DMV Practice LabChange state
SCSouth Carolina
DMV Practice LabAll tests

Air brakes

Selected knowledge practice, not a complete licensing course or practical training.

Air brakes

Practice mode
Question 1 of 60
  1. 1. What supplies compressed air to the brake reservoirs while the engine is running?
  2. 2. The air pressure reaches the compressor governor’s cut-out setting. What should happen?
  3. 3. Your compressor has its own oil supply rather than sharing engine lubrication. What additional maintenance check is needed?
  4. 4. Why can an air-braked vehicle still have several brake applications available after the compressor stops pumping?
  5. 5. Why is water left in an air-brake reservoir especially dangerous in freezing weather?
  6. 6. Your vehicle has manual air-tank drains. What does the South Carolina CDL handbook advise?
  7. 7. A truck has an alcohol evaporator for its air system. How should it be treated?
  8. 8. The air reservoir’s safety relief valve opens because system pressure is too high. What does this indicate?
  9. 9. In normal air-brake operation, pressing the brake pedal farther generally does what?
  10. 10. When you release the air-brake pedal after a normal application, what happens to the air used for that application?
  11. 11. Why should you avoid pumping air brakes unnecessarily?
  12. 12. What actually slows the wheel when a drum-type service brake applies?
  13. 13. In an S-cam drum brake, what does rotation of the S-cam do?
  14. 14. What does a supply air-pressure gauge tell the driver?
  15. 15. On a vehicle fitted with an application-pressure gauge, more pressure than usual is needed to maintain the same speed downhill. What may this suggest?
  16. 16. How are the stop lights commonly activated in the air-brake system described by the handbook?
  17. 17. In a spring-brake chamber, what normally holds the powerful parking spring back so the brake can release?
  18. 18. Why can poor brake adjustment weaken both service braking and spring-brake holding on the same wheel?
  19. 19. On a vehicle with the standard yellow diamond-shaped parking-brake control, how do you apply the parking brakes?
  20. 20. Why should you avoid a strong service-brake application while spring brakes are applied on a vehicle without anti-compounding protection?
  21. 21. A bus is equipped with a modulating valve for spring brakes. What is that control designed to permit?
  22. 22. Some buses have a separate reserve that can release the spring brakes in an emergency. What limitation matters?
  23. 23. An older vehicle has a driver-operated front-brake limiting valve marked NORMAL and SLIPPERY. For normal stopping, which setting does the handbook specify?
  24. 24. What makes a dual air-brake system different from one single air circuit?
  25. 25. Before moving a vehicle with two service-air circuits, what should you check on the pressure gauges?
  26. 26. One circuit in a dual air-brake system loses pressure during a trip. Why is continuing the trip unsafe?
  27. 27. What should be inspected when the air compressor is belt-driven?
  28. 28. An automatic slack adjuster repeatedly needs manual readjustment. What is the sound response?
  29. 29. Is one inch the universal legal maximum applied pushrod stroke for every commercial-vehicle brake chamber?
  30. 30. During inspection, you find oil or grease on a brake lining. How should you regard it?
  31. 31. An air hose is rubbing against a sharp metal edge. What should you recognize?
  32. 32. Before releasing parking brakes for the handbook’s stationary applied-air-leak test, what is essential?
  33. 33. During South Carolina’s applied-air-leak test with the engine off and the vehicle secured, which brake condition is maintained for the one-minute measurement?
