsCorrect
Why: Most heavy vehicles use a dual air brake system: two separate systems (a primary and a secondary) that share one set of controls. Each system has its own tanks, lines, and hoses. If one system fails, the other can still stop the vehicle.
Based on: CDL Manual §5.2 — Dual Air Brake Systems
21. Before driving a vehicle with a dual air brake system, you should wait until the pressure in both systems reaches at least what level?
- A50 psi
- B100 psiCorrect
- C150 psi
- DThere is no minimum; drive as soon as the engine starts
Why: With a dual air brake system, let the air compressor build up a minimum of 100 psi in both the primary and secondary systems before you drive. Starting with too little pressure means you may not have enough air for safe braking.
Based on: CDL Manual §5.2 — Dual Air Brake Systems
22. In a dual air brake system, when should the low-air-pressure warning come on?
- AOnly when both systems are completely empty
- BWhen either system rises above 125 psi
- CBefore the pressure in either system drops below 55 psiCorrect
- DOnly when the parking brake is applied
Why: In a dual system, the low-air warning must come on before the pressure in either the primary or secondary system falls below 55 psi. If the warning comes on while driving, stop safely right away, because part of your braking may already be lost.
Based on: CDL Manual §5.2 — Dual Air Brake Systems
23. What is the main cause of brake fade?
- AOverheating the brakes from using them too muchCorrect
- BToo much air pressure in the storage tanks
- CDraining the tanks too often
- DDriving with the parking brake released
Why: Brake fade happens when the brakes overheat, usually from using the service brakes too much, such as riding them down a long grade. Heat makes the drums expand and the linings less effective, so the brakes lose their stopping power.
Based on: CDL Manual §5.4 — Using Air Brakes (Brake Fading)
24. What is the most important thing to help control your speed going down a long, steep grade?
- APumping the brake pedal on and off the whole way down
- BShifting into neutral to save fuel
- CKeeping the parking brake partly applied
- DBeing in the proper low gear and using engine brakingCorrect
Why: The primary way to control speed on a downgrade is choosing a safe speed and being in a low gear so engine braking helps hold you back. The service brakes should only supplement this. Relying on the brakes alone will overheat them and cause fade.
Based on: CDL Manual §5.4 — Using Air Brakes (Going Down Long, Steep Downgrades Safely)
25. Using the recommended braking method on a downgrade, when should you release the brakes?
- AAs soon as you feel any slowing at all
- BWhen your speed has been reduced to about 5 mph below your safe speedCorrect
- COnly after coming to a complete stop
- DEvery two seconds, no matter your speed
Why: The recommended method is to apply the brakes hard enough to feel a definite slowdown, then release them once you are about 5 mph below your safe speed. When you speed back up to the safe speed, repeat. This keeps the brakes from overheating.
Based on: CDL Manual §5.4 — Using Air Brakes (Proper Braking Technique)
26. How is stab braking done in an emergency stop on a vehicle without antilock brakes?
- APress the pedal lightly and hold it steady until you stop
- BPump the pedal quickly and gently the whole time
- CFully apply the brakes, and when the wheels lock, release until they roll, then fully apply againCorrect
- DApply only the trailer brakes with the hand valve
Why: Stab braking is an emergency method for vehicles without ABS. You apply the brakes all the way, and when the wheels lock up, you release the brakes until the wheels start rolling again, then apply fully once more. This helps you stop while keeping steering control.
Based on: CDL Manual §5.4 — Using Air Brakes (Emergency Stops / Stab Braking)
27. Why should you avoid 'fanning' the brakes (rapidly pressing and releasing the pedal) during normal driving?
- AIt can use up the air pressure faster than the compressor can replace itCorrect
- BIt makes the brake lights stay off
- CIt causes the governor to reach cut-out too soon
- DIt overfills the air tanks with pressure
Why: Pressing and releasing the brakes over and over lets air out each time. Fanning can use air faster than the compressor can build it back, which may lower pressure until the spring brakes come on. Except for stab braking, avoid fanning the brakes.
