DMV

Air Brakes Practice Questions

6 free sample cdl questions about air brakes, with the correct answer, a full explanation, and a source reference for each. Want to test yourself instead of just reading the answers? Find your state's free practice test.

easy

1. What happens to spring brakes when air pressure in the system drops too low?

AThey automatically release completely, letting the vehicle roll freely
BThey stay locked in whatever position they were in before the pressure first dropped
CNothing happens until the driver manually applies the parking brake
DThey automatically apply to stop the vehicle, since spring pressure can’t be held back without air
medium

2. Federal rules require a low air pressure warning device to activate before pressure drops below what level?

A60 psi, giving the driver an early warning before the brakes are affected
B20 psi, right before the brakes would fail completely
C100 psi, well above the level where brakes still work normally
DThere is no federally required warning threshold for air pressure
easy

3. In general terms, how does an air brake system apply the brakes?

AHydraulic fluid alone pushes the brake pads against the wheels
BCompressed air pushes on brake chambers that force the brake shoes or pads against the wheels
CA steel cable directly connects the brake pedal to the wheels
DElectric motors mounted at each individual wheel apply the brakes when signaled
easy

4. What is the purpose of an anti-lock braking system (ABS) on a commercial vehicle?

AIt replaces the need for the vehicle’s regular service brakes entirely on its own
BIt helps prevent wheel lockup during hard braking, so the driver keeps steering control
CIt only functions above normal highway driving speeds
DIt automatically applies maximum braking force at all times
medium

5. Why do most heavy vehicles use a dual air brake system, with two separate air supplies?

AA dual system exists mainly to reduce brake noise during normal driving
BIf one system fails, the other can still provide some braking capability
CA dual system is required only for vehicles above a specific weight threshold
DA dual system is designed to increase the vehicle’s overall top speed
hard

6. What is "brake lag" in an air brake system?

AA delay in the parking brake releasing right after the engine starts up
BThe time it takes for tires to wear unevenly across an axle
CA brief delay between pressing the brake pedal and the brakes applying full force
DAnother term for the low air pressure warning device activating