  34. 34. A single vehicle loses 4 psi in one minute after the full applied-brake pressure has stabilized, with the engine off and the test properly set up. How does this compare with the South Carolina handbook limit?
  35. 35. In the South Carolina handbook’s applied-leak check for a truck-and-one-trailer combination, pressure falls 5 psi in one minute after stabilizing. What does that mean?
  36. 36. Why does the handbook’s engine-off low-air warning test keep the electrical power on?
  37. 37. The low-air warning comes on while you are driving. What should you do before pressure falls farther?
  38. 38. Why must a driver use the manufacturer’s specified pressure range when checking automatic spring-brake application?
  39. 39. For the ordinary dual-air-system example in the South Carolina handbook, what buildup check is described at operating rpm?
  40. 40. After building air pressure and following the safe test setup, how does the handbook check that the parking brake holds?
  41. 41. During a safe, low-speed service-brake check, the vehicle pulls to one side and the response feels delayed. What should you conclude?
  42. 42. What extra part of stopping distance must an air-brake driver allow for, beyond perception, reaction and wheel-braking distance?
  43. 43. The handbook warns against applying parking brakes when they are very hot. What must a driver still ensure while following the cooling procedure?
  44. 44. Under the cited current FMVSS 121 requirement, ignition is in RUN and service-reservoir pressure is below 60 psi. What must the warning signal do?
  45. 45. For a non-exempt air-braked power unit not subject to FMVSS 121 when manufactured, the governor cuts out at 100 psi. Under 49 CFR 393.51(c)(2), at what pressure and below is continuous warning required?
  46. 46. Under the cited FMVSS 121 gauge requirement, what pressure information must be readily visible from the normal driving position?
  47. 47. Under federal commercial-vehicle brake equipment rules, how must corresponding brake chambers on the two ends of an axle compare?
  48. 48. Which minimum thickness must commercial-vehicle brake drums and rotors meet under 49 CFR 393.47(g)?
  49. 49. Which part of the air-brake system is used for ordinary stops made with the brake pedal?
  50. 50. How does the CDL manual describe the emergency brake system on an air-braked vehicle?
  51. 51. In the ordinary governor example in South Carolina Section 5, approximately which cut-in and cut-out pressures are given, subject to manufacturer specifications?
  52. 52. At approximately what pressure does the South Carolina manual say the air-tank safety relief valve is usually set to open?
  53. 53. For the dual-air-system procedure in South Carolina Section 5.2, what minimum buildup does the manual specify in both primary and secondary systems before driving?
  54. 54. A secured single vehicle has its engine off, parking brake released and service brake released for the manual’s released-brake leakage check. A one-minute drop is 2.5 psi. How does that compare with the stated criterion?
  55. 55. A secured truck-and-one-trailer combination loses 3.5 psi in one minute with the engine off and the brakes released during the manual’s released-brake leakage check. Which conclusion fits that setup?
  56. 56. Why does the CDL manual give a special parking precaution when brakes are very wet and temperatures are freezing?
  57. 57. What can excessive heat do to a drum brake that contributes to brake fade?
  58. 58. A driver assumes ABS guarantees a shorter stop on every surface. What does the air-brake section actually say?
  59. 59. Where does the South Carolina CDL manual tell a driver to look for a trailer’s yellow ABS malfunction lamp?
  60. 60. An ABS malfunction is detected, but there is no separate evidence that the regular service brakes have failed. What distinction should the driver understand?
Review all 60 questions with answers

