Minnesota CDL — Air Brakes Practice Test
25 questions · pass with 20 correct. You get instant feedback and an explanation after every answer.
Minnesota CDL — Air Brakes questions with answers
20 sample questions from the Minnesota CDL — Air Brakes question bank — each with the correct answer, a short explanation, and the section of the official handbook it comes from.
1. If the ABS on your vehicle malfunctions, what happens to your braking?
Correct answer: You still have your normal brakes and should get the system serviced soon
ABS is an add-on to your normal brakes; if it fails you still have your regular brakes and should just get the system serviced soon.
Source: Minnesota Commercial Driver's License Manual, Section 5.1.16 - Antilock Braking Systems (ABS)
2. Why can a heavily loaded vehicle take a long distance to stop once the spring brakes come on?
Correct answer: The spring brakes do not act on all of the axles
The spring brakes do not act on all of the axles, so a heavy load has less braking available and needs a long distance to stop.
Source: Minnesota Commercial Driver's License Manual, Section 5.4.7 - Low Air Pressure
3. How does an air brake system turn on the brake lights to warn drivers behind you?
Correct answer: An electric switch operated by air pressure closes when you brake
An electric stop light switch that is operated by air pressure closes when you apply the brakes, lighting the brake lamps.
Source: Minnesota Commercial Driver's License Manual, Section 5.1.12 - Stop Light Switch
4. On a long, steep downgrade, the service brakes should be used in what way?
Correct answer: Only as a supplement to the braking effect of the engine
On a long or steep downgrade the brakes are only a supplement to the braking effect of the engine, which does most of the work in a low gear.
Source: Minnesota Commercial Driver's License Manual, Section 5.4.6 - Proper Braking Technique
5. Why should you avoid pressing the brake pedal while the spring brakes are already applied?
Correct answer: The combined force of the springs and air pressure could damage the brakes
Doing so adds air pressure on top of the spring force, and the combined forces could damage the brakes.
Source: Minnesota Commercial Driver's License Manual, Section 5.1.15 - Parking Brake Controls
6. What is the most common brake-related problem found during roadside inspections?
Correct answer: Brakes that are out of adjustment
Out-of-adjustment brakes are the problem found most often at roadside inspections, which is why checking the slack adjusters matters.
Source: Minnesota Commercial Driver's License Manual, Section 5.3.2 - Walk-around Inspection
7. What is a dual air brake system?
Correct answer: Two separate air brake systems that share one set of brake controls
It is two completely separate air brake systems that share a single set of brake controls; each has its own tanks, hoses, and lines.
Source: Minnesota Commercial Driver's License Manual, Section 5.2 - Dual Air Brake
8. If the safety valve lets air escape from the system, what does that tell you?
Correct answer: Something is wrong and a mechanic should fix it
A safety valve releasing air means something is wrong, and the fault should be corrected by a mechanic.
Source: Minnesota Commercial Driver's License Manual, Section 5.1.6 - Safety Valve
9. What is the 'wig wag' low-air warning device?
Correct answer: A mechanical arm that drops into view when air pressure gets low
A wig wag is a mechanical arm that drops down into the driver's view when system pressure falls below about 55 psi.
Source: Minnesota Commercial Driver's License Manual, Section 5.1.11 - Low Air Pressure Warning
10. Compared with hydraulic brakes, air brakes add an extra delay called brake lag. Why does it happen?
Correct answer: It takes time for the air to travel through the lines to the brakes
Hydraulic brakes act instantly, but air needs a little time (a half second or more) to travel through the lines to the brakes, adding brake lag.
Source: Minnesota Commercial Driver's License Manual, Section 5.4.4 - Stopping Distance
11. How are the spring brakes kept released while you drive normally?
Correct answer: Air pressure holds the powerful springs back
While driving, air pressure holds the powerful springs back; if that air is lost, the springs push the brakes on.
Source: Minnesota Commercial Driver's License Manual, Section 5.1.14 - Spring Brakes
12. What most commonly causes air brakes to fade?
Correct answer: Excessive heat from overusing the service brakes instead of the engine to slow down
Brake fade comes from excessive heat, usually from overusing the service brakes instead of letting the engine help slow the vehicle.
Source: Minnesota Commercial Driver's License Manual, Section 5.4.5 - Brake Fading or Failure
13. On a tractor or straight truck, the spring brakes will come fully on when air pressure falls into which range?
Correct answer: 20 to 45 psi
Spring brakes on tractors and straight trucks fully engage once air pressure drops to somewhere in the 20 to 45 psi range.
Source: Minnesota Commercial Driver's License Manual, Section 5.1.14 - Spring Brakes
14. What does the supply pressure gauge tell the driver?
Correct answer: How much pressure is currently in the air tanks
The supply pressure gauge shows how much air pressure is currently stored in the tanks.
Source: Minnesota Commercial Driver's License Manual, Section 5.1.9 - Supply Pressure Gauges
15. What happens as the brake shoes and linings press against the drum to slow the vehicle?
Correct answer: Friction slows the vehicle and produces heat
Pressing the linings against the drum produces friction that slows the vehicle, and that friction also generates a lot of heat.
Source: Minnesota Commercial Driver's License Manual, Section 5.1.8 - Foundation Brakes
16. On newer air brake vehicles, what does the parking brake control usually look like?
Correct answer: A yellow, diamond-shaped push-pull knob
Newer vehicles use a yellow, diamond-shaped push-pull knob; you pull it out to set the parking (spring) brakes and push it in to release them.
Source: Minnesota Commercial Driver's License Manual, Section 5.1.15 - Parking Brake Controls
17. In a dual air brake system, what are the two systems called?
Correct answer: The primary and secondary systems
One is the primary system and the other is the secondary system; both supply air to a trailer if one is attached.
Source: Minnesota Commercial Driver's License Manual, Section 5.2 - Dual Air Brake
18. Which sequence describes stab braking?
Correct answer: Apply the brakes fully, release when the wheels lock, then reapply fully once the wheels roll again
Stab braking is applying the brakes fully, releasing when the wheels lock, then reapplying fully once the wheels start rolling again.
Source: Minnesota Commercial Driver's License Manual, Section 5.4.3 - Emergency Stops
19. A visible low-air-pressure warning must come on before tank pressure falls below what level?
Correct answer: 55 psi
On vehicles with air brakes, a warning you can see is required to activate before pressure drops under 55 psi.
Source: Minnesota Commercial Driver's License Manual, Section 5.1.11 - Low Air Pressure Warning
20. If one half of a dual air system loses most of its pressure, what should you expect?
Correct answer: It will take longer to stop, so stop safely and get it fixed
Losing one half means either the front or rear brakes will not work fully, so it takes longer to stop; bring the vehicle to a safe stop and have it repaired.
Source: Minnesota Commercial Driver's License Manual, Section 5.2 - Dual Air Brake