{
  "slug": "air-brakes",
  "jurisdiction": "US",
  "name": "CDL Air Brakes",
  "kind": "air-brakes",
  "title": "CDL Air Brakes Practice",
  "language": "en",
  "edition": "March 2025",
  "questionCount": 38,
  "writtenTest": {
    "questionsAsked": 25,
    "correctToPass": 20,
    "confirmed": {
      "questionsAsked": false,
      "correctToPass": false,
      "passShare": true
    },
    "ruleSource": {
      "title": "49 CFR 383.135(b)(1) (80 percent to pass a CDL knowledge test)",
      "url": "https://www.ecfr.gov/current/title-49/subtitle-B/chapter-III/subchapter-B/part-383/subpart-F/section-383.135"
    },
    "note": "Federal rules require at least an 80 percent score on a CDL knowledge test. Twenty-five air brakes questions is the common state test size and the practice target here; the real test size and pass mark vary by state."
  },
  "disclaimer": "This is a study aid written by examprepare.help from the AAMVA model Commercial Driver's License Manual (Version March 2025), the model commercial driver's license manual that most states adopt. It is not the real CDL knowledge test, it is not affiliated with FMCSA, AAMVA or any state agency, and your state's test may differ. Check your state driver licensing agency before you take the real test.",
  "source": {
    "publisher": "American Association of Motor Vehicle Administrators (AAMVA), distributed by the Oklahoma Department of Public Safety; supported by the Federal Motor Carrier Safety Administration",
    "title": "Commercial Driver's License Manual, Modernized Testing System",
    "formNumber": null,
    "edition": "March 2025",
    "url": "https://aem-prod.oklahoma.gov/content/dam/service-oklahoma/Documents/CDLDriverManual.pdf",
    "pageUrl": "https://www.fmcsa.dot.gov/registration/commercial-drivers-license/cdl-manual",
    "retrieved": "2026-10-05",
    "textSha256": "1faa1b36ebbfaacdccfc7876fc5e75d02b5ea9d3b852dd22708d70b059c92672",
    "via": null
  },
  "facts": [
    {
      "key": "governor",
      "label": "Air compressor governor cut-in and cut-out",
      "value": "Cut-in around 100 psi, cut-out around 125 psi",
      "section": "5.1.2 – Air Compressor Governor",
      "page": 72
    },
    {
      "key": "safety-valve",
      "label": "Pressure the safety valve opens at",
      "value": "150 psi",
      "section": "5.1.6 – Safety Valve",
      "page": 73
    },
    {
      "key": "low-air-warning",
      "label": "Low air pressure warning threshold",
      "value": "Must come on before pressure falls below 55 psi",
      "section": "5.1.11 – Low Air Pressure Warning",
      "page": 74
    },
    {
      "key": "spring-brakes",
      "label": "Pressure at which spring brakes come fully on",
      "value": "20 to 45 psi",
      "section": "5.1.14 – Spring Brakes",
      "page": 74
    },
    {
      "key": "slack-adjuster",
      "label": "Slack adjuster movement that needs adjustment",
      "value": "More than about one inch where the push rod attaches",
      "section": "5.3.2 – During Step 5 Walk-around Inspection",
      "page": 77
    },
    {
      "key": "leak-rate",
      "label": "Maximum air leakage rate without braking",
      "value": "Less than 2 psi per minute single, less than 3 psi per minute combination",
      "section": "5.3.3 – Step 7 Final Air Brake Check",
      "page": 77
    },
    {
      "key": "foot-brake-leak",
      "label": "Maximum air drop with the foot brake held one minute",
      "value": "3 psi single, 4 psi combination",
      "section": "5.3.3 – Step 7 Final Air Brake Check",
      "page": 78
    },
    {
      "key": "pressure-buildup",
      "label": "Dual air system pressure buildup rate",
      "value": "85 to 100 psi within 45 seconds",
      "section": "5.3.3 – Step 7 Final Air Brake Check",
      "page": 78
    },
    {
      "key": "brake-lag",
      "label": "Brake lag distance at 55 mph",
      "value": "About 32 feet",
      "section": "5.4.4 – Stopping Distance",
      "page": 79
    },
    {
      "key": "air-stop-55",
      "label": "Total stopping distance with air brakes at 55 mph",
      "value": "Over 450 feet",
      "section": "5.4.4 – Stopping Distance",
      "page": 79
    },
    {
      "key": "downgrade-air-braking",
      "label": "Proper air brake technique on a downgrade",
      "value": "Apply to feel a slowdown, release about five mph below the safe speed",
      "section": "5.4.6 – Proper Braking Technique",
      "page": 80
    }
  ],
  "questions": [
    {
      "id": "air-brakes-001",
      "number": 1,
      "section": "5.1 – The Parts of an Air Brake System",