  1. What supplies compressed air to the brake reservoirs while the engine is running?

    Correct answer: The air compressor.

    The compressor pumps air into the reservoirs. The other components transmit braking force or hold the brakes, rather than replenish stored air.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.1; printed 5-1; physical PDF page 87; reviewed September 21, 2026. ↗

  2. The air pressure reaches the compressor governor’s cut-out setting. What should happen?

    Correct answer: The compressor stops pumping air into the system.

    The governor controls pumping at the cut-in and cut-out settings. Cut-out does not mean the engine stops or the reservoirs are emptied; use the vehicle’s specified pressure settings.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.2; printed 5-1; physical PDF page 87; reviewed September 21, 2026. ↗

  3. Your compressor has its own oil supply rather than sharing engine lubrication. What additional maintenance check is needed?

    Correct answer: Check that compressor’s oil level.

    Some compressors have a separate oil supply. An acceptable engine oil level does not establish that a separately lubricated compressor has enough oil.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.1; printed 5-1; physical PDF page 87; reviewed September 21, 2026. ↗

  4. Why can an air-braked vehicle still have several brake applications available after the compressor stops pumping?

    Correct answer: The reservoirs hold a supply of compressed air.

    Reservoirs store braking air. That reserve is limited: it does not make continued driving with a failed air supply safe.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.3; printed 5-1; physical PDF page 87; reviewed September 21, 2026. ↗

  5. Why is water left in an air-brake reservoir especially dangerous in freezing weather?

    Correct answer: It can freeze and interfere with brake operation.

    Moisture can freeze in the system and cause brake trouble. The handbook calls for draining water and oil from tanks using the equipment’s proper procedure.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.4; printed 5-1; physical PDF page 87; reviewed September 21, 2026. ↗

  6. Your vehicle has manual air-tank drains. What does the South Carolina CDL handbook advise?

    Correct answer: Drain the tanks at the end of each working day.

    Manual drains remove collected water and oil. Waiting for a malfunction allows contamination to build up; follow the vehicle’s drain procedure and maintenance instructions.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.4; printed 5-1; physical PDF page 87; reviewed September 21, 2026. ↗

  7. A truck has an alcohol evaporator for its air system. How should it be treated?

    Correct answer: Maintain it as specified; it does not eliminate tank-drain checks.

    An alcohol evaporator is fitted only on some systems. Follow the equipment’s instructions and continue required moisture-management checks; do not improvise additives for other systems.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.5; printed 5-2; physical PDF page 88; reviewed September 21, 2026. ↗

  8. The air reservoir’s safety relief valve opens because system pressure is too high. What does this indicate?

    Correct answer: A system problem needs attention by a mechanic.

    The safety valve protects against excessive pressure. Its operation is a reason to find the fault, not disable the protection or assume the brakes are otherwise sound.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.6; printed 5-2; physical PDF page 88; reviewed September 21, 2026. ↗

  9. In normal air-brake operation, pressing the brake pedal farther generally does what?

    Correct answer: Applies more air pressure to the service brakes.

    The foot valve meters service-brake pressure according to pedal demand. It does not adjust the governor’s pressure setting.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.7; printed 5-2; physical PDF page 88; reviewed September 21, 2026. ↗

  10. When you release the air-brake pedal after a normal application, what happens to the air used for that application?

    Correct answer: It is exhausted from the brake chambers.

    Releasing the pedal vents the application air so the service brakes can release. New air must be supplied from the reservoirs for later applications.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.7; printed 5-2; physical PDF page 88; reviewed September 21, 2026. ↗

  11. Why should you avoid pumping air brakes unnecessarily?

    Correct answer: Repeated applications can use air faster than it is replenished.

    Each application consumes air. Repeated unnecessary applications can reduce the available reserve, especially if the compressor cannot keep up.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.7; printed 5-2; physical PDF page 88; reviewed September 21, 2026. ↗

  12. What actually slows the wheel when a drum-type service brake applies?

    Correct answer: Friction between the brake lining and the drum.

    Air pressure operates the brake mechanism, but contact between lining and drum creates the friction that slows the wheel and produces heat.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.8; printed 5-2; physical PDF page 88; reviewed September 21, 2026. ↗

  13. In an S-cam drum brake, what does rotation of the S-cam do?

    Correct answer: Pushes the brake shoes outward against the drum.

    The chamber’s pushrod and slack adjuster turn the camshaft. The S-shaped cam spreads the shoes against the inside of the drum. This describes S-cam brakes, not every air-brake design.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.8; printed 5-2; physical PDF page 88; reviewed September 21, 2026. ↗

  14. What does a supply air-pressure gauge tell the driver?

    Correct answer: How much air pressure is available in the reservoir system.

    Supply gauges monitor stored air pressure. They do not measure lining wear or show the individual braking force at each wheel.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.9; printed 5-2; physical PDF page 88; reviewed September 21, 2026. ↗

  15. On a vehicle fitted with an application-pressure gauge, more pressure than usual is needed to maintain the same speed downhill. What may this suggest?

    Correct answer: Brake fade or another braking-system problem.

