52 Air Brakes Practice Questions & Answers

1. 1. An air brake vehicle really contains three separate braking systems. Which three?

   • A.Service, parking and exhaust-brake systems
   • B.Service, hydraulic and emergency brakes
   • C.Primary, secondary and retarder systems
   • D.Service, parking and emergency brakes✓ Answer

   Compressed air is only the medium; the manual describes air brakes as three systems working together. The service brake applies and releases with the brake pedal in normal driving, the parking brake applies and releases with the parking brake control, and the emergency brake stops the vehicle if the brake system fails.

   Source: AAMVA Model CDL Manual, Section 5 (Air Brakes) — the three braking systems; 49 CFR 383.111(b)(1)Report a problem with this question

2. 2. What makes up the emergency brake system on an air brake vehicle?

   • A.A hydraulic backup circuit that is independent of the air system
   • B.A completely separate third set of brakes installed only for emergencies
   • C.The engine brake and the transmission retarder working together
   • D.Parts of the service brake and parking brake systems, used to stop the vehicle if the brake system fails✓ Answer

   There is no extra set of brake hardware for emergencies. The emergency system borrows components already present in the service and parking brake systems — most importantly the spring brakes, which are held off by air and set mechanically when air pressure is lost.

   Source: AAMVA Model CDL Manual, Section 5 — emergency brakes and spring brakesReport a problem with this question

3. 3. Which statement best describes the air compressor on an air brake vehicle?

   • A.It stores compressed air and releases it to the brake chambers when you press the pedal
   • B.It pumps air into the storage tanks, and if it has its own oil supply the oil level must be checked before driving✓ Answer
   • C.It regulates how much pressure reaches the brakes during an application
   • D.It is driven by the alternator and needs no oil or drive belt service

   The compressor is the pump of the system: it is driven off the engine by gears or a v-belt and fills the storage tanks. Storage is the tanks' job, and pressure to the brakes is set by the foot valve, not the compressor. Compressors with their own oil supply need that level checked like any other engine-driven pump.

   Source: AAMVA Model CDL Manual, Section 5 — air compressorReport a problem with this question

4. 4. At about what tank pressure does the governor normally stop the compressor from pumping air into the tanks (cut-out)?

   • A.About 100 psi
   • B.About 125 psi✓ Answer
   • C.About 60 psi
   • D.About 150 psi

   The governor is the pressure switch for the compressor. It cuts the compressor out at about 125 psi so the tanks are not over-pressurized; 150 psi is the safety valve setting and 100 psi is the cut-in point where pumping resumes.

   Source: AAMVA Model CDL Manual, Section 5 — air compressor governor (cut-out)Report a problem with this question

5. 5. At about what tank pressure does the governor let the compressor start pumping again (cut-in)?

   • A.About 125 psi
   • B.About 85 psi
   • C.About 60 psi
   • D.About 100 psi✓ Answer

   As brake applications use air, tank pressure falls. At about 100 psi the governor cuts the compressor back in so pressure is rebuilt long before it approaches the 60 psi low-air warning level.

   Source: AAMVA Model CDL Manual, Section 5 — air compressor governor (cut-in)Report a problem with this question

6. 6. The safety relief valve is installed in the first tank the compressor pumps to. What is its usual setting, and what does it mean if it releases air?

   • A.100 psi; it means the governor has cut in normally
   • B.60 psi; it means the low air pressure warning is about to come on
   • C.150 psi; it means something is wrong and the vehicle should be seen by a mechanic✓ Answer
   • D.125 psi; it means the compressor has reached cut-out

   The safety valve is a last-resort protection against over-pressurization and normally never opens, because the governor cuts the compressor out at about 125 psi. If it blows off at 150 psi, the governor or compressor is faulty and the problem must be repaired.

   Source: AAMVA Model CDL Manual, Section 5 — safety valve (first tank, 150 psi)Report a problem with this question

7. 7. Why do water and oil collect in air brake tanks, and why is that dangerous?

   • A.Rainwater enters through the drain valve, and the added weight unbalances the axle
   • B.Engine coolant leaks into the tanks and corrodes the brake chambers
   • C.The alcohol evaporator adds liquid that must be pumped out before each trip
   • D.Compressed air carries water vapor and compressor oil that settle in the bottom of the tank; in cold weather the water can freeze and cause brake failure✓ Answer

   Compressing air concentrates the moisture already in it, and a little compressor oil comes along with it. The mixture condenses in the tanks, and ice forming in tanks, lines or valves can block the air the brakes need to work.

   Source: AAMVA Model CDL Manual, Section 5 — air storage tanks and tank drains; 49 CFR 383.111(b)(2)Report a problem with this question

8. 8. On a vehicle whose air tanks have manually operated drain valves, how often must the driver drain the tanks?

