Kentucky CDL — Air Brakes Practice Test
25 questions · pass with 20 correct. You get instant feedback and an explanation after every answer.
Kentucky CDL — Air Brakes questions with answers
20 sample questions from the Kentucky CDL — Air Brakes question bank — each with the correct answer, a short explanation, and the section of the official handbook it comes from.
1. For a smooth normal stop with a manual transmission, when should you push in the clutch?
Correct answer: Not until the engine rpm drops close to idle
Control the pedal for a smooth stop and, with a manual gearbox, wait to press the clutch until the engine speed is near idle, then choose a starting gear.
Source: Section 5.4.1 - Normal Stops
2. During a walk-around, a manually adjusted s-cam slack adjuster probably needs adjustment if it moves more than about how much?
Correct answer: About one inch
With the parking brakes released, pull hard on the slack adjuster; movement of more than roughly one inch where the push rod attaches signals it likely needs adjusting.
Source: Section 5.3.2 - Walk-around Inspection
3. At 55 mph on dry pavement, about how many feet does air brake lag add to the stopping distance?
Correct answer: About 32 feet
Brake lag alone contributes roughly 32 feet at 55 mph on dry pavement, which helps push the total stopping distance to over 450 feet.
Source: Section 5.4.4 - Stopping Distance
4. What is the purpose of the stop light switch in an air brake system?
Correct answer: To light the brake lamps whenever you apply the brakes
The stop light switch is an air-operated electric switch that lights the brake lamps so the drivers behind you know you are braking.
Source: Section 5.1.12 - Stop Light Switch
5. A brake drum may be unsafe if it has a crack of what size?
Correct answer: A crack longer than half the width of the friction surface
Drums or discs must not have cracks exceeding half the width of the friction surface; linings also must not be loose, oil-soaked, or dangerously thin.
Source: Section 5.3.2 - Walk-around Inspection
6. After a loss of air, why does a heavily loaded vehicle need a long distance to stop once the spring brakes apply?
Correct answer: The spring brakes act on only some of the axles
Spring brakes act on only some axles, so a heavy load takes a long way to stop; it is far safer to pull over while there is still air for the foot brakes.
Source: Section 5.4.7 - Low Air Pressure
7. About what pressure allows the governor to let the compressor begin pumping again (cut-in)?
Correct answer: Around 100 psi
Cut-in happens near 100 psi and restarts the compressor, while cut-out near 125 psi stops it once more.
Source: Section 5.1.2 - Air Compressor Governor
8. On newer air brake vehicles, what does the parking brake control knob look like?
Correct answer: A yellow, diamond-shaped push-pull knob
Newer vehicles use a yellow, diamond-shaped push-pull knob; pull it out to set the parking (spring) brakes and push it in to release them.
Source: Section 5.1.15 - Parking Brake Controls
9. What is the most common brake problem found during roadside inspections?
Correct answer: Brakes that are out of adjustment
Out-of-adjustment brakes turn up more than any other brake defect in roadside inspections, and too much slack can make a vehicle very hard to stop.
Source: Section 5.3.2 - Walk-around Inspection
10. On a long, steep downgrade, the service brakes should be used in what role?
Correct answer: As a supplement to the engine's braking effect
On a long or steep grade the brakes only supplement engine braking; get into a low gear first, then use the brakes in short applications.
Source: Section 5.4.6 - Proper Braking Technique
11. How does a "wig wag" low-pressure warning device work?
Correct answer: It drops a mechanical arm into the driver's view when pressure falls below 55 psi
A wig wag lowers a mechanical arm into your line of sight once pressure drops under 55 psi; the automatic version lifts back out of view when pressure climbs above that level.
Source: Section 5.1.11 - Low Air Pressure Warning
12. What is a dual air brake system?
Correct answer: Two separate air brake systems worked by one set of controls
A dual setup is really two independent air brake systems, each with its own tanks and lines, worked through a single set of controls, one primary and one secondary.
Source: Section 5.2 - Dual Air Brake Systems
13. In "controlled braking" during an emergency, how hard do you apply the brakes?
Correct answer: As hard as possible without letting the wheels lock
Controlled braking means braking as hard as you can while keeping the wheels turning and steering movements small; if a wheel locks or you must steer more, ease off then reapply.
Source: Section 5.4.3 - Emergency Stops
14. Why should you avoid pressing the brake pedal while the spring brakes are already applied?
Correct answer: The combined spring and air forces could damage the brakes
Adding pedal air pressure on top of the spring force can overload and damage the brakes; some systems guard against this, but not all, so build the habit of not doing it.
Source: Section 5.1.15 - Parking Brake Controls
15. Which four elements add up to the total stopping distance for an air brake vehicle?
Correct answer: Perception, reaction, brake lag, and braking distances
For air brakes the total is the sum of perception distance, reaction distance, the added brake lag distance, and the actual braking distance.
Source: Section 5.4.4 - Stopping Distance
16. What information does a supply pressure gauge give the driver?
Correct answer: How much air pressure is stored in the tanks
The supply gauge, fitted to every air brake vehicle, shows the amount of pressure held in the tanks; a dual system uses one gauge per half or a two-needle gauge.
Source: Section 5.1.9 - Supply Pressure Gauges
17. The brake pedal on an air brake vehicle is also known by what other names?
Correct answer: The foot valve or treadle valve
The pedal you press to brake is also called the foot valve or treadle valve; pushing it harder sends more air pressure to the brakes.
Source: Section 5.1.7 - The Brake Pedal
18. Using the proper downhill braking technique, how long should each brake application last?
Correct answer: About three seconds
Once in a low gear, brake hard enough to feel a clear slowdown for roughly three seconds, dropping about five mph below your safe speed, then release.
Source: Section 5.4.6 - Proper Braking Technique
19. When should you avoid setting the parking brakes?
Correct answer: When the brakes are very hot or very wet in freezing temperatures
Hot brakes can be damaged and wet brakes can freeze solid, so in those cases use wheel chocks instead; let hot brakes cool, or dry wet ones by driving lightly in a low gear.
Source: Section 5.4.8 - Parking Brakes
20. What is "brake lag" in an air brake system?
Correct answer: The short delay while air travels to the brakes after you press the pedal
Unlike hydraulic brakes, air brakes need a moment, about half a second or more, for air to travel through the lines, adding a delay before braking begins.
Source: Section 5.4.4 - Stopping Distance