    An unusual increase in required application pressure can indicate fade, adjustment trouble, leaks or other mechanical problems. The gauge alone does not identify the exact fault.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.10; printed 5-3; physical PDF page 89; reviewed September 21, 2026. ↗

  16. How are the stop lights commonly activated in the air-brake system described by the handbook?

    Correct answer: Air pressure operates an electrical switch.

    An air-operated electrical switch turns on the stop lights when the service brakes are applied. The warning system and parking control serve different purposes.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.12; printed 5-3; physical PDF page 89; reviewed September 21, 2026. ↗

  17. In a spring-brake chamber, what normally holds the powerful parking spring back so the brake can release?

    Correct answer: Air pressure.

    Air holds the parking spring compressed. When that holding pressure is released or lost, the spring can apply the brake; this differs from service brakes that use air to apply.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.14; printed 5-3; physical PDF page 89; reviewed September 21, 2026. ↗

  18. Why can poor brake adjustment weaken both service braking and spring-brake holding on the same wheel?

    Correct answer: Both rely on properly adjusted foundation-brake components.

    The spring brake and service brake depend on the wheel’s brake mechanism. A spring application does not overcome improper foundation-brake adjustment.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.14; printed 5-3; physical PDF page 89; reviewed September 21, 2026. ↗

  19. On a vehicle with the standard yellow diamond-shaped parking-brake control, how do you apply the parking brakes?

    Correct answer: Pull the control out.

    The standard yellow diamond control is pulled to apply and pushed to release. Older or special equipment may use a different control, so know the vehicle before operation.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.15; printed 5-3; physical PDF page 89; reviewed September 21, 2026. ↗

  20. Why should you avoid a strong service-brake application while spring brakes are applied on a vehicle without anti-compounding protection?

    Correct answer: The combined forces can damage brake components.

    Service and spring forces can add together at the brake mechanism. Some vehicles have protection, but the driver must not assume every vehicle does.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.15; printed 5-3; physical PDF page 89; reviewed September 21, 2026. ↗

  21. A bus is equipped with a modulating valve for spring brakes. What is that control designed to permit?

    Correct answer: Gradual control of spring-brake application.

    A modulating control can vary spring-brake application rather than providing only an on/off action. Operation and securing the control for parking depend on the vehicle’s instructions.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.15; printed 5-3; physical PDF page 89; reviewed September 21, 2026. ↗

  22. Some buses have a separate reserve that can release the spring brakes in an emergency. What limitation matters?

    Correct answer: The reserve permits only a limited number of releases.

    An emergency-release reserve is finite and vehicle-specific. Its presence is not permission to continue passenger service with an unsafe braking system.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.15; printed 5-4; physical PDF page 90; reviewed September 21, 2026. ↗

  23. An older vehicle has a driver-operated front-brake limiting valve marked NORMAL and SLIPPERY. For normal stopping, which setting does the handbook specify?

    Correct answer: NORMAL.

    The handbook specifies NORMAL to preserve normal stopping power. This question concerns the older manually controlled limiter, not ABS or a modern automatic system.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.13; printed 5-3; physical PDF page 89; reviewed September 21, 2026. ↗

  24. What makes a dual air-brake system different from one single air circuit?

    Correct answer: It has two air systems with separate reservoirs and lines.

    Dual systems have separate air circuits operated through a common brake control. The exact axle arrangement depends on the vehicle.

    Source: South Carolina CDL Manual, body March 2025, section 5.2; printed 5-5; physical PDF page 91; reviewed September 21, 2026. ↗

  25. Before moving a vehicle with two service-air circuits, what should you check on the pressure gauges?

    Correct answer: That both circuits have reached the required operating pressure.

    The handbook says to allow both systems to build adequate pressure. One satisfactory reading does not establish that the other circuit is ready; follow the vehicle’s specified procedure.

    Source: South Carolina CDL Manual, body March 2025, section 5.2; printed 5-5; physical PDF page 91; reviewed September 21, 2026. ↗

  26. One circuit in a dual air-brake system loses pressure during a trip. Why is continuing the trip unsafe?

    Correct answer: Some braking may be lost and stopping distance can increase.