   • A.Every 10,000 miles or at the next scheduled service
   • B.Once a week
   • C.Only when the low air pressure warning comes on
   • D.At the end of each day of driving✓ Answer

   Manual drains (a quarter-turn valve or a pull cable) do nothing unless the driver opens them, and water and oil accumulate every day the compressor runs. Draining at the end of each driving day keeps contamination from building up and freezing overnight.

   Source: AAMVA Model CDL Manual, Section 5 — manual tank drains; 49 CFR 393.50 (condensate drain valve on each reservoir)Report a problem with this question

9. 9. What is the purpose of an alcohol evaporator in an air brake system?

   • A.To put alcohol into the air system to reduce the risk of ice forming in the air brake valves in cold weather✓ Answer
   • B.To eliminate the need to drain the air tanks during winter
   • C.To absorb compressor oil before it reaches the tanks
   • D.To cool the compressed air so the tanks build pressure faster

   Alcohol lowers the freezing point of the moisture carried in the air, protecting valves from icing up. It is an anti-icing aid only: the container must be checked and filled daily in cold weather, and the tanks still have to be drained.

   Source: AAMVA Model CDL Manual, Section 5 — alcohol evaporatorReport a problem with this question

10. 10. What is the job of an air dryer and the supply ("wet") tank in an air brake system?

    • A.They warm the air lines so the service brakes apply faster
    • B.They raise the governor cut-out pressure so the tanks can hold more air
    • C.They remove moisture and oil from the compressed air before it reaches the service tanks✓ Answer
    • D.They measure how much air is delivered to the brake chambers

    Air leaving the compressor is hot and wet. The supply tank and air dryer sit first in line so condensate and oil drop out there instead of contaminating the service tanks, lines and valves. They reduce contamination but do not replace regular tank draining.

    Source: AAMVA Model CDL Manual, Section 5 — air supply system / contaminated air supply; 49 CFR 383.111(b)(2)Report a problem with this question

11. 11. How does the foot (treadle) valve control an air brake system?

    • A.The harder you press the pedal, the more air pressure is applied to the brakes; letting up releases air pressure and the brakes✓ Answer
    • B.The pedal pulls a mechanical cable that spreads the brake shoes directly
    • C.The pedal opens the governor so the compressor pumps straight to the brake chambers
    • D.Pedal position has no effect: the brakes are either fully on or fully off

    The treadle valve meters tank air to the brake chambers in proportion to pedal travel, so application pressure is under the driver's control. Because each application uses air from the tanks, repeated hard pumping wastes air and lowers system pressure.

    Source: AAMVA Model CDL Manual, Section 5 — foot brake (treadle) valve and application pressureReport a problem with this question

12. 12. What does the application pressure gauge show?

    • A.The governor cut-out setting
    • B.The temperature of the brake drums
    • C.How much air pressure you are applying to the brakes✓ Answer
    • D.How much air is left in the storage tanks

    Tank contents are shown by the separate supply pressure gauge; the application gauge reads only the pressure the treadle valve is sending to the chambers. Because of that, needing steadily more application pressure to hold the same speed is a warning that the brakes are fading or out of adjustment.

    Source: AAMVA Model CDL Manual, Section 5 — application pressure gaugeReport a problem with this question

13. 13. On a vehicle with air brakes, the low air pressure warning signal must come on before tank pressure drops below what level?

    • A.60 psi✓ Answer
    • B.100 psi
    • C.85 psi
    • D.45 psi

    A warning you can see — usually a red light, often with a buzzer — is required so the driver learns of a leak while there is still enough air for controlled braking. Sixty psi is well above the 20 to 45 psi range where the spring brakes come on by themselves. Large buses commonly warn earlier, at 80 to 85 psi.

    Source: AAMVA Model CDL Manual, Section 5 — low air pressure warning; 49 CFR 383.111(b)(4)Report a problem with this question

14. 14. What does a "wig wag" low air pressure warning device do?

    • A.It drops a mechanical arm into the driver's view when pressure falls below about 60 psi✓ Answer
    • B.It flashes the trailer marker lights when air pressure is low
    • C.It sounds a horn whenever the parking brake is applied
    • D.It automatically drains the air tanks when moisture is detected

    The wig wag is a mechanical version of the low air warning, placed where the driver cannot miss it. The automatic type lifts back out of view once pressure rises above about 60 psi; the manual-reset type must be pushed up by hand and will not stay put until pressure is above 60 psi.

    Source: AAMVA Model CDL Manual, Section 5 — wig wag low pressure warning deviceReport a problem with this question

15. 15. In an s-cam drum brake, what happens after you press the brake pedal?

