Oklahoma CDL — Air Brakes Exam Simulation
25 questions · 20 correct to pass. You won't see any feedback until you finish — just like the real test.
Oklahoma CDL — Air Brakes questions with answers
20 sample questions from the Oklahoma CDL — Air Brakes question bank — each with the correct answer, a short explanation, and the section of the official handbook it comes from.
1. In a vehicle without ABS, what happens when the steering-axle wheels lock up during hard braking?
Correct answer: You lose steering control
Without ABS, hard braking on slippery surfaces can lock the wheels; when the steering wheels lock you lose the ability to steer, and other locked wheels can cause a skid, jackknife, or spin.
Source: Section 5.4.2 - Braking with Antilock Brakes
2. What braking equipment must every truck, truck tractor, and bus carry?
Correct answer: Emergency and parking brakes held on by mechanical force
These vehicles must have emergency and parking brakes, and since air can eventually leak away, those brakes are held on by mechanical force, usually by spring brakes.
Source: Section 5.1.14 - Spring Brakes
3. At 55 mph on dry pavement, roughly how much distance does air brake lag add by itself?
Correct answer: About 32 feet
The manual states that the brake lag distance at 55 mph on dry pavement adds about 32 feet.
Source: Section 5.4.4 - Stopping Distance
4. In a dual air system with the engine at operating rpm, air pressure should build within which range and time?
Correct answer: From 85 to 100 psi within 45 seconds
For a dual air system, with the engine at operating speed the pressure should climb from 85 to 100 psi within 45 seconds; larger-than-minimum tanks may take longer and still be safe.
Source: Section 5.3.3 - Final Air Brake Check (Rate of Air Pressure Buildup)
5. What is the job of an alcohol evaporator in some air brake systems?
Correct answer: To reduce the risk of ice forming in the valves in cold weather
By feeding alcohol into the system, the evaporator lowers the chance of ice forming in air brake valves and other parts during cold weather, since ice can keep the brakes from working.
Source: Section 5.1.5 - Alcohol Evaporator
6. On large buses, at what pressure do the low pressure warning devices commonly activate?
Correct answer: 80 to 85 psi
While most vehicles warn below 55 psi, it is common for the low pressure warning on large buses to signal at 80 to 85 psi.
Source: Section 5.1.11 - Low Air Pressure Warning
7. When should you avoid setting the parking brakes?
Correct answer: When the brakes are very hot or very wet in freezing temperatures
Don't set the parking brakes if they are very hot from a steep grade or very wet in freezing weather, since heat can damage them and freezing can lock them; use wheel chocks instead.
Source: Section 5.4.8 - Parking Brakes
8. What is the main cause of brake fade?
Correct answer: Excessive heat from overusing the service brakes
Brakes fade when they overheat from being used too much instead of relying on engine braking; the heat changes the lining chemically and expands the drums.
Source: Section 5.4.5 - Brake Fading or Failure
9. Truck tractors with air brakes built on or after which date are required to have antilock brakes?
Correct answer: March 1, 1997
Air brake truck tractors manufactured on or after March 1, 1997 must carry ABS; other air brake vehicles are covered from March 1, 1998.
Source: Section 5.1.16 - Antilock Braking Systems (ABS)
10. Before driving a vehicle with a dual air system, what minimum pressure should build in both the primary and secondary systems?
Correct answer: 100 psi
Give the compressor time to build at least 100 psi in both the primary and secondary systems before you start driving.
Source: Section 5.2 - Dual Air Brake
11. With the engine off and the parking brake released, what is the largest allowable air loss rate for a single vehicle?
Correct answer: Less than two psi in one minute
With a fully charged system, the engine off, and the parking brake released, a single vehicle's air loss should stay under two psi per minute; for combination vehicles the limit is under three psi.
Source: Section 5.3.3 - Final Air Brake Check (Air Leakage Rate)
12. Why is it important to drain the air storage tanks completely?
Correct answer: To clear out water and oil that harm the system and can freeze
Compressed air carries water and a little compressor oil that settle at the bottom of the tank; the water can freeze in cold weather and cause the brakes to fail, so the tanks must be emptied fully.
Source: Section 5.1.4 - Air Tank Drains
13. How does the air brake system turn on the brake lights for traffic behind you?
Correct answer: Through an electric switch operated by air pressure
An electric stop light switch that reacts to air pressure lights up the brake lamps whenever the air brakes go on, warning drivers behind you.
Source: Section 5.1.12 - Stop Light Switch
14. On tractors and straight trucks, spring brakes come fully on when air pressure falls into which range?
Correct answer: 20 to 45 psi
The spring brakes on tractors and straight trucks apply fully once air pressure drops to somewhere in the 20 to 45 psi range, often 20 to 30 psi.
Source: Section 5.1.14 - Spring Brakes
15. How does excessive heat produce brake fade in a drum brake?
Correct answer: It makes the drums expand so the shoes travel farther and press with less force
Overheating both reduces the lining's friction and expands the drums; as the drums grow, the shoes and linings must reach farther to touch them, and they press with less force.
Source: Section 5.4.5 - Brake Fading or Failure
16. The driver-operated front brake limiting valve was fitted on some vehicles built during which era?
Correct answer: Before 1975
A cab-controlled front brake limiting valve, with a switch marked 'normal' and 'slippery,' appeared on some older vehicles made before 1975.
Source: Section 5.1.13 - Front Brake Limiting Valve
17. For an average driver at 55 mph in good conditions, the total stopping distance of an air brake vehicle is about how far?
Correct answer: More than 450 feet
Adding all four factors together, the total stopping distance at 55 mph for an average driver with good traction and brakes works out to more than 450 feet.
Source: Section 5.4.4 - Stopping Distance
18. With the foot brake fully applied and held, what is the largest acceptable one-minute pressure drop for a combination vehicle?
Correct answer: Four psi
Holding the foot brake down for a minute, the pressure should not drop more than three psi for a single vehicle or more than four psi for a combination vehicle.
Source: Section 5.3.3 - Final Air Brake Check (Air Leakage Rate)
19. Which statement about slack adjusters on vehicles built since 1994 is accurate?
Correct answer: They are automatic but still have to be checked
Every vehicle made since 1994 uses automatic slack adjusters; even though they self-adjust during full brake applications, the driver still has to inspect them.
Source: Section 5.3.2 - Walk-around Inspection
20. What does a supply pressure gauge tell the driver?
Correct answer: How much pressure is stored in the air tanks
Every air brake vehicle has a supply pressure gauge tied to the air tank; it reports how much pressure the tanks are holding.
Source: Section 5.1.9 - Supply Pressure Gauges