    A surviving circuit provides limited protection during failure, not normal service capability. Stop safely and have the problem corrected.

    Source: South Carolina CDL Manual, body March 2025, section 5.2; printed 5-5; physical PDF page 91; reviewed September 21, 2026. ↗

  27. What should be inspected when the air compressor is belt-driven?

    Correct answer: The belt’s condition and proper tension.

    A worn, damaged or improperly tensioned drive belt can affect compressor operation. This check applies when the compressor uses a belt, not a gear drive.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.1; printed 5-6; physical PDF page 92; reviewed September 21, 2026. ↗

  28. An automatic slack adjuster repeatedly needs manual readjustment. What is the sound response?

    Correct answer: Have a qualified person find and repair the underlying fault.

    Repeated manual readjustment can hide a problem in the adjuster, installation or foundation brake. It does not replace diagnosis and proper repair.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.2; printed 5-6; physical PDF page 92; reviewed September 21, 2026. ↗

  29. Is one inch the universal legal maximum applied pushrod stroke for every commercial-vehicle brake chamber?

    Correct answer: No; use the limit for the actual actuator type and size.

    Federal limits depend on the actuator. The handbook’s approximate manual free-play check must not be confused with a universal applied-stroke limit. Unlisted actuator types have the manufacturer-related limits specified by the rule.

    Source: 49 CFR 393.47(e)(1)–(2); eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗

  30. During inspection, you find oil or grease on a brake lining. How should you regard it?

    Correct answer: As a brake defect that needs attention before safe operation.

    Brake linings rely on controlled friction. Oil or grease contamination is an unsafe condition, not a desirable lubricant for the friction surface.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.2; printed 5-6; physical PDF page 92; reviewed September 21, 2026. ↗

  31. An air hose is rubbing against a sharp metal edge. What should you recognize?

    Correct answer: The rubbing can damage the hose and cause air loss.

    Brake hoses must be secured against chafing, kinking and other mechanical damage. Rubbing on a sharp edge can damage the hose and cause pressure loss; a warning device does not repair the defect.

    Source: 49 CFR 393.45(b)(1)–(3) and (d); eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗

  32. Before releasing parking brakes for the handbook’s stationary applied-air-leak test, what is essential?

    Correct answer: Secure the vehicle against rolling, including proper wheel chocks.

    Some air-system checks require releasing the parking brakes. Establish proper rollaway protection first and follow the vehicle’s full inspection procedure.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.3 (continued on physical page 93); printed 5-6; physical PDF page 92; reviewed September 21, 2026. ↗

  33. During South Carolina’s applied-air-leak test with the engine off and the vehicle secured, which brake condition is maintained for the one-minute measurement?

    Correct answer: The foot brake is fully applied and held.

    The South Carolina procedure measures air loss while a full foot-brake application is held for one minute, with the engine off and the parking and trailer-supply controls released as directed. Secure the wheels against movement before releasing brakes. This is not a released-brake leakage test.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-7; physical PDF page 93; reviewed September 21, 2026. ↗

  34. A single vehicle loses 4 psi in one minute after the full applied-brake pressure has stabilized, with the engine off and the test properly set up. How does this compare with the South Carolina handbook limit?

    Correct answer: It exceeds the 3-psi-per-minute single-vehicle limit.

    For this applied check, the handbook gives no more than 3 psi in one minute for a single vehicle. A 4-psi loss exceeds that criterion and requires correction before driving.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-7; physical PDF page 93; reviewed September 21, 2026. ↗

  35. In the South Carolina handbook’s applied-leak check for a truck-and-one-trailer combination, pressure falls 5 psi in one minute after stabilizing. What does that mean?

    Correct answer: It exceeds the stated 4-psi-per-minute combination limit.

    The handbook’s combination-vehicle criterion is no more than 4 psi in one minute during the applied test. This question does not assign that limit to every possible multi-trailer configuration.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-7; physical PDF page 93; reviewed September 21, 2026. ↗

  36. Why does the handbook’s engine-off low-air warning test keep the electrical power on?

    Correct answer: So the electrical warning devices can operate as pressure is reduced.