    • A.Air enters the brake chamber, the push rod moves out, the slack adjuster turns the brake camshaft, and the s-cam forces the shoes against the drum✓ Answer
    • B.Air enters the brake chamber, the s-cam turns the slack adjuster, and the push rod spreads the shoes
    • C.The push rod turns a power screw and the caliper squeezes the shoes against the camshaft
    • D.The slack adjuster moves the push rod, air enters the chamber, and the drum clamps a rotor

    The s-cam brake converts air pressure into mechanical force in a fixed order: chamber, push rod, slack adjuster, camshaft, s-cam, shoes. The cam is named for its S shape, and when the pedal is released the cam rotates back and a return spring pulls the shoes away from the drum.

    Source: AAMVA Model CDL Manual, Section 5 — s-cam foundation brakesReport a problem with this question

16. 16. Why does excessive push rod stroke (brakes out of adjustment) matter on an s-cam air brake?

    • A.It makes the governor cut out early and starves the tanks of air
    • B.It has no effect as long as the low air pressure warning stays off
    • C.The s-cam must rotate farther before the shoes reach the drum, so the brake delivers less force; out-of-adjustment brakes are the most common defect found in roadside inspections, and spring brake power also depends on brakes being in adjustment✓ Answer
    • D.It affects only the parking brake, never the service brakes

    Braking force depends on the cam being able to press the shoes hard against the drum within its available travel. Too much slack wastes that travel, so a vehicle with brakes out of adjustment is much harder to stop, and the mechanical spring brakes are weakened for the same reason.

    Source: AAMVA Model CDL Manual, Section 5 — slack adjusters and brake adjustment; 49 CFR 393.47 (pushrod stroke limits)Report a problem with this question

17. 17. In an air-operated disc brake, what takes the place of the s-cam?

    • A.A wedge driven between the ends of the two brake shoes
    • B.A power screw that makes the caliper clamp the brake pads against the rotor✓ Answer
    • C.A hydraulic master cylinder fed by the air chamber
    • D.A second brake chamber mounted inside the drum

    Air disc brakes use the same chamber and slack adjuster as an s-cam brake, but the cam is replaced by a power screw that squeezes the caliper like a large c-clamp. The wedge described in option 3 belongs to wedge brakes, where the push rod drives a wedge directly between the shoe ends; both wedge and disc designs are less common than s-cam.

    Source: AAMVA Model CDL Manual, Section 5 — disc brakes and wedge brakesReport a problem with this question

18. 18. Which statement about the air lines and glad hands between a tractor and trailer is true?

    • A.The blue glad hand is the service line and the red one is the emergency (supply) line; crossing them can leave you with no trailer brakes when you need them✓ Answer
    • B.Both lines carry the same air, so it makes no difference if they are crossed
    • C.The blue line is the emergency (supply) line and the red line is the service line
    • D.Crossed glad hands are harmless because the tractor protection valve prevents any loss of air

    Color coding exists precisely because the two lines do different jobs: the service line carries the pressure created by the treadle valve, while the supply line charges the trailer tanks and holds the trailer spring brakes off. Crossed connections can let the trailer roll with no service braking, so the couplers are also shaped and positioned to discourage mis-mating.

    Source: AAMVA Model CDL Manual — air line and glad hand color coding (service = blue, emergency/supply = red); 49 CFR 383.111(b)(3)Report a problem with this question

19. 19. What does the tractor protection valve do?

    • A.It drains moisture out of the trailer air tanks
    • B.It limits front-axle brake pressure on slippery roads
    • C.It keeps air in the tractor's brake system if the trailer breaks away or a bad leak develops, closing automatically when air pressure falls to somewhere between about 20 and 45 psi✓ Answer
    • D.It releases the trailer spring brakes whenever the tractor is parked

    Without this valve, a trailer breakaway would vent the tractor's air through the open lines and leave the tractor unable to stop. Federal rules require the device to operate automatically in the 20 to 45 psi range, which is also where the trailer's own spring brakes set.

    Source: AAMVA Model CDL Manual — tractor protection valve; 49 CFR 393.43Report a problem with this question

20. 20. The red, eight-sided knob on the dash of a tractor controls what?

    • A.The front brake limiting valve
    • B.The trailer air supply: pushed in it sends air to the trailer, pulled out it shuts the air off and sets the trailer emergency and spring brakes✓ Answer
    • C.The alcohol evaporator
    • D.The tractor parking brake only

    The red octagonal knob is the trailer air supply control and it also operates the tractor protection valve. It must be pushed in before the trailer will charge and roll, and it pops out on its own if air pressure drops far enough.

