U.S. CDL · AIR BRAKES · QUESTIONS & ANSWERS

U.S. CDL Air Brakes Practice — questions and answers

All 30 original Rig Toolbox practice questions for this test, printed in full with the correct answer and an explanation for each one. Read them here, then put them to work in Practice or the Exam Simulator — no sign-up, ever.

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Independent practice: these are Rig Toolbox's own study questions, not official exam questions. Always confirm rules in the official handbook for the licence you are taking.

About this question set

How many questions are here? 30 — the full bank for this test. The interactive test draws 20 of them at a time.

What is this test? Original air-brake system practice based on the air brakes section of state CDL manuals.

Every question, with its answer

Question 1

At about what pressure does the compressor governor cut out and stop pumping air into the tanks?

About 150 psi
About 85 psi
About 60 psi
About 120 psi

Correct answer: About 120 psi

The governor cuts the compressor out at roughly 120 psi and cuts it back in near 100 psi.

Question 2

What does the safety valve on an air storage tank do?

Drains water from the tank
Adds air when pressure is low
Opens to release air if pressure rises too high, at about 150 psi
Cuts in the compressor

Correct answer: Opens to release air if pressure rises too high, at about 150 psi

The safety valve protects the system from excess pressure and is normally set at 150 psi.

Question 3

Why must air storage tanks be drained regularly?

Draining raises the cut-out pressure
It is only needed on new vehicles
Water and oil collect in the tanks and can freeze or damage parts
To make room for fuel vapour

Correct answer: Water and oil collect in the tanks and can freeze or damage parts

Compressor oil and condensed water settle in the tanks; in cold weather the water can freeze and cause brake failure.

Question 4

The low air pressure warning signal must come on before tank pressure falls below:

60 psi
85 psi
30 psi
100 psi

Correct answer: 60 psi

The warning light and buzzer activate before pressure drops below 60 psi.

Question 5

If the low air pressure warning comes on while you are driving, you should:

Turn off the engine warning lights
Keep driving to the next scheduled stop
Fan the brake pedal to rebuild pressure
Stop safely and park as soon as possible

Correct answer: Stop safely and park as soon as possible

A low-air warning means stopping power is being lost; stop safely before the spring brakes apply on their own.

Question 6

Spring brakes come on automatically when air pressure in the tanks falls to roughly:

100–110 psi
80–90 psi
50–60 psi
20–45 psi

Correct answer: 20–45 psi

When pressure drops into the 20 to 45 psi range, the spring brakes apply automatically.

Question 7

Why must you never push the brake pedal down when the spring brakes are already on?

It drains the fuel system
It turns off the ABS
It releases the parking brake
The combined forces can damage the brake system

Correct answer: The combined forces can damage the brake system

Service-brake air pressure added on top of spring force can overload and damage brake components.

Question 8

What holds the spring brakes in the released position while driving?

The brake pedal
Air pressure in the system
The trailer hand valve
The slack adjuster

Correct answer: Air pressure in the system

Compressed air holds the powerful springs back; losing air lets the springs apply the brakes.

Question 9

What is an advantage of a dual air brake system?

If one system fails, the other still provides braking to bring the vehicle to a stop
It never needs draining
It doubles the vehicle speed
It removes the need for a pre-trip check

Correct answer: If one system fails, the other still provides braking to bring the vehicle to a stop

Primary and secondary systems have separate supplies, so one failure leaves partial braking instead of none.

Question 10

During a build-up check, air pressure should rise from 85 to 100 psi within:

10 minutes
45 seconds
5 minutes
2 minutes

Correct answer: 45 seconds

With the engine at operating rpm, a healthy dual system builds from 85 to 100 psi within 45 seconds.

Question 11

With the engine off and brakes released, the static air leakage rate for a single vehicle must not exceed:

2 psi per minute
1 psi per minute
5 psi per minute
10 psi per minute

Correct answer: 2 psi per minute

After pressure stabilises, loss should be no more than 2 psi in one minute for a single vehicle (3 psi for a combination).

Question 12

With the brakes fully applied, the leakage rate for a combination vehicle must not exceed:

8 psi per minute
4 psi per minute
3 psi per minute
2 psi per minute

Correct answer: 4 psi per minute

Applied leakage limits are 3 psi per minute for single vehicles and 4 psi per minute for combinations.

Question 13

How can you check that a brake is out of adjustment using the slack adjuster?

Pump the pedal ten times
Listen for the buzzer
Pull hard on the slack adjuster; if it moves more than about 1 inch, adjustment is needed
Measure the tire tread

Correct answer: Pull hard on the slack adjuster; if it moves more than about 1 inch, adjustment is needed

Excess slack-adjuster movement at the pushrod shows the brake stroke is beyond its limit.

Question 14

What is brake lag in an air brake system?

Wear in the clutch
The time air takes to travel through the lines and apply the brakes
The delay before the engine starts
A defect in the steering

Correct answer: The time air takes to travel through the lines and apply the brakes

Unlike hydraulic brakes, air brakes need time for air to flow to the brake chambers, which adds stopping distance.

Question 15

At 55 mph, roughly how much extra distance does brake lag add to a stop?

About 100 feet
About 32 feet
About 3 feet
About 320 feet

Correct answer: About 32 feet

At 55 mph the vehicle travels roughly 32 additional feet while the brakes are still applying.