      "stem": "Air brakes are really how many different braking systems?",
      "choices": [
        "Three: service, parking, and emergency",
        "Two: service and parking",
        "Four: service, parking, emergency, and trailer",
        "One combined system"
      ],
      "answer": 0,
      "explanation": "The manual says air brakes are really three different braking systems: service brake, parking brake, and emergency brake.",
      "source": {
        "section": "5.1 – The Parts of an Air Brake System",
        "page": 72,
        "printedPage": "5-1"
      }
    },
    {
      "id": "air-brakes-002",
      "number": 2,
      "section": "5.1.1 – Air Compressor",
      "stem": "What does the air compressor do in an air brake system?",
      "choices": [
        "It pumps air into the air storage tanks",
        "It cools the brake drums",
        "It releases the spring brakes",
        "It measures the air pressure"
      ],
      "answer": 0,
      "explanation": "The air compressor pumps air into the air storage tanks, also called reservoirs.",
      "source": {
        "section": "5.1.1 – Air Compressor",
        "page": 72,
        "printedPage": "5-1"
      }
    },
    {
      "id": "air-brakes-003",
      "number": 3,
      "section": "5.1.2 – Air Compressor Governor",
      "stem": "At about what pressure does the governor allow the air compressor to start pumping again?",
      "choices": [
        "About 100 psi",
        "About 55 psi",
        "About 150 psi",
        "About 200 psi"
      ],
      "answer": 0,
      "explanation": "When tank pressure falls to the cut-in pressure of around 100 psi, the governor allows the compressor to start pumping again.",
      "source": {
        "section": "5.1.2 – Air Compressor Governor",
        "page": 72,
        "printedPage": "5-1",
        "fact": "governor"
      }
    },
    {
      "id": "air-brakes-004",
      "number": 4,
      "section": "5.1.3 – Air Storage Tanks",
      "stem": "Why do air storage tanks hold enough air for several brake applications?",
      "choices": [
        "So the brakes can still be used several times if the compressor stops working",
        "So the vehicle can be driven without the engine",
        "So the tires can be inflated",
        "So the horn can be used for long periods"
      ],
      "answer": 0,
      "explanation": "The tanks hold enough air to allow the brakes to be used several times even if the compressor stops working.",
      "source": {
        "section": "5.1.3 – Air Storage Tanks",
        "page": 72,
        "printedPage": "5-1"
      }
    },
    {
      "id": "air-brakes-005",
      "number": 5,
      "section": "5.1.4 – Air Tank Drains",
      "stem": "How often must you drain air tanks that have manual drain valves?",
      "choices": [
        "At the end of each day of driving",
        "Once a month",
        "Only when the low pressure warning sounds",
        "Every 100,000 miles"
      ],
      "answer": 0,
      "explanation": "With manually operated drain valves you must drain the tanks yourself at the end of each day of driving so water and oil do not build up.",
      "source": {
        "section": "5.1.4 – Air Tank Drains",
        "page": 72,
        "printedPage": "5-1"
      }
    },
    {
      "id": "air-brakes-006",
      "number": 6,
      "section": "5.1.4 – Air Tank Drains",
      "stem": "Why is it important to drain water from air tanks?",
      "choices": [
        "Water can freeze in cold weather and cause brake failure",
        "Water makes the compressor run faster",
        "Water increases the air pressure",
        "Water lubricates the valves"
      ],
      "answer": 0,
      "explanation": "Compressed air carries water and compressor oil that can freeze in cold weather and cause brake failure, so the tanks should be drained.",
      "source": {
        "section": "5.1.4 – Air Tank Drains",
        "page": 73,
        "printedPage": "5-2"
      }
    },
    {
      "id": "air-brakes-007",
      "number": 7,
      "section": "5.1.5 – Alcohol Evaporator",
      "stem": "What is an alcohol evaporator used for in an air brake system?",
      "choices": [
        "To reduce the risk of ice forming in air brake valves in cold weather",
        "To increase the braking power",
        "To clean the brake drums",
        "To lower the air pressure"
      ],
      "answer": 0,
      "explanation": "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.",
      "source": {
        "section": "5.1.5 – Alcohol Evaporator",