    Source: AAMVA Model CDL Manual — trailer air supply control (red eight-sided knob)Report a problem with this question

21. 21. The yellow, diamond-shaped knob is the parking brake control. How is it operated?

    • A.It works only while the engine is running and pressure is above 125 psi
    • B.Pull it out to apply the parking brakes and push it in to release them✓ Answer
    • C.Push it in to apply the parking brakes and pull it out to release them
    • D.Turn it clockwise to apply and counterclockwise to release

    Pulling the knob out exhausts air from the spring brake chambers so the springs set the brakes; pushing it in re-applies air and holds the springs back. Because the springs hold the vehicle mechanically, never press the brake pedal while the spring brakes are applied — the combined force can damage the brakes.

    Source: AAMVA Model CDL Manual, Section 5 — parking brake control (yellow diamond-shaped knob)Report a problem with this question

22. 22. Spring brakes on a truck or tractor are held released by air pressure while driving. If air is lost, at about what pressure do they come fully on?

    • A.Somewhere in the 20 to 45 psi range✓ Answer
    • B.Only after pressure reaches zero
    • C.Between 60 and 85 psi
    • D.Between 90 and 100 psi

    Parking and emergency brakes must be held on by mechanical force because air eventually leaks away, so powerful springs do the work and air only holds them back. Truck and tractor spring brakes set in the 20 to 45 psi range, typically 20 to 30 psi — well below the 60 psi warning point, which is why a driver should stop while air remains rather than wait for the springs.

    Source: AAMVA Model CDL Manual, Section 5 — spring brakes; 49 CFR 393.43 (20 to 45 psi)Report a problem with this question

23. 23. What is a modulating control valve used for?

    • A.To drain the supply tank automatically
    • B.To apply the spring (parking) brakes gradually so the driver keeps some control if the service brakes fail✓ Answer
    • C.To cut front-axle brake pressure in half on slippery roads
    • D.To meter alcohol into the air lines in cold weather

    Spring brakes are normally all-or-nothing. A spring-loaded modulating handle on the dash lets the driver feed them on in degrees, which is what makes them usable as a controlled emergency stop; when parking, the lever is moved as far as it goes and locked.

    Source: AAMVA Model CDL Manual, Section 5 — modulating control valveReport a problem with this question

24. 24. Which statement about antilock braking systems (ABS) on air-brake vehicles is true?

    • A.ABS always shortens stopping distance
    • B.If the ABS fails, the vehicle has no brakes at all
    • C.ABS replaces the normal service brakes
    • D.ABS is an addition to the normal brakes: it does not change normal braking capability and works only when the wheels are about to lock, helping you stay in control and steer✓ Answer

    ABS only intervenes at the threshold of wheel lock, releasing and reapplying pressure so a rolling wheel can still be steered. It preserves control rather than guaranteeing a shorter stop, and if it malfunctions the normal air brakes remain fully available.

    Source: AAMVA Model CDL Manual, Section 5 — antilock braking systems; 49 CFR 383.111(b)(7)(ii)Report a problem with this question

25. 25. What color is the ABS malfunction lamp, and where is it located on a trailer?

    • A.Blue, next to the glad hands
    • B.Green, inside the tractor cab only
    • C.Red, on the rear of the trailer
    • D.Yellow, on the left side of the trailer (front or rear corner)✓ Answer

    ABS malfunction lamps are yellow: on the instrument panel for tractors, trucks and buses, and on the left side of a trailer or converter dolly so the driver can see it in the mirror. The lamp normally does a bulb check at start-up and goes out; if it stays on or comes on while driving, ABS may be lost at one or more wheels while the normal brakes still work.

    Source: AAMVA Model CDL Manual, Section 5 — ABS malfunction lampsReport a problem with this question

26. 26. Why does stopping with air brakes take longer than stopping with hydraulic brakes at the same speed?

    • A.Air brakes will not work until the governor cuts in
    • B.Air brake linings are designed with a lower coefficient of friction
    • C.It takes about a half second or more for air to travel through the lines to the brakes, adding a brake lag distance of roughly 32 feet at 55 mph✓ Answer
    • D.Air brakes apply less force to the shoes than hydraulic brakes can

    Hydraulic fluid is nearly incompressible and acts almost instantly, but air must flow and build pressure in every chamber first. That delay adds a fourth element to total stopping distance — perception, reaction, brake lag and effective braking distance — so at 55 mph a loaded rig needs over 450 feet to stop.

    Source: AAMVA Model CDL Manual, Section 5 — brake lag and total stopping distanceReport a problem with this question

27. 27. When you test the low air pressure warning signal during the final air brake check, how must the vehicle be set up before you begin stepping on and off the brake pedal?

