All exams/Driving & Licenses/CDL Air Brakes Endorsement/Free questions
Tap an answer to see if you are right, and why. 20 questions across 8 topics, mostly System parts (3).
The real test
Multiple-choice knowledge test · 80% to pass (49 CFR 383.135) · item count set by your state.
80% to pass · question count set by your state
System parts1 of 20
The answer
Answer: B. Air brakes are described as three systems working together: service brakes slow the moving vehicle, parking brakes hold it stopped, and emergency brakes use parts of both to stop it after a failure. Treating them as one system hides the separate controls and air paths you must inspect.
Source: 5.1 - The Parts of an Air Brake System
Compressor governor tanks2 of 20
The answer
Answer: C. At the cut-out point, roughly 125 psi, the governor halts compressor output so the tanks do not keep rising. When pressure later falls to the cut-in point, about 100 psi, it lets the compressor pump again. Opening the safety valve is a separate protection that works only if the pressure keeps climbing past the governor's setting, and draining the tanks is a manual job done at the end of the day.
Source: 5.1.2 - Air Compressor Governor
Dual brakes spring3 of 20
The answer
Answer: C. The two circuits have their own tanks, hoses and lines, but the driver works them through one set of brake controls. That separation is what lets one circuit keep braking when the other loses pressure. The tanks, hoses and lines are not shared, which is why choosing 'one common set of tanks' or a shared drain would defeat the purpose of having two circuits.
Source: 5.2 - Dual Air Brake
Inspection leak tests4 of 20
The answer
Answer: C. A slack adjuster that travels more than about an inch where the push rod attaches is likely out of adjustment. Park on level ground, chock the wheels and release the parking brakes before you check it. Three or six inches of travel would mean the brake was badly out of adjustment, and 'any movement at all' is too strict because some slack is normal.
Source: 5.3.2 - During Step 5 Walk-around Inspection
Low air warning5 of 20
The answer
Answer: C. Federal model guidance puts the low pressure warning threshold at 55 psi: the visible signal must appear before tank pressure drops under that figure. On older vehicles the trigger may instead be one half of the governor cut-out pressure. The figures 75, 95 and 125 psi are all too high: 125 psi is about where the governor cuts out, so a warning there would sound all the time.
Source: 5.1.11 - Low Air Pressure Warning
Stopping distance6 of 20
The answer
Answer: D. The four components add together: perception distance, reaction distance, brake lag distance and braking distance. Air brakes add the lag term because pressure must travel through the lines before the brakes apply. The wrong lists mix in terms the manual does not use, such as steering distance, unloading distance, engine drag and air loss.
Source: 5.4.4 - Stopping Distance
Brake fade use7 of 20
The answer
Answer: B. Overheated linings change chemically and the drums expand, so friction drops and the shoes must travel farther to meet the drum. Keep using the brakes hard and the fade can progress until the vehicle will not stop. A loose parking brake cable, cold air or a low cut-in setting does not overheat the linings, and overheating is what causes fade.
Source: 5.4.5 - Brake Fading or Failure
Restrictions abs8 of 20
The answer
Answer: A. Scoring below 80 percent on the air brake knowledge portion triggers an air brake restriction under 49 CFR 383.135(a)(2), keyed to 383.95(a). The licence then bars operation of a CMV equipped with any type of air brakes. The other restrictions concern transmission type, a tractor-trailer connection or weight, and none of them is linked to the air brake knowledge test.
Source: 49 CFR 383.135(a)(2) - Passing knowledge and skills tests; 383.95(a)
System parts9 of 20
The answer
Answer: D. The service brake is the everyday circuit: pedal pressure sends air to the chambers and releasing the pedal lets it out. Parking and emergency braking are separate functions even though they share components. The yellow knob is the parking control, and spring brakes or the trailer protection valve belong to emergency and parking braking, not to normal stops.
