CCDL Permit Prep

Air Brakes · page 2 of 4

Air Brakes questions 21–40

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. In the static air-leak test, which stated limit is correct for a single vehicle?

    • ALess than 10 psi in one minute
    • BThere is no limit
    • CLess than 25 psi in one minute
    • DLess than 2 psi in one minuteCorrect

    For the static test, the manual states less than 2 psi in one minute for a single vehicle. A greater loss indicates an air leak that must be fixed before driving.

    FMCSA-hosted Commercial Driver License Manual (July 2014) · 5.3.3 — Test Air Leakage Rate · verified 2026-09-07

  2. When should you avoid applying the parking (spring) brakes until conditions are corrected?

    • AThe vehicle is parked on level ground
    • BThe tanks are fully charged
    • CThe brakes are very hot, such as right after a steep downgradeCorrect
    • DThe engine is off and parked on a hill

    Parking brakes can be damaged if applied while the brakes are very hot. On level ground, chock the wheels and let the brakes cool before applying them. Wet brakes in freezing conditions are a separate freeze risk.

    FMCSA Commercial Driver's License Manual · 5.4.8 — Parking Brakes · verified 2026-09-07

  3. In the correct low gear on a long, steep downgrade, how should service brakes supplement engine braking?

    • ABrake enough for a definite slowdown, then release about 5 mph below the safe speedCorrect
    • BRide the brakes lightly the whole way down
    • CCoast in neutral and brake only at the bottom
    • DUse the trailer brakes alone and not the tractor brakes

    Apply at the selected safe speed, slow about 5 mph below it, and release. Repeat when speed returns to the safe speed. Do not ride the pedal or use trailer brakes alone.

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

  4. Under 49 CFR §393.51(c)(2), when must the warning signal provide a continuous warning for a power unit to which FMVSS No. 121 did not apply on its manufacture date?

    • AWhenever service-reservoir pressure is at or below the lower of 55 psi and one-half of compressor-governor cutout pressure.Correct
    • BOnly after spring brakes apply.
    • COnly below 40 psi, regardless of governor cutout pressure.
    • DNo warning signal is required if a pressure gauge is present.

    Paragraph (c)(2) requires continuous warning at or below the lower of 55 psi and one-half of compressor-governor cutout pressure for power units to which FMVSS No. 121 did not apply on the manufacture date.

    Electronic Code of Federal Regulations: 49 CFR §393.51 · §393.51(c)(2): Air-brake warning signal for power units not subject to FMVSS No. 121 · verified 2026-09-27

  5. In a combination vehicle, how much air loss per minute is allowed with the engine off and brakes applied?

    • ANo more than 4 psiCorrect
    • BNo more than 12 psi
    • CNo more than 20 psi
    • DAny loss under 30 psi

    For the applied-brake leakage check, the combination-vehicle limit is no more than 4 psi in one minute. The single-vehicle limit is 3 psi in one minute. A greater drop exceeds the manual’s check and needs inspection.

    FMCSA 2005 CDL Testing System Model Manual (July 2014) · Section 5.3.3 — Applied Air-Leak Test; Oregon Manual § 5.3.3 cross-check · verified 2026-09-19

  6. For a dual air system with standard-size reservoirs, what is the usual time allowed to build from 85 to 100 psi at operating rpm?

    • AAbout five minutes at idle
    • BAbout 45 seconds at operating rpmCorrect
    • CImmediately when the engine starts
    • DOnly after the first brake use

    The usual check allows 45 seconds. Larger-than-minimum reservoirs can take longer, so consult the vehicle specifications before judging a result. Do not drive with a failed applicable buildup check.

    Oregon DMV Commercial Driver Manual · 5.3.3 Final Air Brake Check; 5.1.11 Low Air Pressure Warning · verified 2026-09-07

  7. What does brake fade mean during repeated or prolonged braking?

    • ALoss of braking power from overheatingCorrect
    • BAir escaping from a cracked line
    • CA slow leak at the glad hands
    • DUneven wear across the linings

    Repeated or prolonged braking can overheat the brakes. Heat can reduce lining friction and expand brake drums, which reduces braking force and can progress until the vehicle cannot be slowed or stopped.

    FMCSA Commercial Driver License Manual · 5.4.5 — Brake Fading or Failure · verified 2026-09-07

  8. What does the supply pressure gauge tell the driver?

    • AHow worn the brake linings are
    • BHow much air is in the tanksCorrect
    • CHow hot the brake drums have become
    • DHow far the slack adjusters have moved

    The supply pressure gauge shows the air pressure available in the storage tanks for the brake system. Monitor the gauge and the low-pressure warning; if pressure becomes very low while driving, stop safely and have the system repaired.

    FMCSA Commercial Driver's License Manual (2005 CDL Testing System) · Section 5.1 — Air Brake System Components; Section 5.2 — Dual Air Brake · verified 2026-09-08

  9. During a secured air-brake check, the vehicle specification gives a valve pop-out range. Within which pressure range should the applicable controls pop out as you fan the pressure down?

    • ABetween 20 and 45 psiCorrect
    • BBetween 60 and 80 psi
    • CBetween 85 and 100 psi
    • DBetween 100 and 120 psi

    The applicable parking-brake and tractor-protection controls should pop out within the manufacturer's specified range, causing the spring brakes to apply. This check follows the earlier low-air warning test; brake adjustment still affects spring-brake force.