Based on: CDL Manual §5.4 — Using Air Brakes (Proper Braking Technique)
28. Compared with hydraulic brakes, air brakes add an extra delay to stopping. What is this delay called?
- APerception distance
- BReaction distance
- CEffective braking distance
- DBrake lag distanceCorrect
Why: Air brakes have brake lag: it takes about half a second for the air to travel through the lines and work the brakes after you push the pedal. This adds brake lag distance to your total stopping distance, which also includes perception, reaction, and effective braking distances.
Based on: CDL Manual §5.4 — Using Air Brakes (Total Stopping Distance)
29. At about 55 mph on dry pavement, roughly how far does an air-braked vehicle travel from the time you see a hazard until it stops?
- AAbout 100 feet
- BAbout 400 feet, longer than a football fieldCorrect
- CAbout 60 feet
- DAbout 1,000 feet
Why: At 55 mph on dry pavement, total stopping distance for an air-braked vehicle is roughly 400 feet, which is longer than a football field. This adds up perception distance, reaction distance, brake lag distance, and effective braking distance.
Based on: CDL Manual §5.4 — Using Air Brakes (Total Stopping Distance)
30. What does an antilock braking system (ABS) do for you?
- AIt shortens your stopping distance significantly
- BIt replaces the need to slow down for curves
- CIt keeps your wheels from locking up so you can keep steering control during hard brakingCorrect
- DIt lets you skip the low-air-pressure warning check
Why: ABS keeps your wheels from locking up during hard braking, which helps you keep steering control and stay in your lane. ABS does not shorten your stopping distance or make you a better driver; it is a tool for keeping control in an emergency.
Based on: CDL Manual §5.4 — Using Air Brakes (Antilock Braking Systems)
31. In a dual air system, how fast should air pressure build in the tanks when you check the buildup rate with the engine at operating rpm?
- AFrom 85 to 100 psi within about 45 secondsCorrect
- BFrom 0 to 150 psi within 10 seconds
- CFrom 20 to 45 psi within 5 minutes
- DIt does not matter how fast it builds
Why: During the air brake check, with the engine at operating rpm, air pressure in a typical dual system should build from about 85 to 100 psi within roughly 45 seconds. Slow buildup means the system may not keep up with your air use and needs to be fixed.
Based on: CDL Manual §5.3 — Inspecting Air Brake Systems (Check Air Pressure Buildup Rate)
32. With the engine off, the parking brake released, and the service brakes NOT applied, what is the most air a single vehicle should lose in one minute?
- A10 psi
- B6 psi
- C0 psi — any loss fails
- D2 psiCorrect
Why: For the leakage test with brakes released, a single vehicle should lose less than 2 psi in one minute. A combination vehicle should lose less than 3 psi in one minute. More loss than that means there is a leak that must be fixed.
Based on: CDL Manual §5.3 — Inspecting Air Brake Systems (Test Air Leakage Rate)
33. With the air system fully charged and the service brakes fully applied, what is the maximum air a combination vehicle should lose in one minute?
- A2 psi
- B4 psiCorrect
- C8 psi
- D12 psi
Why: With the brakes applied, air loss should be no more than 3 psi in one minute for a single vehicle and no more than 4 psi in one minute for a combination vehicle. Losing more than that means the system leaks too much and is unsafe to drive.
Based on: CDL Manual §5.3 — Inspecting Air Brake Systems (Test Air Leakage Rate)
34. When checking the air compressor governor cut-in and cut-out during the air brake inspection, you build pressure to cut-out, then do what to check cut-in?
- ATurn the engine off and wait ten minutes
- BDrain both tanks completely
- CStep on and off the brake to reduce pressure and watch for the compressor to start againCorrect
- DPull the parking brake knob out
Why: After building to cut-out (around 125 psi), you reduce pressure by stepping on and off the brake pedal. The governor should let the compressor cut in and start pumping again at the manufacturer's cut-in pressure, then build back up to cut-out.
