CCDL Permit Prep

Air Brakes · page 3 of 4

Air Brakes questions 41–60

Part of the 67-question Air Brakes pool. Each answer cites the FMCSA CDL Manual or the federal rule it comes from.

Practise these as a timed set

  1. Which air tanks must be drained completely?

    • AEach tank, including the supply tankCorrect
    • BOnly the last tank in line
    • COnly the trailer tank
    • DOnly when a warning shows

    FMCSA Section 5.1.4 says to drain the air tanks completely. Manually drained tanks must be drained at the end of each day; automatic drains expel water and oil automatically.

    FMCSA Commercial Driver License Manual · Section 5.1.4: Air Tank Drains · verified 2026-09-08

  2. What does an automatic drain valve do?

    • AIncreases the tank pressure
    • BExpels water without manual drainingCorrect
    • CWarns of low air supply
    • DLocks the parking brakes

    FMCSA Section 5.1.4 says automatic drains expel water and oil automatically. Some tanks may also be equipped for manual draining.

    FMCSA Commercial Driver License Manual · Section 5.1.4: Air Tank Drains · verified 2026-09-08

  3. What does the application pressure gauge show?

    • AThe temperature of the drums
    • BHow much air remains in the tanks
    • CHow much air the brakes are usingCorrect
    • DThe wear left on the linings

    FMCSA Section 5.1.10 says the application pressure gauge shows how much air pressure is being applied to the brakes. Increasing pressure to hold the same speed means the brakes may be fading.

    FMCSA Commercial Driver License Manual · Section 5.1.10: Application Pressure Gauge · verified 2026-09-08

  4. After a hazard becomes visible, which distance ends when the driver recognizes it?

    • AHow far you travel before recognising a hazardCorrect
    • BThe distance the brakes need to stop
    • CThe gap the air takes to reach the wheels
    • DThe length of the vehicle itself

    FMCSA Section 2.6.1 defines perception distance as the distance the vehicle travels from the time the driver's eyes see a hazard until the brain recognizes it.

    FMCSA Commercial Driver License Manual · Section 2.6.1: Stopping Distance · verified 2026-09-08

  5. How does doubling your speed affect braking distance?

    • AIt leaves it unchanged
    • BIt roughly doubles it
    • CIt roughly quadruples itCorrect
    • DIt roughly halves it

    FMCSA Section 2.6.1 includes a knowledge check asking whether doubling speed increases stopping distance by two or four times. Under otherwise comparable conditions, the answer is about four times.

    FMCSA Commercial Driver License Manual · Section 2.6.1: Stopping Distance · verified 2026-09-08

  6. Why can an empty truck require more stopping distance than the same truck when fully loaded?

    • AAn empty truck has less tire tractionCorrect
    • BIts brakes do not work until cargo is added
    • CIts air compressor runs more slowly
    • DIts brake lag always doubles

    A heavier vehicle makes the brakes do more work, but truck brakes, tires, springs, and shock absorbers are designed to work best when fully loaded. An empty truck can require more stopping distance because it has less traction.

    FMCSA Commercial Driver License Manual · 2.6.1 — Stopping Distance · verified 2026-09-07

  7. Where is the trailer's ABS malfunction lamp usually mounted?

    • AOn the tractor's bumper
    • BInside the trailer body
    • COn the trailer's left sideCorrect
    • DUnder the fifth wheel

    FMCSA Section 2.18.3 says trailers have yellow ABS malfunction lamps on the left side, either at the front or rear corner.

    FMCSA Commercial Driver License Manual · Section 2.18.3: How to Know If Your Vehicle Is Equipped with ABS · verified 2026-09-08

  8. What does an automatic slack adjuster do?

    • AKeeps pushrod travel within rangeCorrect
    • BDrains the air tanks
    • CCools the brake drums
    • DApplies the parking brake

    FMCSA Section 5.3.2 says automatic slack adjusters adjust themselves during full brake applications, but they must still be checked.

    FMCSA Commercial Driver License Manual · Section 5.3.2: During Step 5 Walk-around Inspection · verified 2026-09-08

  9. Why should you not adjust an automatic slack adjuster manually?

    • AIt voids the vehicle registration
    • BIt can give false security without fixing the underlying problemCorrect
    • CIt releases the spring brakes
    • DIt drains the air tanks

    FMCSA Section 5.3.2 says manual adjustment of an automatic slack adjuster is dangerous because it may give a false sense of security and usually does not fix the underlying adjustment problem. It is only a temporary emergency measure.

    FMCSA Commercial Driver License Manual · Section 5.3.2: During Step 5 Walk-around Inspection · verified 2026-09-08

  10. What should you check about the compressor drive belt?

    • AThat it turns freely by hand
    • BThat it is painted black
    • CGood condition and proper tensionCorrect
    • DThat it is removed in summer

    FMCSA Section 5.3.1 says to check a belt-driven air compressor's drive belt for condition and tightness and to ensure it is in good condition.

    FMCSA Commercial Driver License Manual · Section 5.3.1: During Step 2 Engine Compartment Checks · verified 2026-09-08

  11. Why should you avoid riding the brakes on a long descent?

    • AIt uses more compressed air
    • BConstant heat causes brake fadeCorrect
    • CIt wears the tires unevenly
    • DIt drains the battery

    FMCSA Sections 2.6.6 and 5.4.5 say engine braking should be the principal control on downgrades. Excessive service-brake use creates heat and can cause brake fade or failure.