        "page": 73,
        "printedPage": "5-2"
      }
    },
    {
      "id": "air-brakes-008",
      "number": 8,
      "section": "5.1.6 – Safety Valve",
      "stem": "At what pressure is the safety valve usually set to open?",
      "choices": [
        "150 psi",
        "55 psi",
        "100 psi",
        "250 psi"
      ],
      "answer": 0,
      "explanation": "The safety relief valve is usually set to open at 150 psi, and if it releases air something is wrong and must be fixed by a mechanic.",
      "source": {
        "section": "5.1.6 – Safety Valve",
        "page": 73,
        "printedPage": "5-2",
        "fact": "safety-valve"
      }
    },
    {
      "id": "air-brakes-009",
      "number": 9,
      "section": "5.1.8 – Foundation Brakes",
      "stem": "How does the most common foundation brake, the s-cam brake, work?",
      "choices": [
        "Air pushes the rod out, turning the s-cam so it forces the brake shoes against the drum",
        "Air pressure pulls the shoes away from the drum",
        "A spring pulls the shoes against the drum",
        "Water pressure pushes the shoes apart"
      ],
      "answer": 0,
      "explanation": "When the brake pedal is pushed, the s-cam forces the brake shoes away from one another and against the inside of the brake drum.",
      "source": {
        "section": "5.1.8 – Foundation Brakes",
        "page": 73,
        "printedPage": "5-2"
      }
    },
    {
      "id": "air-brakes-010",
      "number": 10,
      "section": "5.1.11 – Low Air Pressure Warning",
      "stem": "A visible low air pressure warning signal must come on before the air pressure falls below what level?",
      "choices": [
        "55 psi",
        "100 psi",
        "20 psi",
        "150 psi"
      ],
      "answer": 0,
      "explanation": "A warning signal you can see must come on before the air pressure in the tanks falls below 55 psi, usually as a red light with a buzzer.",
      "source": {
        "section": "5.1.11 – Low Air Pressure Warning",
        "page": 74,
        "printedPage": "5-3",
        "fact": "low-air-warning"
      }
    },
    {
      "id": "air-brakes-011",
      "number": 11,
      "section": "5.1.13 – Front Brake Limiting Valve",
      "stem": "What position should the front brake limiting valve control be in for normal stopping power?",
      "choices": [
        "Normal",
        "Slippery",
        "Automatic",
        "Off"
      ],
      "answer": 0,
      "explanation": "The limiting valve cuts normal air pressure to the front brakes in half when set to slippery, so keep the control in the normal position for normal stopping power.",
      "source": {
        "section": "5.1.13 – Front Brake Limiting Valve",
        "page": 74,
        "printedPage": "5-3"
      }
    },
    {
      "id": "air-brakes-012",
      "number": 12,
      "section": "5.1.14 – Spring Brakes",
      "stem": "At what air pressure do tractor and straight truck spring brakes come fully on?",
      "choices": [
        "20 to 45 psi",
        "55 to 70 psi",
        "80 to 100 psi",
        "120 to 140 psi"
      ],
      "answer": 0,
      "explanation": "Spring brakes come fully on when air pressure drops into the range of 20 to 45 psi, typically 20 to 30 psi.",
      "source": {
        "section": "5.1.14 – Spring Brakes",
        "page": 74,
        "printedPage": "5-3",
        "fact": "spring-brakes"
      }
    },
    {
      "id": "air-brakes-013",
      "number": 13,
      "section": "5.1.14 – Spring Brakes",
      "stem": "Why should you never push the brake pedal down when the spring brakes are on?",
      "choices": [
        "The brakes could be damaged by the combined forces of the springs and air pressure",
        "It will release the spring brakes suddenly",
        "It will drain the air tanks in seconds",
        "It will set off the ABS warning"
      ],
      "answer": 0,
      "explanation": "Never push the brake pedal down when the spring brakes are on, because the brakes could be damaged by the combined forces of the springs and the air pressure.",
      "source": {
        "section": "5.1.14 – Spring Brakes",
        "page": 74,
        "printedPage": "5-3"
      }
    },
    {
      "id": "air-brakes-014",
      "number": 14,
      "section": "5.1.15 – Parking Brake Controls",
      "stem": "How do you apply the parking brakes on a newer vehicle with air brakes?",
      "choices": [
        "Pull out the diamond-shaped yellow push-pull control knob",
        "Push in the round blue knob",
