Take CDL Air Brakes Knowledge Test

QUESTION 1 / 25

What is the primary function of the air compressor in an air brake system?

Choose one answer below.

ATo pump air into the storage tanks✓BTo release air from the brake chambers✓CTo control the governor cut-out pressure✓DTo pressurize the brake pedal✓

QUESTION 2 / 25

At what pressure does the governor typically cut out the air compressor?

Choose one answer below.

A60–80 psi✓B100–125 psi✓C150–175 psi✓D200–225 psi✓

QUESTION 3 / 25

At what pressure does the governor cut in and signal the compressor to resume pumping?

Choose one answer below.

A20–40 psi✓B50–60 psi✓C80–85 psi✓D100–110 psi✓

QUESTION 4 / 25

What is the purpose of an air dryer in the air brake system?

Choose one answer below.

ATo cool compressed air✓BTo remove moisture and contaminants from the air supply✓CTo increase system pressure✓DTo store compressed air✓

QUESTION 5 / 25

How is the air compressor typically driven on a commercial vehicle?

Choose one answer below.

ABy an electric motor✓BBy the vehicle's engine via belt or gear✓CBy hydraulic pressure✓DBy the transmission output shaft✓

QUESTION 6 / 25

What can happen if the air compressor governor fails in the open position?

Choose one answer below.

AThe compressor will stop working entirely✓BAir pressure will drop to zero✓CThe compressor will run continuously and may over-pressurize the system✓DThe brakes will lock up automatically✓

QUESTION 7 / 25

What is the primary purpose of the safety relief valve on an air tank?

Choose one answer below.

ATo release air if the tank pressure gets too high✓BTo drain moisture from the tank automatically✓CTo regulate air flow to the brake chambers✓DTo equalize pressure between the primary and secondary tanks✓

QUESTION 8 / 25

At what pressure is the safety relief valve on an air tank typically set to open?

Choose one answer below.

A100 psi✓B125 psi✓C150 psi✓D175 psi✓

QUESTION 9 / 25

How often should air tanks with manual drain valves be drained?

Choose one answer below.

AOnce a week✓BAt the end of each day of use✓CEvery 500 miles✓DOnce a month✓

QUESTION 10 / 25

Which air tank is the first to receive compressed air from the compressor?

Choose one answer below.

APrimary tank✓BSecondary tank✓CSupply (wet) tank✓DRear axle tank✓

QUESTION 11 / 25

What is the function of a one-way check valve between air tanks?

Choose one answer below.

AIt limits maximum pressure in each tank✓BIt prevents air from flowing backward from one tank to another✓CIt automatically drains water from the tanks✓DIt controls the rate of air flow to the brakes✓

QUESTION 12 / 25

Why is it important to drain air tanks regularly in an air brake system?

Choose one answer below.

ATo increase air pressure in the system✓BTo remove water and oil that can cause brake failure or freeze in cold weather✓CTo reset the governor cut-in pressure✓DTo allow the compressor to build pressure faster✓

QUESTION 13 / 25

What does a drain cock (drain valve) allow you to do on an air tank?

Choose one answer below.

AManually release air pressure from the tank to drain moisture✓BAdjust the pressure at which the governor cuts out✓CControl air flow to the primary circuit only✓DRefill the tank with compressed air from an external source✓

QUESTION 14 / 25

What is the purpose of a slack adjuster in an air brake system?

Choose one answer below.

ATo adjust air pressure delivered to the brake chambers✓BTo maintain proper brake shoe-to-drum clearance as brake linings wear✓CTo regulate the governor cut-out pressure✓DTo control air flow to the spring brakes✓

QUESTION 15 / 25

What is the maximum push rod stroke allowed before brakes are considered out of adjustment?

Choose one answer below.

AWhatever the driver feels is sufficient based on pedal feedback✓BVaries by brake chamber size, but typically 1¾ to 2 inches maximum✓C3 inches maximum for all brake chamber sizes✓D4 inches maximum for all brake chamber sizes✓

QUESTION 16 / 25

What is an automatic slack adjuster (ASA)?

Choose one answer below.

AA device that automatically adjusts the governor cut-out pressure✓BA self-adjusting mechanism that maintains proper brake adjustment without manual intervention✓CAn ABS component that adjusts brake pressure during skids✓DA computer-controlled system that monitors brake temperature✓

QUESTION 17 / 25

Even with automatic slack adjusters installed, what must a driver do during pre-trip inspection?

Choose one answer below.

ANothing — automatic adjusters require no inspection whatsoever✓BManually adjust each slack adjuster to the desired setting✓CVisually inspect and check push rod stroke to verify proper adjustment✓DReplace all brake linings at each inspection interval✓

QUESTION 18 / 25

What is the minimum brake lining thickness allowed on drum brakes before replacement is required?

Choose one answer below.

A¼ inch for drum brakes✓B⅛ inch for drum brakes✓C¾ inch for drum brakes✓DThere is no minimum — replace only when brakes stop working✓

QUESTION 19 / 25

What condition requires immediate brake lining replacement regardless of lining thickness?

Choose one answer below.

ALining has been in service for more than 6 months✓BLining is contaminated with oil or grease✓CLining color has changed from its original appearance✓DVehicle has traveled more than 50,000 miles since last replacement✓

QUESTION 20 / 25

What is brake fade?

Choose one answer below.

AA gradual increase in braking efficiency over time✓BThe loss of braking power caused by overheated brakes✓CThe automatic adjustment of brake pressure on hills✓DThe reduction in stopping distance on wet roads✓

QUESTION 21 / 25

What is the correct technique for driving down a long, steep grade with air brakes?

Choose one answer below.

ARide the brakes continuously while staying in a higher gear✓BSelect a low gear before starting down and use controlled, intermittent brake applications✓CUse only the engine brake and avoid the air brakes entirely✓DShift to neutral to save fuel and use brakes as needed✓

QUESTION 22 / 25

What is 'snub braking' when used on a downgrade?

Choose one answer below.

APumping the brakes rapidly to build air pressure✓BApplying brakes firmly to reduce speed by about 5 mph, then releasing until speed rises again✓CUsing only trailer brakes on steep grades✓DApplying maximum brake pressure continuously throughout the descent✓

QUESTION 23 / 25

What is a safe speed limit when driving down a long, steep grade?

Choose one answer below.

AThe posted speed limit for the grade✓B5 mph above the speed you could manage without brakes✓C55 mph regardless of grade steepness✓DA speed at which you could still control the vehicle if brakes failed completely✓

QUESTION 24 / 25

How does brake fade affect stopping distance?

Choose one answer below.

AStopping distance decreases as brakes get hotter✓BStopping distance remains the same regardless of temperature✓CStopping distance increases significantly as brakes overheat✓DStopping distance only changes on wet roads✓

QUESTION 25 / 25

What is 'glazing' in the context of brake drums?

Choose one answer below.

AApplying a protective coating to brake drums✓BA smooth, hard surface that forms on overheated brake drums, reducing friction✓CIce formation on brake drums in cold weather✓DThe normal wear pattern on new brake drums✓