Missouri CDL — Air Brakes: Handling Emergencies Practice Test

Question 1

At about what pressure do a tractor or straight truck's spring brakes fully engage on their own?

Accepted Answer

20 to 45 psi — Spring brakes on tractors and straight trucks come fully on when air pressure falls into the 20-to-45 psi range.

Answer

55 to 75 psi

Answer

100 to 125 psi

Answer

80 to 90 psi

Question 2

When the low air warning light and buzzer first come on, what should you do?

Accepted Answer

Stop the vehicle safely without delay, while you still have control over the brakes — Do not wait for the spring brakes to trip automatically; stop safely as soon as the warning appears, while you still have braking control.

Answer

Keep driving until the spring brakes apply by themselves

Answer

Pump the pedal to rebuild pressure and continue

Answer

Release the parking brake and coast

Question 3

What is the safest response if the low-pressure warning lights up while you are driving?

Accepted Answer

Pull over and park safely as soon as you can, while air still remains — A low-air warning may mean a leak; stop and park safely right away, because controlled braking is only possible while enough air is left in the tanks.

Answer

Speed up to reach the next exit

Answer

Pump the pedal to keep the warning off

Answer

Set the parking brake and keep driving

Question 4

What is the benefit of a modulating control valve for the spring brakes?

Accepted Answer

It applies the spring brakes gradually, giving control if the service brakes quit — A modulating valve lets you bring the spring brakes on gradually, so you can still control the vehicle if the service brakes fail.

Answer

It automatically drains the tanks

Answer

It doubles tank pressure in an emergency

Answer

It locks only the front wheels

Question 5

How do spring brakes hold the vehicle when the air pressure is lost?

Accepted Answer

Powerful springs, held off by air while driving, clamp the brakes once the air is gone — Strong springs are normally held back by air pressure; when that air is released or lost, the springs apply the brakes, meeting the mechanical-hold requirement.

Answer

A backup compressor keeps them charged

Answer

Hydraulic pressure takes over automatically

Answer

A parking pawl locks the transmission

Question 6

How does the emergency brake system bring a vehicle to a stop if the brakes fail?

Accepted Answer

It draws on parts shared with the service and parking brake systems — The emergency braking function is not a standalone system; it borrows components from both the service and parking systems to stop the vehicle after a failure.

Answer

It relies on a separate hydraulic backup circuit

Answer

It uses engine compression by itself

Answer

It applies extra drums mounted on the driveshaft

Question 7

Using the stab braking method, what do you do after the wheels lock up?

Accepted Answer

Ease off the brakes until the wheels roll again, then reapply them fully — Stab braking is applying the brakes fully, releasing when the wheels lock, and reapplying once they start rolling again, which can take up to a second.

Answer

Hold the brakes locked until you stop

Answer

Pump the pedal rapidly while steering hard

Answer

Shift to a lower gear and coast

Question 8

The safety relief valve in the first air tank is usually set to open at what pressure?

Accepted Answer

150 psi — The safety valve guards the system against too much pressure and typically opens at 150 psi.

Answer

100 psi

Answer

55 psi

Answer

125 psi

Question 9

What does it indicate if the safety valve lets air escape?

Accepted Answer

Something is wrong and a mechanic should fix it — If the safety valve vents air, it signals a fault that needs to be repaired by a mechanic.

Answer

The tanks are properly charged

Answer

The governor has reached its cut-in point

Answer

The alcohol evaporator has run dry

Question 10

In controlled braking during an emergency, how hard should you press the brakes?

Accepted Answer

Firmly, but not so hard the wheels lock, using only small steering inputs — Controlled braking means applying the brakes as hard as possible short of locking the wheels while keeping steering movements small; release if the wheels lock or you must steer more.

Answer

Fully to the floor until the wheels lock

Answer

Lightly, then release completely

Answer

Only enough to feel a slowdown

Question 11

If one part of a dual air system loses most of its pressure, what is the result?

Accepted Answer

Either the front or rear brakes will not work fully, so stopping takes longer — When one system runs very low, the front or rear brakes lose full function, which lengthens your stopping distance, so you should stop and have it fixed.

Answer

Both systems shut down at once

Answer

The parking brakes release on their own

Answer

The compressor doubles its output

Handling Emergencies

Study questions with answers

1. At about what pressure do a tractor or straight truck's spring brakes fully engage on their own?

2. When the low air warning light and buzzer first come on, what should you do?

3. What is the safest response if the low-pressure warning lights up while you are driving?

4. What is the benefit of a modulating control valve for the spring brakes?

5. How do spring brakes hold the vehicle when the air pressure is lost?

6. How does the emergency brake system bring a vehicle to a stop if the brakes fail?

7. Using the stab braking method, what do you do after the wheels lock up?

8. The safety relief valve in the first air tank is usually set to open at what pressure?

9. What does it indicate if the safety valve lets air escape?

10. In controlled braking during an emergency, how hard should you press the brakes?

11. If one part of a dual air system loses most of its pressure, what is the result?

More CDL — Air Brakes topics

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