        "Press the foot brake to the floor",
        "Turn the safety valve clockwise"
      ],
      "answer": 0,
      "explanation": "On newer vehicles you pull the diamond-shaped yellow control knob out to put on the parking brakes and push it in to release them.",
      "source": {
        "section": "5.1.15 – Parking Brake Controls",
        "page": 74,
        "printedPage": "5-3"
      }
    },
    {
      "id": "air-brakes-015",
      "number": 15,
      "section": "5.2 – Dual Air Brake",
      "stem": "What is a dual air brake system?",
      "choices": [
        "Two separate air brake systems that use a single set of brake controls",
        "A system with two brake pedals",
        "A trailer brake system only",
        "A system that uses hydraulic and air pressure together"
      ],
      "answer": 0,
      "explanation": "A dual air brake system has two separate air brake systems that use a single set of brake controls and each have their own air tanks, hoses, and lines.",
      "source": {
        "section": "5.2 – Dual Air Brake",
        "page": 76,
        "printedPage": "5-5"
      }
    },
    {
      "id": "air-brakes-016",
      "number": 16,
      "section": "5.2 – Dual Air Brake",
      "stem": "In a typical dual air brake system, what do the two systems operate?",
      "choices": [
        "One operates the rear axle brakes and the other the front axle brakes",
        "One operates the brakes and the other the lights",
        "One operates the front and rear brakes and the other the trailer brakes only",
        "One operates the parking brake and the other the horn"
      ],
      "answer": 0,
      "explanation": "One system typically operates the regular brakes on the rear axle or axles, and the other operates the regular brakes on the front axle and possibly one rear axle.",
      "source": {
        "section": "5.2 – Dual Air Brake",
        "page": 76,
        "printedPage": "5-5"
      }
    },
    {
      "id": "air-brakes-017",
      "number": 17,
      "section": "5.2 – Dual Air Brake",
      "stem": "Before driving a vehicle with a dual air system, how much pressure should build up in both systems?",
      "choices": [
        "A minimum of 100 psi",
        "A minimum of 55 psi",
        "A minimum of 150 psi",
        "A minimum of 200 psi"
      ],
      "answer": 0,
      "explanation": "Before driving, allow time for the air compressor to build a minimum of 100 psi pressure in both the primary and secondary systems.",
      "source": {
        "section": "5.2 – Dual Air Brake",
        "page": 76,
        "printedPage": "5-5"
      }
    },
    {
      "id": "air-brakes-018",
      "number": 18,
      "section": "5.2 – Dual Air Brake",
      "stem": "What should you do if the low air pressure warning comes on in either system of a dual air system while driving?",
      "choices": [
        "Stop right away and safely park the vehicle",
        "Continue to the next service station",
        "Speed up to build air pressure",
        "Pull the trailer air supply knob"
      ],
      "answer": 0,
      "explanation": "If the warning comes on while driving, stop right away and safely park the vehicle, because either the front or the rear brakes will not fully operate.",
      "source": {
        "section": "5.2 – Dual Air Brake",
        "page": 76,
        "printedPage": "5-5"
      }
    },
    {
      "id": "air-brakes-019",
      "number": 19,
      "section": "5.3.2 – During Step 5 Walk-around Inspection",
      "stem": "When checking slack adjusters, how much movement means they probably need adjustment?",
      "choices": [
        "More than about one inch where the push rod attaches",
        "More than one quarter inch",
        "More than six inches",
        "Any movement at all"
      ],
      "answer": 0,
      "explanation": "If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjustment.",
      "source": {
        "section": "5.3.2 – During Step 5 Walk-around Inspection",
        "page": 77,
        "printedPage": "5-6",
        "fact": "slack-adjuster"
      }
    },
    {
      "id": "air-brakes-020",
      "number": 20,
      "section": "5.3.2 – During Step 5 Walk-around Inspection",
      "stem": "What is the most common problem found in roadside inspections?",
      "choices": [
        "Out-of-adjustment brakes",
        "Worn tires",
        "Burnt-out lights",
        "Dirty windshields"
      ],
      "answer": 0,
      "explanation": "The manual says out-of-adjustment brakes are the most common problem found in roadside inspections, and vehicles with too much brake slack can be very hard to stop.",