Question 16

Which list gives the full parts of total stopping distance for an air-braked vehicle?

Brake lag and tire wear
Reaction and braking only
Perception, reaction, brake lag and braking distances
Perception and parking distance

Correct answer: Perception, reaction, brake lag and braking distances

Total stopping distance combines perception distance, reaction distance, brake lag distance and braking distance.

Question 17

What causes brake fade on a long downgrade?

Cold brake drums
Too much air pressure
Overheated drums expand away from the shoes, reducing friction
Worn fan belts

Correct answer: Overheated drums expand away from the shoes, reducing friction

Heat makes drums expand away from the linings and the brakes lose their grip, sometimes completely.

Question 18

Using the snub braking method on a long, steep downgrade (vehicle without ABS), you should:

Pump the pedal as fast as possible
Ride the brakes lightly the whole way down
Brake firmly to slow about 5 mph below your safe speed, release, and repeat as speed climbs back
Shift to neutral and brake at the bottom

Correct answer: Brake firmly to slow about 5 mph below your safe speed, release, and repeat as speed climbs back

Snub braking lets the brakes cool between firm applications instead of overheating from constant dragging.

Question 19

How should you brake a vehicle equipped with ABS on a slippery road?

Use the parking brake only
Apply firm, steady pressure and let the system prevent lockup
Turn the ABS off first
Pump the pedal rapidly

Correct answer: Apply firm, steady pressure and let the system prevent lockup

With ABS you brake normally with steady pressure; the system keeps wheels from locking so you can steer.

Question 20

What does the tractor protection valve do?

Inflates the trailer tires
Controls the trailer hand valve
Keeps air in the tractor if the trailer breaks away or develops a bad leak
Releases the spring brakes

Correct answer: Keeps air in the tractor if the trailer breaks away or develops a bad leak

It closes automatically when trailer supply pressure is lost, protecting the tractor air supply so the tractor brakes still work.

Question 21

What colour and shape is the trailer air supply control valve knob?

Black and square
Yellow and diamond
Red and octagonal
Blue and round

Correct answer: Red and octagonal

The trailer air supply control is a red, eight-sided push-pull knob; the yellow diamond knob is the parking brake control.

Question 22

What is the trailer hand control valve (trolley valve) used for?

Supplying air to the trailer tanks
Testing the trailer service brakes
Parking the trailer for the night
Draining the wet tank

Correct answer: Testing the trailer service brakes

The hand valve applies only the trailer service brakes; it must never be used for parking because air can leak off.

Question 23

An air dryer helps the brake system by:

Oiling the slack adjusters
Increasing the cut-out pressure
Removing moisture from the air before it reaches the tanks
Cooling the brake drums

Correct answer: Removing moisture from the air before it reaches the tanks

The dryer strips water vapour from the compressed air, reducing corrosion and freeze-ups.

Question 24

What do the one-way check valves between the compressor and the tanks prevent?

Slack adjusters from moving
The governor from cutting in
Water from entering the cab
Air from flowing back out of the tanks if the supply line fails

Correct answer: Air from flowing back out of the tanks if the supply line fails

Check valves let air into each tank but stop it escaping back toward a leak in the supply side.

Question 25

Before driving a vehicle with dual air brakes, you should wait until:

The safety valve pops off
The engine reaches full temperature only
Both systems reach at least 100 psi
The needle moves at all

Correct answer: Both systems reach at least 100 psi

Do not drive until primary and secondary gauges both show at least 100 psi.

Question 26

Why should you avoid fanning (repeatedly pumping) the brake pedal?

It overheats the tires
It uses air faster than the compressor can replace it, dropping system pressure
It confuses other drivers
It wears out the pedal pad

Correct answer: It uses air faster than the compressor can replace it, dropping system pressure

Each application consumes stored air; fanning can pull pressure down toward the spring-brake range.

Question 27

What does the application pressure gauge show?

Trailer tire pressure
How much air pressure you are applying to the brakes with the pedal
Air pressure stored in the tanks
Compressor oil pressure

Correct answer: How much air pressure you are applying to the brakes with the pedal

The application gauge reads the pressure delivered to the brakes as you press the pedal; rising pressure confirms air is reaching them.

Question 28

On an older tractor with a front brake limiting valve, the valve should normally be kept:

In the normal position, because the slippery setting cuts front braking and lengthens stops on dry roads
In the slippery position at all times
Set halfway
Removed from the vehicle

Correct answer: In the normal position, because the slippery setting cuts front braking and lengthens stops on dry roads

Limiting valves halve front-brake pressure; they help only on slippery surfaces and hurt stopping on dry pavement.

Question 29

What does an S-cam do when you press the brake pedal?

It opens the safety valve
It adjusts tire pressure
It drains the air tanks
It rotates and forces the brake shoes against the drum

Correct answer: It rotates and forces the brake shoes against the drum

Air pressure pushes the pushrod and slack adjuster, turning the S-cam so the shoes press into the drum.

Question 30

A modulating control valve, when fitted, lets the driver:

Modulate the radio volume
Test the ABS lamp
Adjust the governor
Apply the spring brakes gradually if the service brakes fail

Correct answer: Apply the spring brakes gradually if the service brakes fail

It gives gradual control of the spring brakes so they can be used as an emergency brake without locking the wheels.

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