    • A.Engine running at operating rpm with the parking brake released
    • B.Engine shut off and the electrical power turned off
    • C.Engine running at fast idle with the parking brake applied
    • D.Engine shut off, electrical power turned on, and enough air pressure that the warning is already off✓ Answer

    With the engine off the compressor cannot replace air, so fanning the pedal steadily bleeds the tanks down; the electrical power must be on so the warning light or buzzer can actually operate, and you must start above the warning point in order to see it come on.

    Source: AAMVA Model CDL Manual 5.3.3 (Test Low Pressure Warning Signal)Report a problem with this question

28. 28. The low air pressure warning signal must come on before tank pressure falls below what level?

    • A.85 psi
    • B.45 psi
    • C.60 psi✓ Answer
    • D.75 psi

    Federal equipment rules require a warning the driver can see before pressure drops below 60 psi (or one-half the governor cut-out pressure on older vehicles), which leaves enough air in the tanks to make a controlled stop before the spring brakes apply themselves at 20-45 psi.

    Source: AAMVA Model CDL Manual 5.1.10 and 5.3.3; 49 CFR 393.51Report a problem with this question

29. 29. You keep fanning off air pressure after the low air warning comes on. At what pressure should the parking brake valve (and, on a tractor-trailer, the tractor protection valve) pop out?

    • A.60 to 75 psi
    • B.20 to 40 psi✓ Answer
    • C.45 to 60 psi
    • D.10 to 15 psi

    Spring brakes are held released by air pressure, so when pressure falls to the manufacturer's setting of about 20 to 40 psi the valves close and the springs apply the brakes mechanically; this check proves the fail-safe still works.

    Source: AAMVA Model CDL Manual 5.3.3 (Check That Spring Brakes Come On Automatically)Report a problem with this question

30. 30. With the engine at operating rpm, a dual air brake system should build pressure from 85 to 100 psi within how long?

    • A.30 seconds
    • B.3 minutes
    • C.45 seconds✓ Answer
    • D.15 seconds

    The 45-second standard shows the compressor and governor can replace air as fast as normal braking uses it; a slower build means the compressor, belt, or governor is failing and you could run short of air in traffic. (Larger-than-minimum tanks may legitimately take longer, so check manufacturer specs.)

    Source: AAMVA Model CDL Manual 5.3.3 (Check Rate of Air Pressure Buildup)Report a problem with this question

31. 31. On an older vehicle with a single air brake system, what buildup rate shows the compressor is doing its job?

    • A.0 to 125 psi within 30 seconds at full throttle
    • B.60 to 120 psi within 90 seconds at operating rpm
    • C.85 to 100 psi within 45 seconds
    • D.50 to 90 psi within 3 minutes at an engine idle of 600-900 rpm✓ Answer

    Single-circuit systems on pre-1975 vehicles use smaller compressors and are checked at normal idle rather than at operating rpm, so the manual gives a separate, slower benchmark; applying the 45-second dual-system figure to them would wrongly condemn a healthy compressor.

    Source: AAMVA Model CDL Manual 5.3.3 (Check Rate of Air Pressure Buildup)Report a problem with this question

32. 32. How do you check governor cut-in and cut-out during the air brake check, and what readings are correct?

    • A.With the engine off, watch for the compressor to start at 60 psi and stop at 90 psi
    • B.Run at fast idle until the compressor cuts out at about 125 psi, then fan the pedal at idle until it cuts in at about 100 psi✓ Answer
    • C.With the parking brake applied, watch for cut-out at about 150 psi
    • D.At fast idle, watch for cut-in at about 125 psi and cut-out at about 100 psi

    The governor tells the compressor when to pump: it cuts out at about 125 psi and cuts back in at about 100 psi, so you must raise pressure to see cut-out and then bleed it down to see cut-in. The 150 psi figure belongs to the safety relief valve, not the governor.

    Source: AAMVA Model CDL Manual 5.1.2 and 5.3.3 (Check Air Compressor Governor Cut-in and Cut-out Pressures)Report a problem with this question

33. 33. With a fully charged system, the engine shut off and the parking brake released, what is the maximum air pressure loss allowed in one minute?

    • A.2 psi for a single vehicle and 3 psi for a combination vehicle✓ Answer
    • B.4 psi for a single vehicle and 6 psi for a combination vehicle
    • C.3 psi for any vehicle, single or combination
    • D.3 psi for a single vehicle and 4 psi for a combination vehicle

    This is the static (brakes released) test: the engine must be off so the compressor cannot mask a leak, and a combination is allowed one extra psi because it has additional lines, glad hands, and trailer valves that can seep. The 3/4 psi pair applies only after the brakes are applied.