    The warning test requires electrical power for the warning circuit. The engine is off while pressure is reduced according to the safe inspection procedure.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-7; physical PDF page 93; reviewed September 21, 2026. ↗

  37. The low-air warning comes on while you are driving. What should you do before pressure falls farther?

    Correct answer: Make a safe stop and secure the vehicle as soon as possible.

    Act while you still have enough air to control the stop. Waiting for an automatic spring-brake application can leave much less control and may not apply every axle.

    Source: South Carolina CDL Manual, body March 2025, section 5.4.7; printed 5-9; physical PDF page 95; reviewed September 21, 2026. ↗

  38. Why must a driver use the manufacturer’s specified pressure range when checking automatic spring-brake application?

    Correct answer: The application pressure varies with the brake equipment.

    The handbook gives a typical range, not one exact pressure for all systems. The brakes may apply with air still in the tanks; use the specified procedure and limits.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-7; physical PDF page 93; reviewed September 21, 2026. ↗

  39. For the ordinary dual-air-system example in the South Carolina handbook, what buildup check is described at operating rpm?

    Correct answer: 85 to 100 psi within 45 seconds, with larger-reservoir exceptions.

    The handbook describes the 85-to-100-psi, 45-second check and says unusually large reservoirs may take longer. Follow manufacturer specifications; this is not a universal federal vehicle-design test limit.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-7; physical PDF page 93; reviewed September 21, 2026. ↗

  40. After building air pressure and following the safe test setup, how does the handbook check that the parking brake holds?

    Correct answer: Gently try to pull against the applied parking brake in a low gear.

    The handbook uses a gentle low-gear pull against the applied parking brake. Use a safe area and the vehicle’s procedure; a control’s appearance does not prove holding ability.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-7; physical PDF page 93; reviewed September 21, 2026. ↗

  41. During a safe, low-speed service-brake check, the vehicle pulls to one side and the response feels delayed. What should you conclude?

    Correct answer: A braking problem needs investigation and correction.

    Pulling, delayed response or unusual feel can reveal trouble. The check identifies a symptom, not one certain component failure.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-7; physical PDF page 93; reviewed September 21, 2026. ↗

  42. What extra part of stopping distance must an air-brake driver allow for, beyond perception, reaction and wheel-braking distance?

    Correct answer: Distance traveled while the air brakes respond to the application.

    Air pressure takes time to reach and operate the brakes. The distance traveled during that delay adds to stopping distance and varies with speed and conditions.

    Source: South Carolina CDL Manual, body March 2025, section 5.4.4; printed 5-8; physical PDF page 94; reviewed September 21, 2026. ↗

  43. The handbook warns against applying parking brakes when they are very hot. What must a driver still ensure while following the cooling procedure?

    Correct answer: The vehicle is securely prevented from rolling, including suitable chocks.

    The hot-brake caution is not permission to leave a vehicle unsecured. Use a level location, appropriate chocks and the vehicle’s safe parking procedure while the brakes cool.

    Source: South Carolina CDL Manual, body March 2025, section 5.4.8; printed 5-9; physical PDF page 95; reviewed September 21, 2026. ↗

  44. Under the cited current FMVSS 121 requirement, ignition is in RUN and service-reservoir pressure is below 60 psi. What must the warning signal do?

    Correct answer: Give a continuous low-air warning.

    S5.1.5 requires a continuous signal other than the gauge below 60 psi with ignition in RUN. It may be visible, or both audible and visible. Applicability depends on the vehicle and the standard in effect when manufactured.

    Source: 49 CFR 571.121, S5.1.5; eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗

  45. For a non-exempt air-braked power unit not subject to FMVSS 121 when manufactured, the governor cuts out at 100 psi. Under 49 CFR 393.51(c)(2), at what pressure and below is continuous warning required?

    Correct answer: 50 psi, the lower of 55 psi and half of cut-out pressure.