      "source": {
        "section": "5.3.2 – During Step 5 Walk-around Inspection",
        "page": 77,
        "printedPage": "5-6"
      }
    },
    {
      "id": "air-brakes-021",
      "number": 21,
      "section": "5.3.2 – During Step 5 Walk-around Inspection",
      "stem": "All vehicles built since what year have automatic slack adjusters?",
      "choices": [
        "1994",
        "1975",
        "1998",
        "2005"
      ],
      "answer": 0,
      "explanation": "All vehicles built since 1994 have automatic slack adjusters, though they must still be checked because they can be damaged or out of adjustment.",
      "source": {
        "section": "5.3.2 – During Step 5 Walk-around Inspection",
        "page": 77,
        "printedPage": "5-6"
      }
    },
    {
      "id": "air-brakes-022",
      "number": 22,
      "section": "5.3.2 – During Step 5 Walk-around Inspection",
      "stem": "How long may a crack in a brake drum be?",
      "choices": [
        "No longer than one half the width of the friction area",
        "No longer than the whole width of the drum",
        "Up to two inches",
        "Cracks of any length are allowed"
      ],
      "answer": 0,
      "explanation": "Brake drums or discs must not have cracks longer than one half the width of the friction area.",
      "source": {
        "section": "5.3.2 – During Step 5 Walk-around Inspection",
        "page": 77,
        "printedPage": "5-6"
      }
    },
    {
      "id": "air-brakes-023",
      "number": 23,
      "section": "5.3.3 – Step 7 Final Air Brake Check",
      "stem": "With a fully charged air system and the engine off, what is the maximum air leakage rate for a single vehicle?",
      "choices": [
        "Less than two psi in one minute",
        "Less than ten psi in one minute",
        "Less than five psi in one minute",
        "Less than one psi in five minutes"
      ],
      "answer": 0,
      "explanation": "The loss rate should be less than two psi in one minute for single vehicles and less than three psi in one minute for combination vehicles.",
      "source": {
        "section": "5.3.3 – Step 7 Final Air Brake Check",
        "page": 77,
        "printedPage": "5-6",
        "fact": "leak-rate"
      }
    },
    {
      "id": "air-brakes-024",
      "number": 24,
      "section": "5.3.3 – Step 7 Final Air Brake Check",
      "stem": "With the engine off and the foot brake fully applied for one minute, how much may the air pressure drop on a combination vehicle?",
      "choices": [
        "No more than four pounds in one minute",
        "No more than ten pounds in one minute",
        "No more than one pound in one minute",
        "No more than two pounds in one minute"
      ],
      "answer": 0,
      "explanation": "With the foot brake held for one minute, the pressure may drop no more than three pounds on a single vehicle or four pounds on a combination vehicle.",
      "source": {
        "section": "5.3.3 – Step 7 Final Air Brake Check",
        "page": 78,
        "printedPage": "5-7",
        "fact": "foot-brake-leak"
      }
    },
    {
      "id": "air-brakes-025",
      "number": 25,
      "section": "5.3.3 – Step 7 Final Air Brake Check",
      "stem": "When testing the low pressure warning signal, before what pressure must the signal come on?",
      "choices": [
        "Before the pressure drops to less than 55 psi",
        "Before the pressure drops to less than 100 psi",
        "Before the pressure drops to less than 20 psi",
        "Before the pressure rises above 55 psi"
      ],
      "answer": 0,
      "explanation": "The low air pressure warning signal must come on before the pressure drops to less than 55 psi in the air tank with the lowest pressure.",
      "source": {
        "section": "5.3.3 – Step 7 Final Air Brake Check",
        "page": 78,
        "printedPage": "5-7"
      }
    },
    {
      "id": "air-brakes-026",
      "number": 26,
      "section": "5.3.3 – Step 7 Final Air Brake Check",
      "stem": "How can you check that the spring brakes come on automatically?",
      "choices": [
        "Fan off the air pressure and watch for the tractor protection and parking brake valves to pop out at 20 to 45 psi",
        "Push the brake pedal to the floor and listen",
        "Pull the yellow parking knob and release it",
        "Open the drain valves and watch the gauge"
      ],
      "answer": 0,