    Source: AAMVA Model CDL Manual 5.3.3 (Test Air Leakage Rate)Report a problem with this question

34. 34. After the static leakage check, you press and hold the brake pedal. How much pressure must you apply, and what applied leakage rate is too much?

    • A.Apply 90 psi; more than 2 psi for a single vehicle or 3 psi for a combination
    • B.Apply 90 psi or more; more than 3 psi in one minute for a single vehicle or 4 psi for a combination✓ Answer
    • C.Apply 125 psi; more than 5 psi in one minute for any vehicle
    • D.Apply 60 psi; more than 2 psi for a single vehicle or 3 psi for a combination

    Holding at least 90 psi pressurizes the service side of the system, so leaks in the brake chambers, hoses and application valves show up that a released-brake test would miss; the allowable rates are one psi higher than the static limits because more of the system is under pressure.

    Source: AAMVA Model CDL Manual 5.3.3 (Test Air Leakage Rate)Report a problem with this question

35. 35. Before driving a vehicle with a dual air brake system, how much pressure must you let the compressor build?

    • A.At least 85 psi total
    • B.At least 125 psi in the secondary system only
    • C.At least 60 psi in the primary system
    • D.A minimum of 100 psi in both the primary and the secondary systems✓ Answer

    Each half of a dual system feeds different axles from its own tanks, so 100 psi in each confirms both halves are charging and leaves reserve for several brake applications; if one side is low, those axles will not brake fully and the vehicle will take longer to stop.

    Source: AAMVA Model CDL Manual 5.2 (Dual Air Brake Systems)Report a problem with this question

36. 36. Before pulling on slack adjusters to check free play, what must you do?

    • A.Apply the parking brake and leave the engine running
    • B.Charge the system to 125 psi and hold the brake pedal down
    • C.Raise the axle with a jack and remove the wheel
    • D.Park on level ground, chock the wheels, release the parking brakes, and wear gloves✓ Answer

    The slack adjuster can only move if the spring brakes are released, but with the parking brake off the vehicle can roll, so chocks on level ground are what keep you safe; gloves protect your hands on sharp, greasy hardware.

    Source: AAMVA Model CDL Manual 5.3.2 (Slack Adjusters on S-cam Brakes)Report a problem with this question

37. 37. You pull hard on a slack adjuster and it moves about an inch and a half where the push rod attaches. What does that tell you?

    • A.The air tanks need to be drained
    • B.The brakes are correctly adjusted
    • C.The compressor drive belt is loose
    • D.The brake probably needs adjustment, and too much slack can make the vehicle very hard to stop✓ Answer

    Movement of more than about one inch means the push rod stroke is too long, so the s-cam rotates less and the shoes press the drum with less force; out-of-adjustment brakes are the most common defect found in roadside inspections precisely because they quietly lengthen stopping distance.

    Source: AAMVA Model CDL Manual 5.3.2 (Slack Adjusters on S-cam Brakes)Report a problem with this question

38. 38. A vehicle has automatic slack adjusters and one shows pushrod stroke beyond the legal adjustment limit. What is the correct response?

    • A.Take the vehicle to a repair facility, because excessive stroke on an automatic adjuster indicates a mechanical problem✓ Answer
    • B.Do nothing, because automatic adjusters correct themselves once the vehicle is driven
    • C.Manually adjust it and keep the vehicle in service indefinitely
    • D.Drain the air tanks and re-check the stroke

    An automatic adjuster that lets stroke go out of limits has failed, or a related foundation brake component has, so manually adjusting it only masks the fault, can cause premature wear, and the condition will return; manual adjustment is acceptable only as a temporary emergency measure.

    Source: AAMVA Model CDL Manual 5.3.2 (Slack Adjusters on S-cam Brakes)Report a problem with this question

39. 39. During the walkaround, how large a crack in a brake drum or disc is still acceptable?

    • A.Any crack that does not go all the way through the drum
    • B.No crack longer than one half the width of the friction area✓ Answer
    • C.Cracks up to the full width of the friction area
    • D.Cracks up to two inches long

    Braking heat and clamping force make cracks grow, so the manual sets the limit at half the width of the friction area; beyond that the drum or disc can fail in service and that wheel loses braking entirely.

    Source: AAMVA Model CDL Manual 5.3.2 (Check Brake Drums (or Discs), Linings, and Hoses)Report a problem with this question

40. 40. Why must the air tanks be drained at the end of each working day on a vehicle that does not have automatic drains?