    FMCSA Commercial Driver License Manual · 5.1.14 and 5.3.3 — Spring Brakes · verified 2026-09-07

  10. Why should you avoid pressing the brake pedal when the spring brakes are already applied?

    • AStopping distance is halved
    • BThe combined forces can damage the brakesCorrect
    • CThe springs release automatically
    • DAir pressure rebuilds faster

    The service-brake air force and spring-brake force can combine on the brake linkage. The manual warns that this can damage the brakes, although some systems prevent the forces from adding together.

    FMCSA Commercial Driver License Manual · 5.1.14 — Spring Brakes · verified 2026-09-07

  11. What does the air compressor governor control?

    • AThe pressure inside the brake drums
    • BHow hard the brakes apply
    • CThe rate the tanks drain moisture
    • DWhen the compressor pumps air into the air-storage tanksCorrect

    The governor controls when the compressor pumps air into the storage tanks: it stops at the cut-out pressure and allows the compressor to start again at the cut-in pressure. Use the vehicle manufacturer's specified pressures.

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

  12. Why must air tanks be drained?

    • ATo remove water and compressor oil that collect in the tanksCorrect
    • BTo raise the governor cut-out pressure
    • CTo apply the spring brakes
    • DTo cool the brake drums

    Compressed air can carry water and compressor oil into the tanks. Drain the tanks as required for the vehicle because water can freeze and cause brake failure, and oil is harmful to the air-brake system.

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

  13. What does the safety valve on the first air tank do?

    • AHolds pressure when the engine stops
    • BVents air if pressure gets too highCorrect
    • CApplies the spring brakes on demand
    • DDrains moisture automatically

    The safety relief valve protects the first tank and the rest of the system from too much pressure. It is usually set to open around 150 psi, but the vehicle's specification controls; if it releases air, have the fault fixed by a mechanic.

    FMCSA Commercial Driver License Manual · Section 5.1.6: Safety Valve · verified 2026-09-08

  14. What does an illuminated ABS malfunction lamp mean?

    • ANormal braking remains, but antilock operation may notCorrect
    • BThe brakes have failed completely
    • CThe parking brake is still applied
    • DAir pressure has dropped below 60 psi

    An ABS malfunction does not remove normal brake function. Brake normally, avoid relying on antilock operation, and have the ABS serviced as soon as possible; investigate any separate brake or air-pressure warning on its own merits.

    FMCSA Commercial Driver's License Manual (July 2014) · 2.18.5/2.18.7 ABS malfunction; 5.1.16 Antilock Braking Systems · verified 2026-09-25

  15. Why should the trailer hand valve not be used while driving?

    • AIt can make the trailer skidCorrect
    • BIt disconnects the electrical line
    • CIt raises the landing gear
    • DIt locks the fifth wheel open

    The trailer hand valve should be used only to test the trailer brakes. Using it while driving can make the trailer skid; use the foot brake for normal service braking.

    FMCSA Commercial Driver License Manual · Section 6.2.1: Trailer Hand Valve · verified 2026-09-08

  16. In Figure 5.4 of the July 2014 FMCSA CDL Manual, which component is labeled on the air path between the compressor and air storage tanks?

    • AOne-way check valve.Correct
    • BTrailer hand brake.
    • CFuel pump.
    • DEngine coolant valve.

    Figure 5.4 labels this component as a one-way check valve. The question asks only for the figure label and makes no claim about the valve’s function. The source is a historical July 2014 manual.

    FMCSA Commercial Driver License Manual, July 2014 edition (historical) · Section 5, Figure 5.4: Air brake system components and location; historical July 2014 edition · verified 2026-09-27

  17. Which plan reduces brake overheating while controlling speed on a long downgrade?

    • AChoose a safe low gear before descent and use the recommended apply-and-release cycleCorrect
    • BCoast in neutral and brake late
    • CRide the brakes the whole way down
    • DUse the parking brake to slow

    Use engine braking as the main control. Supplement it with a definite slowdown to about 5 mph below the selected safe speed, then release until speed returns.

    Oregon DMV Commercial Driver Manual · 2.16.2–2.16.4 · verified 2026-09-07

  18. Under the Oregon CDL vehicle-inspection procedure, what should you listen and look for when inspecting air lines?

    • AFresh paint on the couplers
    • BAudible leaks and cut, chafed, cracked, or worn air linesCorrect
    • CThat the lines hang loosely
    • DThat the seals are removed

    The Oregon procedure says to listen for air leaks and inspect air hoses or lines for cuts, chafing, cracks, and wear, with couplings and fittings secure. Other jurisdictions may use different inspection wording.

    Oregon Commercial Driver Manual · Section 11: Vehicle Inspection Test — Brake Lines / Hoses / Leaks; Air/Electric Lines · verified 2026-09-08

  19. In a dual air system, what do the primary and secondary circuits serve?

    • AOnly the trailer and only the dolly
    • BThe same axles twice over
    • CDifferent axle groups, with both circuits supplying trailer air when presentCorrect
    • DOnly the parking brakes

    FMCSA Section 5.2 says one system typically operates the rear axle or axles, while the other operates the front axle and possibly one rear axle. Both systems supply air to the trailer when present.

    FMCSA Commercial Driver License Manual · Section 5.2: Dual Air Brake · verified 2026-09-08

  20. What is the alcohol evaporator for?

    • ACooling the brake drums
    • BCleaning the brake linings
    • CBoosting the compressor output
    • DReducing ice in the air systemCorrect

    FMCSA Section 5.1.5 says an alcohol evaporator puts alcohol into the air system to reduce the risk of ice in air-brake valves and other parts during cold weather.

    FMCSA Commercial Driver License Manual · Section 5.1.5: Alcohol Evaporator · verified 2026-09-08