      "explanation": "Continue to fan off the air pressure; the tractor protection valve and parking brake valve should close or pop out at 20 to 45 psi, causing the spring brakes to come on.",
      "source": {
        "section": "5.3.3 – Step 7 Final Air Brake Check",
        "page": 78,
        "printedPage": "5-7"
      }
    },
    {
      "id": "air-brakes-027",
      "number": 27,
      "section": "5.3.3 – Step 7 Final Air Brake Check",
      "stem": "In a dual air system with the engine at operating rpm, how fast should pressure build from 85 to 100 psi?",
      "choices": [
        "Within 45 seconds",
        "Within 3 minutes",
        "Within 10 minutes",
        "Within 5 seconds"
      ],
      "answer": 0,
      "explanation": "In dual air systems the pressure should build from 85 to 100 psi within 45 seconds; older single systems build from 50 to 90 psi within 3 minutes.",
      "source": {
        "section": "5.3.3 – Step 7 Final Air Brake Check",
        "page": 78,
        "printedPage": "5-7",
        "fact": "pressure-buildup"
      }
    },
    {
      "id": "air-brakes-028",
      "number": 28,
      "section": "5.3.3 – Step 7 Final Air Brake Check",
      "stem": "How should you test the parking brake?",
      "choices": [
        "Stop the vehicle, apply the parking brake, and gently pull against it in a low gear",
        "Roll backward down a hill and apply it",
        "Apply it at highway speed",
        "Check that the knob is yellow"
      ],
      "answer": 0,
      "explanation": "To test the parking brake, stop the vehicle, put the parking brake on, and gently pull against it in a low gear to make sure it holds.",
      "source": {
        "section": "5.3.3 – Step 7 Final Air Brake Check",
        "page": 78,
        "printedPage": "5-7"
      }
    },
    {
      "id": "air-brakes-029",
      "number": 29,
      "section": "5.4.1 – Normal Stops",
      "stem": "With a manual transmission, when should you push the clutch in during a normal stop with air brakes?",
      "choices": [
        "When the engine rpm is down close to idle",
        "As soon as you touch the brake pedal",
        "Before you begin braking",
        "Only after the vehicle has stopped completely"
      ],
      "answer": 0,
      "explanation": "Control the brake pressure for a smooth, safe stop, and with a manual transmission do not push the clutch in until the engine rpm is down close to idle.",
      "source": {
        "section": "5.4.1 – Normal Stops",
        "page": 79,
        "printedPage": "5-8"
      }
    },
    {
      "id": "air-brakes-030",
      "number": 30,
      "section": "5.4.4 – Stopping Distance",
      "stem": "What does brake lag add to stopping distance at 55 mph on dry pavement?",
      "choices": [
        "About 32 feet",
        "About 4 feet",
        "About 216 feet",
        "About 100 feet"
      ],
      "answer": 0,
      "explanation": "Brake lag is the time for air to flow through the lines to the brakes, and at 55 mph on dry pavement it adds about 32 feet.",
      "source": {
        "section": "5.4.4 – Stopping Distance",
        "page": 79,
        "printedPage": "5-8",
        "fact": "brake-lag"
      }
    },
    {
      "id": "air-brakes-031",
      "number": 31,
      "section": "5.4.4 – Stopping Distance",
      "stem": "At 55 mph, what is the total stopping distance for an air brake vehicle under good conditions?",
      "choices": [
        "Over 450 feet",
        "About 216 feet",
        "About 250 feet",
        "About 100 feet"
      ],
      "answer": 0,
      "explanation": "At 55 mph for an average driver under good traction and brake conditions, the total stopping distance with air brakes is over 450 feet.",
      "source": {
        "section": "5.4.4 – Stopping Distance",
        "page": 79,
        "printedPage": "5-8",
        "fact": "air-stop-55"
      }
    },
    {
      "id": "air-brakes-032",
      "number": 32,
      "section": "5.4.5 – Brake Fading or Failure",
      "stem": "What causes brake fade?",
      "choices": [
        "Excessive heat that chemically changes the lining and expands the drums",
        "Cold brake drums",
        "Too much engine oil in the air system",
        "Brake adjustment that is too tight"
      ],
      "answer": 0,
      "explanation": "Brake fade comes from excessive heat causing chemical changes in the brake lining and expansion of the brake drums, which reduce friction.",
      "source": {
        "section": "5.4.5 – Brake Fading or Failure",
        "page": 80,
        "printedPage": "5-9"
      }
    },
    {
      "id": "air-brakes-033",