Based on: CDL Manual §5.3 — Inspecting Air Brake Systems (Check Air Compressor Governor Cut-in and Cut-out Pressures)
35. During the air brake check, how do you test that the low-air-pressure warning signal works?
- AWith the engine off and key on, fan the brakes to lower pressure and watch for the warning to come on before it drops below 55 psiCorrect
- BRev the engine until the tanks reach cut-out pressure
- CDrain the tanks and look for water
- DPull out the trailer air supply knob
Why: With the electrical power on and the engine off, you step on and off the brake pedal to release air and lower the tank pressure. The low-air warning (light and buzzer, or wig wag) must come on before pressure drops below 55 psi.
Based on: CDL Manual §5.3 — Inspecting Air Brake Systems (Test Low Pressure Warning Signal)
36. How do you check that the spring brakes come on automatically during the air brake test?
- ARev the engine until pressure reaches cut-out
- BLook for the safety valve to open
- CPush the parking brake knob in and drive forward
- DKeep fanning off air so pressure keeps dropping until the parking brake knob pops out (spring brakes come on) in the 20 to 45 psi rangeCorrect
Why: Continue to reduce air pressure by stepping on and off the brake. On a tractor or straight truck, the parking brake knob should pop out and the spring brakes should come on automatically when pressure falls into about the 20 to 45 psi range.
Based on: CDL Manual §5.3 — Inspecting Air Brake Systems (Check that Spring Brakes Come On Automatically)
37. Why do the air storage tanks hold a large supply of compressed air?
- ATo power the horn and other accessories
- BSo there is enough air to make several stops even if the compressor stops workingCorrect
- CTo keep the tires inflated
- DSo the brakes never need adjusting
Why: The reservoirs store enough compressed air to make several stops even if the compressor suddenly stops working. This built-in reserve gives you a chance to stop safely if a problem develops in the air supply.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Air Storage Tanks)
38. When you press the brake pedal (the foot valve) harder, what happens?
- AAir is drained from the tanks to reset the system
- BThe compressor is shut off by the governor
- CMore air pressure is applied to the brakes, giving more braking powerCorrect
- DThe parking spring brakes are released
Why: The brake pedal, also called the foot valve or treadle valve, controls how much air goes to the brakes. Pressing harder sends more air pressure, giving more braking power. Letting up sends less. Pressing too hard can cause wheels to lock and skid.
Based on: CDL Manual §5.1 — The Parts of an Air Brake System (Foot Brake)
39. During the walk-around and air brake inspection, brakes that are badly out of adjustment are dangerous mainly because they:
- AReduce the vehicle's stopping power and can lead to brake failureCorrect
- BCause the air tanks to overfill
- CMake the low-air warning sound continuously
- DForce the compressor to run backwards
Why: Out-of-adjustment brakes are one of the most common causes of poor braking. When the pushrod travels too far, the S-cam cannot press the shoes firmly against the drum, so the vehicle loses stopping power and the brakes can fail, especially when hot.
Based on: CDL Manual §5.1 / §5.3 — Slack Adjusters and Inspecting Air Brake Systems
40. As part of the final air brake check, how do you test the service brakes before driving?
- ARev the engine and watch the supply gauge
- BDrain the wet tank while holding the pedal down
- CPump the pedal 20 times with the engine off
- DMove forward slowly at about 5 mph, then push the brake pedal firmly to see that the vehicle stops without pulling to one sideCorrect
Why: To test the service brakes, wait for normal air pressure, release the parking brake, move ahead slowly (about 5 mph), and apply the brake pedal firmly. The vehicle should stop straight without pulling to one side. Pulling, delay, or an unusual pedal feel means the brakes need attention.
Based on: CDL Manual §5.3 — Inspecting Air Brake Systems (Test Service Brakes)