Tennessee CDL — Air Brakes: Handling Emergencies Practice Test

Question 1

While driving, the low-pressure warning suddenly comes on. What is your safest move?

Accepted Answer

Stop and safely park as soon as possible — Controlled braking is only possible while enough air remains, and the spring brakes come on around 20 to 45 psi, so you should stop and safely park as soon as possible while you still have foot brakes.

Answer

Speed up to reach your destination before pressure runs out

Answer

Pump the brakes hard to keep pressure up

Answer

Set the parking brake while still moving fast

Question 2

On a tractor or straight truck, spring brakes typically come fully on when the air pressure falls into what range?

Accepted Answer

20 to 45 psi — The spring brakes on a tractor or straight truck engage fully once pressure drops into the 20 to 45 psi range, most often somewhere between 20 and 30 psi.

Answer

80 to 100 psi

Answer

100 to 125 psi

Answer

45 to 60 psi

Question 3

How is the "stab braking" method performed in an emergency?

Accepted Answer

Apply fully, release when wheels lock, then apply fully again once they roll — In stab braking you press the brakes all the way, release them when the wheels lock, and reapply fully once the wheels start rolling again.

Answer

Hold steady, light pressure the entire time

Answer

Apply the parking brake in short bursts

Answer

Steer hard while holding the wheels locked

Question 4

What is "controlled braking" during an emergency stop?

Accepted Answer

Braking as hard as possible without letting the wheels lock, using tiny steering inputs — Controlled braking means pressing as firmly as possible short of locking the wheels, while keeping steering inputs tiny; if a wheel locks or you must steer more, release and reapply.

Answer

Slamming the brakes fully and holding them locked

Answer

Pumping the pedal rapidly regardless of wheel lock

Answer

Braking only with the parking brake control

Question 5

When using stab braking, why must you wait before re-applying the brakes after releasing them?

Accepted Answer

The wheels can take up to a second to start rolling, and you must let them — After you let off, the wheels may need as much as a second to start turning again; re-applying before they roll keeps the vehicle from straightening out.

Answer

The tanks need time to refill above cut-out pressure

Answer

The stop lights need a moment to reset

Answer

The governor must cut in before you can brake again

Question 6

Why are spring brakes held off by air pressure while you drive rather than held on by air?

Accepted Answer

Because air pressure can leak away, so mechanical force must hold the brakes on — Because air pressure can eventually leak away, the emergency and parking brakes are held on by mechanical force from powerful springs; air pressure simply holds those springs back during driving.

Answer

Because springs are cheaper than air tanks

Answer

Because the governor cannot supply air when parked

Answer

Because the compressor only runs while moving

Question 7

Does ABS guarantee a shorter stopping distance?

Accepted Answer

No; its main value is keeping the vehicle under control during hard braking — ABS does not necessarily shorten how far you take to stop; its real value is helping the driver stay in command of the rig while braking hard.

Answer

Yes; it always cuts stopping distance roughly in half

Answer

Yes; it stops the vehicle instantly with no lag

Answer

No; it actually makes every stop longer

Question 8

What is the purpose of a modulating control valve on the dashboard?

Accepted Answer

To let you apply the spring brakes gradually if the service brakes fail — A modulating valve lets you apply the spring brakes gradually; it is spring-loaded so you can feel the braking, and moving the lever farther applies the spring brakes harder, useful if the service brakes fail.

Answer

To automatically drain the air tanks each day

Answer

To keep the compressor from cutting out early

Answer

To flash the brake lights during hard stops

Question 9

If one side of a dual air system loses most of its pressure, what happens to your stopping?

Accepted Answer

It takes longer to stop because some brakes are not working fully — If either system runs very low, the brakes it feeds—front or rear—won't work fully, so stopping will take you longer and you should safely park for repair.

Answer

Nothing changes; the other system covers all brakes

Answer

The vehicle stops faster than normal

Answer

The parking brakes take over automatically at speed

Question 10

As soon as the low-pressure warning light and its buzzer come on, what is the correct action?

Accepted Answer

Bring the vehicle to a safe stop while you can still control the brakes — Don't wait for the spring brakes to set themselves; pull over and stop safely at once, while you still have enough air left to control the brakes.

Answer

Keep driving until the spring brakes apply by themselves

Answer

Pump the pedal quickly to rebuild pressure

Answer

Ignore it until the gauge reads zero

Handling Emergencies

Study questions with answers

1. While driving, the low-pressure warning suddenly comes on. What is your safest move?

2. On a tractor or straight truck, spring brakes typically come fully on when the air pressure falls into what range?

3. How is the "stab braking" method performed in an emergency?

4. What is "controlled braking" during an emergency stop?

5. When using stab braking, why must you wait before re-applying the brakes after releasing them?

6. Why are spring brakes held off by air pressure while you drive rather than held on by air?

7. Does ABS guarantee a shorter stopping distance?

8. What is the purpose of a modulating control valve on the dashboard?

9. If one side of a dual air system loses most of its pressure, what happens to your stopping?

10. As soon as the low-pressure warning light and its buzzer come on, what is the correct action?

More CDL — Air Brakes topics

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