    This particular older-vehicle rule uses 55 psi or half the governor cut-out pressure, whichever is less. Half of 100 is 50; do not use this exception as the warning threshold for modern FMVSS 121 vehicles.

    Source: 49 CFR 393.51(c)(2), with paragraph (f) exceptions excluded by the question; eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗

  46. Under the cited FMVSS 121 gauge requirement, what pressure information must be readily visible from the normal driving position?

    Correct answer: The reservoir pressure in each service-brake system.

    S5.1.4 requires a pressure gauge in each service-brake system. The reading is reservoir pressure, and the separate low-air warning is not replaced by the gauge.

    Source: 49 CFR 571.121, S5.1.4; eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗

  47. Under federal commercial-vehicle brake equipment rules, how must corresponding brake chambers on the two ends of an axle compare?

    Correct answer: The service chambers must match in size, and the spring chambers must match in size.

    49 CFR 393.47(b) requires matching service-brake chamber sizes across the axle and matching spring-brake chamber sizes across the axle. It does not say a service chamber and spring chamber must be identical to each other.

    Source: 49 CFR 393.47(b); eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗

  48. Which minimum thickness must commercial-vehicle brake drums and rotors meet under 49 CFR 393.47(g)?

    Correct answer: The limits established by the drum or rotor manufacturer.

    The rule uses the drum or rotor manufacturer’s limits. Do not replace that standard with a guessed universal dimension or a visual impression that the part still looks usable.

    Source: 49 CFR 393.47(g); eCFR title 49 current through September 17, 2026; reviewed September 21, 2026. ↗

  49. Which part of the air-brake system is used for ordinary stops made with the brake pedal?

    Correct answer: The service brake system.

    The manual distinguishes service, parking and emergency braking. Ordinary pedal-controlled stops use the service brake system; the parking control serves a different function.

    Source: South Carolina CDL Manual, body March 2025, section 5 (introduction); printed 5-1; physical PDF page 87; reviewed September 21, 2026. ↗

  50. How does the CDL manual describe the emergency brake system on an air-braked vehicle?

    Correct answer: It uses parts of the service and parking systems during a brake-system failure.

    The emergency function draws on parts of the service and parking brake systems to help stop during a system failure. The warning devices alert the driver but do not themselves provide braking force.

    Source: South Carolina CDL Manual, body March 2025, section 5 (introduction); printed 5-1; physical PDF page 87; reviewed September 21, 2026. ↗

  51. In the ordinary governor example in South Carolina Section 5, approximately which cut-in and cut-out pressures are given, subject to manufacturer specifications?

    Correct answer: Cut in at 100 psi and cut out at 125 psi.

    The manual uses about 100 psi for cut-in and 125 psi for cut-out and instructs the driver to check manufacturer specifications. These example settings are not the spring-brake application range or a universal value for every vehicle.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-6; physical PDF page 92; reviewed September 21, 2026. ↗

  52. At approximately what pressure does the South Carolina manual say the air-tank safety relief valve is usually set to open?

    Correct answer: 150 psi.

    The handbook gives 150 psi as the usual safety-valve setting. The valve protects against excessive pressure; air being released by it calls for a mechanic to correct the fault, not treating it as the normal governor cut-out.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.6; printed 5-2; physical PDF page 88; reviewed September 21, 2026. ↗

  53. For the dual-air-system procedure in South Carolina Section 5.2, what minimum buildup does the manual specify in both primary and secondary systems before driving?

    Correct answer: 100 psi in each system.

    Section 5.2 calls for at least 100 psi in both systems before driving. The low-air warning going out or one gauge rising is not the same as both circuits reaching the required pressure; also follow the vehicle manufacturer’s procedure.

    Source: South Carolina CDL Manual, body March 2025, section 5.2; printed 5-5; physical PDF page 91; reviewed September 21, 2026. ↗

  54. A secured single vehicle has its engine off, parking brake released and service brake released for the manual’s released-brake leakage check. A one-minute drop is 2.5 psi. How does that compare with the stated criterion?

    Correct answer: It fails the released-brake criterion of less than 2 psi per minute.