    FMCSA Commercial Driver License Manual · Section 2.6.6: Mountain Driving; Section 5.4.5: Brake Fading or Failure · verified 2026-09-08

  12. Your vehicle is in the correct low gear and reaches its selected safe speed on a long downgrade. What service-brake cycle applies?

    • AOnly with the trailer hand valve
    • BLightly and continuously
    • CSlow about 5 mph below the safe speed, release, and repeat when safe speed returnsCorrect
    • DOnly with the parking brake

    Use enough pressure for a definite slowdown. Service brakes supplement engine braking; this is not a continuous light application or trailer-only braking.

    Oregon DMV Commercial Driver Manual · 2.16.4 · verified 2026-09-07

  13. What should you do if air pressure keeps falling while parked?

    • AIgnore it if the warning is off
    • BTop it up and set off
    • CRelease the parking brake
    • DFind the leak before drivingCorrect

    FMCSA Section 5.3.3 says excessive air loss must be checked for leaks and fixed before driving; otherwise the vehicle could lose its brakes while driving.

    FMCSA Commercial Driver License Manual · Section 5.3.3: Step 7 Final Air Brake Check · verified 2026-09-08

  14. Why are air brakes fitted with more than one reservoir?

    • ATo make the vehicle heavier
    • BTo store enough air for repeated stopsCorrect
    • CTo cool the compressed air
    • DTo hold the parking brake fluid

    FMCSA Section 5.1.3 says air storage tanks hold enough air to allow the brakes to be used several times even if the compressor stops working.

    FMCSA Commercial Driver License Manual · Section 5.1.3: Air Storage Tanks · verified 2026-09-08

  15. What should you do before testing the low-pressure warning?

    • ADrain every tank completely
    • BRelease the parking brake and drive
    • CBuild full pressure and shut the engine offCorrect
    • DDisconnect the trailer lines

    FMCSA Section 5.3.3 says to shut the engine off when there is enough air pressure that the warning is not on, turn electrical power on, and fan down the pressure with the brake pedal.

    FMCSA Commercial Driver License Manual · Section 5.3.3: Step 7 Final Air Brake Check · verified 2026-09-08

  16. What does the governor cut-out pressure mean?

    • AThe point the springs engage
    • BThe point the brakes fully apply
    • CThe point the warning activates
    • DThe point the compressor stops pumpingCorrect

    FMCSA Sections 5.1.2 and 5.3.3 describe cut-out as the pressure at which the governor stops the compressor from pumping; the compressor starts again at the lower cut-in pressure.

    FMCSA Commercial Driver License Manual · Section 5.1.2: Air Compressor Governor; Section 5.3.3: Step 7 Final Air Brake Check · verified 2026-09-08

  17. Why must you build normal operating pressure and complete the air-brake checks before releasing the parking brake to move?

    • ATo warm the compressor so it can reach the governor cut-out setting
    • BTo confirm enough stored air for controlled service braking and safe system operationCorrect
    • CTo let the spring brakes cool before they are released for the trip
    • DTo drain moisture from the tanks, which only happens at full pressure

    Low air does not mean all service braking disappears at once, but braking becomes limited as pressure falls and spring brakes may apply. Build normal pressure and complete the required checks before moving so the system has enough stored air for controlled braking.

    FMCSA Commercial Driver License Manual · 5.2 and 5.3.3 — Dual Systems and Final Air Brake Check · verified 2026-09-29

  18. Under 49 CFR §396.7(a), which vehicle condition is prohibited from operation?

    • AA motor vehicle in a condition likely to cause an accident or breakdown.Correct
    • BA vehicle with a clean inspection report.
    • CA vehicle with a working service-brake system.
    • DA vehicle that has reached its scheduled inspection date but is safe to operate.

    Section 396.7(a) prohibits operating a motor vehicle in a condition likely to cause an accident or breakdown. Paragraph (b) has a narrow exception for moving an unsafe vehicle discovered on the highway to the nearest repair place.

    Electronic Code of Federal Regulations: 49 CFR §396.7 · §396.7(a)–(b): Unsafe operations forbidden and nearest-repair exception · verified 2026-09-27

  19. At what pressure does the governor normally start the compressor pumping again?

    • AAbout 60 psi
    • BAbout 20 psi
    • CAbout 150 psi
    • DAbout 100 psiCorrect

    Cut-in is the low end of the governor's range, around 100 psi, and cut-out the high end near 125 psi. The compressor loads and unloads between those two points instead of running constantly.

    FMCSA 2005 CDL Testing System Model Manual (July 2014) · Section 5.1.2 — Air Compressor Governor; Oregon Manual § 5.1.2 cross-check · verified 2026-09-19

  20. Under the federal CDL rule, when must an air-brake restriction be indicated on a CLP or CDL, if issued?

    • AAfter failing the air-brake knowledge component or taking the skills test in a vehicle without air brakesCorrect
    • BAfter failing the general knowledge test twice
    • CAfter holding the license for less than one year
    • DAfter declining the hazardous-materials endorsement

    49 CFR 383.95(a) requires the restriction after either condition. The standardized code L means no air-brake-equipped CMV.

    Electronic Code of Federal Regulations · 49 CFR 383.95(a); 49 CFR 383.153(a)(10)(i) · verified 2026-09-08