    • A.To relieve pressure so that the spring brakes will release
    • B.Because water and compressor oil collect in the tanks, and water can freeze in cold weather and cause brake failure✓ Answer
    • C.Because draining resets the governor cut-out setting
    • D.Because otherwise the safety valve will open at 150 psi

    Compressed air carries water vapor and compressor oil that settle in the bottom of the tanks; the sludge fouls valves and reduces usable tank volume, and in freezing weather the water turns to ice that blocks lines and valves so the brakes will not work.

    Source: AAMVA Model CDL Manual 5.1.4 and 5.4.8; 49 CFR 396.3Report a problem with this question

41. 41. What is the correct way to test that the parking brake holds?

    • A.With the vehicle stopped, set the parking brake and gently pull against it in a low gear✓ Answer
    • B.Roll along at 20 mph and pull the parking brake control out
    • C.Apply the parking brake and push hard on the brake pedal at the same time
    • D.Rev the engine in neutral and listen for escaping air

    Gently applying engine torque in a low gear is the only way to prove the spring force plus the brake adjustment can actually hold the loaded vehicle. Never push the brake pedal down while the spring brakes are applied, because the combined spring and air force can damage the brakes.

    Source: AAMVA Model CDL Manual 5.3.3 (Test Parking Brake) and 5.1.15Report a problem with this question

42. 42. During the service brake test you roll forward at about 5 mph, apply the brakes firmly, and the vehicle pulls hard to the right. What does this mean?

    • A.The alcohol evaporator needs to be refilled
    • B.A brake problem, such as brakes out of adjustment or unequal braking, that must be corrected before the vehicle is driven✓ Answer
    • C.Air pressure is above the governor cut-out setting
    • D.Normal behavior on a crowned road surface; no action is needed

    The low-speed service brake test is designed to reveal pulling to one side, unusual pedal feel, or delayed stopping action, all of which mean the brakes are not applying evenly. A driver may not operate a commercial vehicle unless satisfied the service brakes are in good working order.

    Source: AAMVA Model CDL Manual 5.3.3 (Test Service Brakes); 49 CFR 392.7Report a problem with this question

43. 43. What is the correct way to make a normal stop with air brakes?

    • A.Use stab braking so the wheels cannot lock
    • B.Push the pedal all the way to the floor and hold it there
    • C.Push the pedal down just hard enough to bring a smooth, safe stop, controlling the pressure all the way✓ Answer
    • D.Pump the pedal repeatedly to keep tank pressure up

    The brake pedal is a proportional valve: the harder you push, the more air pressure is applied, so smooth control of pedal pressure gives a smooth stop. Fanning the pedal wastes air faster than the compressor can replace it, and a full application invites wheel lockup.

    Source: AAMVA Model CDL Manual 5.4.1 (Normal Stops) and 5.1.7Report a problem with this question

44. 44. What is controlled braking?

    • A.Slowing the combination with the trailer hand valve only
    • B.Repeatedly pressing and releasing the pedal to build tank pressure
    • C.Applying the brakes fully and releasing them each time the wheels lock
    • D.Applying the brakes as hard as you can without locking the wheels, keeping steering wheel movements very small✓ Answer

    Keeping the wheels just short of lockup preserves tire traction, so the vehicle stops in a straight line and you can still steer; if you have to steer more than a little, or the wheels do lock, you release the brakes and re-apply as soon as you can.

    Source: AAMVA Model CDL Manual 5.4.3 (Emergency Stops - Controlled Braking)Report a problem with this question

45. 45. When you use stab braking in an emergency stop, when do you re-apply the brakes?

    • A.Immediately after releasing, without waiting at all
    • B.After counting three full seconds
    • C.As soon as the wheels start rolling again, which can take up to one second after you release✓ Answer
    • D.Only after the vehicle has slowed below 5 mph

    Stab braking is: apply fully, release when the wheels lock, then re-apply once they are rolling again. Rolling wheels are what restore directional stability, so re-applying before they start to roll leaves the vehicle sliding and it will not straighten out.

    Source: AAMVA Model CDL Manual 5.4.3 (Emergency Stops - Stab Braking)Report a problem with this question

46. 46. What is the most important thing to do before you start down a long, steep grade?

    • A.Apply the parking brake lightly to help hold your speed
    • B.Be in the proper low gear before you start down, because the brakes only supplement the engine's braking effect✓ Answer
    • C.Release the trailer hand valve and coast down
    • D.Fan the brakes a few times to make sure they are cool

    The engine, held in a low gear, does most of the speed control on a grade; once the vehicle picks up speed you may not be able to downshift at all, and the service brakes alone will absorb so much heat that they fade. An engine brake or retarder helps but does not replace choosing the right gear first.