Source: 5.1 - The Parts of an Air Brake System
Compressor governor tanks10 of 20
The answer
Answer: D. The compressor is the system's source of compressed air, feeding the reservoirs that the brakes draw from. It is driven by gears or a belt and may be cooled by air or by the engine cooling system. Measuring wheel pressure, releasing spring brakes and drying the supply line are jobs for other parts such as gauges, the supply control and the dryer.
Source: 5.1.1 - Air Compressor
Dual brakes spring11 of 20
The answer
Answer: B. Powerful springs are held compressed by air pressure whenever the system is charged. Release or lose that air and the springs extend to apply the brakes, which is why spring brakes serve as the parking and emergency brake. There is no mechanical latch, hydraulic fluid or vacuum reserve in the system: it is the air itself that keeps the springs compressed.
Source: 5.1.14 - Spring Brakes
Inspection leak tests12 of 20
The answer
Answer: C. With a fully charged system, the engine off and the parking brake released, a single vehicle should lose under 2 psi per minute. A combination vehicle gets a slightly looser limit of under 3 psi per minute. Four, six or ten psi per minute would be too much leakage, and a loss that large means you must find and fix the leak before driving.
Source: 5.3.3 - Step 7 Final Air Brake Check
Low air warning13 of 20
The answer
Answer: D. The automatic version swings its arm into the driver's view when tank pressure falls under 55 psi and lifts it out of sight once pressure is restored. A manual reset wig wag has to be placed out of view by hand and will not stay there until pressure is above 55 psi.
Source: 5.1.11 - Low Air Pressure Warning
Stopping distance14 of 20
The answer
Answer: A. Air must flow through the lines after the pedal moves, so the brakes engage later than hydraulic brakes would. At 55 mph that lag accounts for roughly 32 extra feet before braking distance even begins. Twelve feet is too short for the delay you would feel, and the larger figures of 52 or 72 feet overstate it.
Source: 5.4.4 - Stopping Distance
Brake fade use15 of 20
The answer
Answer: A. Select a safe low gear before the descent and let engine braking carry most of the load. The brakes are then applied only to trim speed, which keeps heat from building beyond what the drums can shed. Using the brakes as the main control, saving them for the bottom or holding one long application all lead to overheated drums and fade.
Source: 5.4.6 - Proper Braking Technique
Restrictions abs16 of 20
The answer
Answer: A. The model manual gives March 1, 1997 for air-braked truck tractors, while other air brake vehicles such as trucks, buses, trailers and dollies follow on March 1, 1998. A certification label shows the build date when you need to confirm equipment.
Source: 5.1.16 - Antilock Braking Systems (ABS)
System parts17 of 20
The answer
Answer: A. Air pressure pushes the rod out, the rod moves the slack adjuster, and the adjuster twists the camshaft. That rotation turns the s-shaped cam so the shoes press against the inside of the drum. The drum, the supply valve and the governor are not moved by the rod: the drum turns with the wheel, and the valve and governor sit elsewhere in the system.
Source: 5.1.8 - Foundation Brakes
Inspection leak tests18 of 20
The answer
Answer: C. Run the engine at fast idle and watch the gauge: cut-in is around 100 psi and cut-out around 125 psi, subject to the manufacturer's specification. If the governor does not behave this way, the system may not keep enough air for safe driving.
Source: 5.3.3 - Step 7 Final Air Brake Check
Dual brakes spring19 of 20
The answer
Answer: C. Allow the compressor time to bring both the primary and secondary circuits to a minimum of 100 psi. Watch both gauges or needles and confirm the low pressure warning has shut off before you move. Fifty-five psi is only the warning level, and 80 or 150 psi do not match the minimum the manual gives for starting out.
Source: 5.2 - Dual Air Brake
Brake fade use20 of 20
The answer
Answer: B. Very hot brakes can be damaged by the parking brake, so let them cool first and use wheel chocks on level ground if needed. Wet brakes in freezing weather are the other exception, because they can freeze and lock the vehicle in place.
Source: 5.4.8 - Parking Brakes
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