      "number": 33,
      "section": "5.4.5 – Brake Fading or Failure",
      "stem": "Why do brakes that are out of adjustment fade faster?",
      "choices": [
        "The adjusted brakes overheat and fade after doing more than their share of the work",
        "They apply too much air pressure",
        "They cool the drums too much",
        "They release the spring brakes"
      ],
      "answer": 0,
      "explanation": "Out-of-adjustment brakes stop doing their share of the work, so the other brakes can overheat and fade until there is not enough braking to control the vehicle.",
      "source": {
        "section": "5.4.5 – Brake Fading or Failure",
        "page": 80,
        "printedPage": "5-9"
      }
    },
    {
      "id": "air-brakes-034",
      "number": 34,
      "section": "5.4.6 – Proper Braking Technique",
      "stem": "On a long downgrade, what is the proper air brake technique?",
      "choices": [
        "Apply the brakes just hard enough to feel a slowdown and release about five mph below the safe speed",
        "Hold the brakes on all the way down the grade",
        "Use the trailer hand brake to control speed",
        "Pump the brakes on and off rapidly the whole way"
      ],
      "answer": 0,
      "explanation": "Apply the brakes just hard enough to feel a definite slowdown, release when about five mph below your safe speed, and repeat when the speed returns to it.",
      "source": {
        "section": "5.4.6 – Proper Braking Technique",
        "page": 80,
        "printedPage": "5-9",
        "fact": "downgrade-air-braking"
      }
    },
    {
      "id": "air-brakes-035",
      "number": 35,
      "section": "5.4.7 – Low Air Pressure",
      "stem": "What should you do if the low air pressure warning comes on?",
      "choices": [
        "Stop and safely park the vehicle as soon as possible",
        "Drive faster to build air pressure",
        "Turn off the engine and coast",
        "Pull the trailer air supply knob out"
      ],
      "answer": 0,
      "explanation": "If the low air pressure warning comes on, stop and safely park as soon as possible, because controlled braking is possible only while enough air remains.",
      "source": {
        "section": "5.4.7 – Low Air Pressure",
        "page": 80,
        "printedPage": "5-9"
      }
    },
    {
      "id": "air-brakes-036",
      "number": 36,
      "section": "5.4.8 – Parking Brakes",
      "stem": "When should you avoid using the parking brakes?",
      "choices": [
        "When the brakes are very hot from a steep grade or very wet in freezing temperatures",
        "Whenever you park on a level surface",
        "Whenever the vehicle is loaded",
        "When the engine is still running"
      ],
      "answer": 0,
      "explanation": "Do not use the parking brakes if the brakes are very hot or very wet in freezing temperatures, because they can be damaged by heat or freeze in place.",
      "source": {
        "section": "5.4.8 – Parking Brakes",
        "page": 80,
        "printedPage": "5-9"
      }
    },
    {
      "id": "air-brakes-037",
      "number": 37,
      "section": "5.4.8 – Parking Brakes",
      "stem": "What should you do if you park a vehicle with wet brakes in freezing weather?",
      "choices": [
        "Use wheel chocks and dry the brakes by using them lightly in a low gear",
        "Set the parking brakes firmly and leave them",
        "Leave the engine running all night",
        "Pour hot water on the brake drums"
      ],
      "answer": 0,
      "explanation": "If the brakes are wet, use wheel chocks to hold the vehicle and use the brakes lightly in a low gear to heat and dry them, rather than setting the parking brake.",
      "source": {
        "section": "5.4.8 – Parking Brakes",
        "page": 80,
        "printedPage": "5-9"
      }
    },
    {
      "id": "air-brakes-038",
      "number": 38,
      "section": "5.4.8 – Parking Brakes",
      "stem": "If your air tanks do not have automatic drains, when should you drain them?",
      "choices": [
        "At the end of each working day",
        "Every week",
        "Only before a long trip",
        "Never, because the tanks are sealed"
      ],
      "answer": 0,
      "explanation": "If your vehicle does not have automatic air tank drains, drain your air tanks at the end of each working day to remove moisture and oil, or the brakes could fail.",
      "source": {
        "section": "5.4.8 – Parking Brakes",
        "page": 80,
        "printedPage": "5-9"
      }
    }
  ]
}