South Carolina CDL — Air Brakes Practice Test
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South Carolina CDL — Air Brakes questions with answers
20 sample questions from the South Carolina CDL — Air Brakes question bank — each with the correct answer, a short explanation, and the section of the official handbook it comes from.
1. How do you test that the parking brake will hold?
Correct answer: Apply the parking brake, then tug lightly against it while in a low gear
Stop the vehicle, apply the parking brake, and tug gently against it in a low gear to confirm it holds.
Source: South Carolina CDL Manual — Section 5.3.3: Final Air Brake Check
2. In an s-cam brake, what happens when you press the brake pedal?
Correct answer: The s-cam turns and forces the brake shoes out against the drum
Pedal pressure sends air to the chamber, which moves the slack adjuster and twists the camshaft; the S-shaped cam then forces the brake shoes apart against the drum to create friction.
Source: South Carolina CDL Manual — Section 5.1.8: Foundation Brakes
3. In a dual air system, when should the low air pressure warning come on?
Correct answer: Before either system's pressure sinks below 60 psi
The warning light and buzzer must activate before pressure in either the primary or secondary system falls under 60 psi; if it does while driving, stop and park safely.
Source: South Carolina CDL Manual — Section 5.2: Dual Air Brake
4. With the engine off and the foot brake fully applied, what is the most air a single vehicle should lose in one minute?
Correct answer: No more than 3 psi
Holding the foot brake down, a single vehicle should not drop more than 3 psi in one minute (a combination vehicle, no more than 4 psi); more than that means leaks to fix before driving.
Source: South Carolina CDL Manual — Section 5.3.3: Final Air Brake Check
5. Vehicles built since which year come equipped with automatic slack adjusters?
Correct answer: 1994
Since 1994, every vehicle has come with automatic slack adjusters, though they still must be checked rather than routinely adjusted by hand.
Source: South Carolina CDL Manual — Section 5.3.2: Walk-around Inspection
6. In a dual air system, what happens if one of the two air systems gets very low on pressure?
Correct answer: Either the front or rear brakes will not work fully, so it takes longer to stop
Losing pressure in one system leaves either the front or rear brakes only partly working, which lengthens your stopping distance; guide the truck safely to a halt and have the brakes serviced.
Source: South Carolina CDL Manual — Section 5.2: Dual Air Brake
7. Total stopping distance for an air brake vehicle is made up of four parts. Which set lists them correctly?
Correct answer: Perception, reaction, brake lag, and braking distance
For air brakes the total is perception distance plus reaction distance plus brake lag distance plus braking distance.
Source: South Carolina CDL Manual — Section 5.4.4: Stopping Distance
8. 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 by setting the parking brakes, and very wet brakes can freeze in cold weather; in those cases let hot brakes cool or dry wet ones, and chock the wheels.
Source: South Carolina CDL Manual — Section 5.4.8: Parking Brakes
9. What is the main job of the air compressor in an air brake system?
Correct answer: It pumps air into the storage tanks that feed the brakes
The compressor forces air into the reservoirs, building up the pressure the brakes depend on; it is driven off the engine by gears or a belt.
Source: South Carolina CDL Manual — Section 5.1.1: Air Compressor
10. How does a 'wig wag' low-pressure warning device alert the driver?
Correct answer: It drops a mechanical arm into the driver's view
The wig wag lowers a mechanical arm into view when pressure falls below 60 psi; an automatic one lifts back out of view once pressure climbs above 60 psi again.
Source: South Carolina CDL Manual — Section 5.1.11: Low Air Pressure Warning
11. During the air leakage test on a single vehicle, with the engine off and the parking brake released, the pressure drop should be no more than how much per minute?
Correct answer: Less than 2 psi in one minute
With a fully charged system and the parking brake released, a single vehicle should lose under 2 psi per minute (a combination vehicle, under 3 psi per minute).
Source: South Carolina CDL Manual — Section 5.3.3: Final Air Brake Check
12. According to the manual, what is the most common defect found during roadside inspections?
Correct answer: Brakes that are out of adjustment
Roadside inspections turn up out-of-adjustment brakes more than any other defect, which is why the slack adjusters deserve a careful look.
Source: South Carolina CDL Manual — Section 5.3.2: Walk-around Inspection
13. Why can a heavily loaded vehicle take a long distance to stop once the spring brakes come on?
Correct answer: Because the spring brakes act on only some of the axles, not all
Spring brakes don't apply at every axle, so a heavy load can roll a long way before stopping; it is safer to stop while the tanks still have air for the foot brakes.
Source: South Carolina CDL Manual — Section 5.4.7: Low Air Pressure
14. Using proper downgrade braking with a 'safe' speed of 40 mph, how should you use the brakes?
Correct answer: At 40 mph, brake firmly down to about 35 mph, then release
Apply firmly enough to feel a definite slowdown, drop to about 5 mph below your safe speed (40 down to 35), release for a few seconds, then repeat as the speed builds back toward 40.
Source: South Carolina CDL Manual — Section 5.4.6: Proper Braking Technique
15. If your vehicle's tanks have manual drains, how often should you drain them?
Correct answer: At the close of each driving day
Manual tanks should be drained completely at the close of each driving day to clear out the water and oil that collect inside.
Source: South Carolina CDL Manual — Section 5.1.4: Air Tank Drains
16. Before checking the slack adjusters on s-cam brakes, what should you do to keep the vehicle from moving?
Correct answer: Chock the wheels while parked on level ground
With the vehicle on level, chocked ground, release the parking brakes so the slack adjusters are free to move when you pull on them.
Source: South Carolina CDL Manual — Section 5.3.2: Walk-around Inspection
17. On many large buses, at what pressure do the low air pressure warning devices commonly signal?
Correct answer: At 80 to 85 psi
While most vehicles warn before 60 psi, large buses often have their low-pressure warnings set to signal at a higher 80 to 85 psi.
Source: South Carolina CDL Manual — Section 5.1.11: Low Air Pressure Warning
18. Federal rules require which feature on air-brake truck tractors manufactured on or after March 1, 1997?
Correct answer: Antilock braking systems (ABS)
ABS is mandated on air-brake truck tractors made on or after March 1, 1997; other air-brake vehicles are covered when made on or after March 1, 1998.
Source: South Carolina CDL Manual — Section 5.1.16: Antilock Braking Systems (ABS)
19. The low-pressure warning activates while you are underway. What is the correct response?
Correct answer: Stop and safely park the vehicle as soon as possible
A low-pressure warning may mean a leak; controlled braking only works while air remains, so stop and park safely as soon as you can, before the spring brakes set at 20 to 45 psi.
Source: South Carolina CDL Manual — Section 5.4.7: Low Air Pressure
20. What is 'brake lag' in an air brake system?
Correct answer: The short delay for air to reach the brakes after you press the pedal
Unlike hydraulic brakes that act instantly, air brakes need roughly half a second (sometimes longer) before the air reaches the brakes through the lines, adding brake lag to the total stopping distance.
Source: South Carolina CDL Manual — Section 5.4.4: Stopping Distance