    Source: AAMVA Model CDL Manual 5.4.6 (Proper Braking Technique)Report a problem with this question

47. 47. Using the manual's downgrade braking technique, when do you release the brakes and about how long should each application last?

    • A.Release when speed has dropped about 5 mph below your safe speed, with each application lasting about three seconds✓ Answer
    • B.Never release; hold light steady pressure the whole way down
    • C.Release when speed drops 15 mph below your safe speed, with applications lasting about ten seconds
    • D.Release as soon as you feel any slowdown, applying for less than one second at a time

    This is snub braking: a firm application held about three seconds sheds speed quickly, and releasing lets air flow through the drums so they cool before the next snub. Riding the brakes with light continuous pressure keeps adding heat with no cooling period, which is what produces brake fade.

    Source: AAMVA Model CDL Manual 5.4.6 (Proper Braking Technique)Report a problem with this question

48. 48. What physically happens inside a drum brake when it overheats and fades?

    • A.The slack adjusters shorten and lock the wheels
    • B.The air lines expand and tank pressure drops below 60 psi
    • C.Heat changes the lining chemically so it loses friction, and the drums expand so the shoes must travel farther to touch them✓ Answer
    • D.The governor cuts out early and the compressor stops pumping

    Both effects reduce braking force at once: less friction from the hot lining and a larger drum that the shoes reach with less remaining stroke. Brakes that are out of adjustment stop doing their share first, so the remaining brakes overheat and fade even sooner.

    Source: AAMVA Model CDL Manual 5.4.5 (Brake Fading or Failure)Report a problem with this question

49. 49. Your brakes are soaked after driving through deep water and the temperature is below freezing. What should you do?

    • A.Set the parking brake so the shoes dry against the drums
    • B.Increase your speed so airflow dries the brakes
    • C.Park and set the parking brake, then check the brakes in the morning
    • D.Dry them by driving slowly in a low gear while applying the brakes lightly, and if you must park, use wheel chocks instead of the parking brake✓ Answer

    Light drag in a low gear generates just enough heat to evaporate the water from the linings. You normally set the parking brake whenever you park, but not when the brakes are very wet in freezing temperatures (they can freeze on) or very hot after a steep grade (heat can damage them) - chock the wheels on level ground instead.

    Source: AAMVA Model CDL Manual 5.4.8 (Parking Brakes / Wet Brakes)Report a problem with this question

50. 50. Total stopping distance with air brakes is made up of which parts?

    • A.Perception distance plus reaction distance plus brake lag distance plus effective braking distance✓ Answer
    • B.Perception distance plus reaction distance plus effective braking distance
    • C.Reaction distance plus braking distance plus following distance
    • D.Perception distance plus braking distance only

    Unlike hydraulic brakes, which act instantly, air must travel through the lines, adding a lag of half a second or more before the brakes work; at 55 mph that lag alone adds about 32 feet, and the total stop for an average driver in good conditions is over 450 feet.

    Source: AAMVA Model CDL Manual 5.4.4 (Stopping Distance)Report a problem with this question

51. 51. The low air pressure warning light comes on while you are driving. What should you do?

    • A.Continue to the nearest terminal, since the spring brakes will stop you if you need them
    • B.Stop and safely park as soon as possible, while you still have enough air to use the service brakes✓ Answer
    • C.Pump the brake pedal repeatedly to build the pressure back up
    • D.Pull the parking brake control out immediately at highway speed

    Controlled braking is possible only while air remains in the tanks. If you keep going, the spring brakes come on by themselves somewhere in the 20 to 45 psi range, and because spring brakes do not work on all axles a loaded vehicle will take a very long distance to stop while a light vehicle or one on a slippery road may skid out of control.

    Source: AAMVA Model CDL Manual 5.4.7 (Low Air Pressure)Report a problem with this question

52. 52. Why should the trailer hand valve never be used to park a combination or to control speed on a downgrade?

    • A.Because it applies only the steering axle brakes
    • B.Because it holds the trailer with air pressure that can leak away while parked, and on a grade it makes the trailer brakes absorb all the heat, inviting fade and a trailer skid or jackknife✓ Answer
    • C.Because it bypasses the low air pressure warning system
    • D.Because it locks out the service brake pedal until it is released

    Parking must be held by mechanical spring force, not by air that can bleed down and release the brakes; and applying only the trailer brakes on a long grade concentrates all the braking heat on one set of brakes and can make the trailer swing out.

    Source: AAMVA Model CDL Manual 6.2.4 (Trailer Hand Valve) and 5.4.8Report a problem with this question

Practice questions based on the AAMVA/FMCSA model CDL manual. Requirements vary by state — study your state's official CDL manual before your exam. Official CDL info →