    The released-brake check uses less than 2 psi per minute for a single vehicle. The separate applied-brake check allows no more than 3 psi; do not substitute its limit for this setup. Secure the vehicle against rolling before releasing parking brakes.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-6; physical PDF page 92; reviewed September 21, 2026. ↗

  55. A secured truck-and-one-trailer combination loses 3.5 psi in one minute with the engine off and the brakes released during the manual’s released-brake leakage check. Which conclusion fits that setup?

    Correct answer: It fails the released-brake criterion of less than 3 psi per minute.

    The manual gives less than 3 psi per minute for the released-brake combination check. The 4-psi figure belongs to the distinct fully applied check. Rollaway protection is essential during either inspection setup.

    Source: South Carolina CDL Manual, body March 2025, section 5.3.3; printed 5-6; physical PDF page 92; reviewed September 21, 2026. ↗

  56. Why does the CDL manual give a special parking precaution when brakes are very wet and temperatures are freezing?

    Correct answer: Applied wet brakes can freeze in place and prevent the vehicle from moving.

    Very wet brakes can freeze if applied for parking in freezing conditions. The manual calls for suitable wheel chocks on a level surface and describes safely drying wet brakes. The exception is not permission to leave a vehicle unsecured.

    Source: South Carolina CDL Manual, body March 2025, section 5.4.8; printed 5-9; physical PDF page 95; reviewed September 21, 2026. ↗

  57. What can excessive heat do to a drum brake that contributes to brake fade?

    Correct answer: Expand the drum and reduce effective friction from the lining.

    The manual describes both heat-related loss of lining friction and drum expansion. Shoes must travel farther as the drum expands, reducing effective braking; continued overuse can leave the vehicle unable to slow or stop.

    Source: South Carolina CDL Manual, body March 2025, section 5.4.5; printed 5-9; physical PDF page 95; reviewed September 21, 2026. ↗

  58. A driver assumes ABS guarantees a shorter stop on every surface. What does the air-brake section actually say?

    Correct answer: ABS helps control, but does not necessarily shorten stopping distance.

    ABS helps prevent wheel lockup and maintain control. The manual explicitly warns that it does not necessarily reduce stopping distance, so the driver must still allow an appropriate following and stopping margin.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.16; printed 5-4; physical PDF page 90; reviewed September 21, 2026. ↗

  59. Where does the South Carolina CDL manual tell a driver to look for a trailer’s yellow ABS malfunction lamp?

    Correct answer: On the trailer’s left side, near the front or rear corner.

    The manual places the trailer ABS malfunction lamp on the left side at a front or rear corner. This is distinct from the instrument-panel lamps on the tractor, truck or bus.

    Source: South Carolina CDL Manual, body March 2025, section 5.1.16; printed 5-4; physical PDF page 90; reviewed September 21, 2026. ↗

  60. An ABS malfunction is detected, but there is no separate evidence that the regular service brakes have failed. What distinction should the driver understand?

    Correct answer: Regular braking can remain available, but ABS needs prompt service.

    ABS is additional control equipment, not a replacement for the ordinary brakes. The manual says normal brake functions can remain when ABS fails and the system should be serviced soon. A separate braking fault or low-air warning still requires its own safe response.

    Source: South Carolina CDL Manual, body March 2025, section 5.4.2; printed 5-8; physical PDF page 94; reviewed September 21, 2026. ↗

Report a problem with this question

Question reference: sc-cdl-air-001

Opens a draft in your email app. Review and send it there; nothing is sent automatically.

Draft a correction email

Contact details & help

Study tools & test information

Home / South Carolina / CDL

Air brakes

60 questions from our CDL question set. Read each explanation to review the material.

All topics and study tools

Study questions checked against SCDMV licensing information, its posted CDL manual and cited federal rules. Review date: September 21, 2026. This is selected study coverage, not a complete course. Follow the cited source and the actual vehicle manufacturer’s procedure; equipment and brake-system differences matter. These quizzes do not replace supervised inspection or hands-on training.