Mississippi CDL — Air Brakes: Safe Driving Practices Practice Test

Question 1

Your tractor-trailer has ABS on only one axle. What does the manual say about that?

Accepted Answer

It still gives you more control during braking — ABS on just the tractor, just the trailer, or even a single axle still improves control while braking.

Answer

It is useless unless every axle has ABS

Answer

It makes a jackknife more likely

Answer

You must brake harder to make up for it

Question 2

Which statement about automatic front-wheel limiting valves is true?

Accepted Answer

They let full air reach the front brakes only during very hard stops, and the driver cannot control them — Automatic limiting valves cut front brake air except during very hard applications of 55 psi or more. The driver has no control over them.

Answer

They are switched with a 'normal' and 'slippery' control in the cab

Answer

They raise front brake pressure on icy roads

Answer

They are found only on vehicles built before 1975

Question 3

You start a truck with a dual air brake system. The primary gauge reads 105 psi, but the secondary reads 80 psi. Are you ready to drive?

Accepted Answer

No; wait until the secondary system also reaches at least 100 psi — Give the compressor time to bring both halves of a dual system to a minimum of 100 psi. Watch both gauges or both needles while it builds.

Answer

Yes, because one system above 100 psi is enough

Answer

Yes, because both systems are above 55 psi

Answer

No; wait until both systems reach 150 psi

Question 4

Which list puts these air system pressure points in order from LOWEST to HIGHEST, using the typical values in the manual?

Accepted Answer

Low air warning, governor cut-in, governor cut-out, safety valve opening — The warning must come on before pressure drops below 55 psi, the governor cuts the compressor back in around 100 psi and out around 125 psi, and the safety valve usually opens at 150 psi. If that valve ever releases air, something is wrong.

Answer

Governor cut-in, low air warning, safety valve opening, governor cut-out

Answer

Low air warning, governor cut-out, governor cut-in, safety valve opening

Answer

Safety valve opening, governor cut-out, governor cut-in, low air warning

Question 5

Of these air brake items, which one belongs in the engine compartment step of the seven-step inspection rather than a later step?

Accepted Answer

The air compressor drive belt, if the compressor is belt-driven — Step 2 is where you look at a belt-driven compressor's belt for good condition and proper tension. Slack adjusters are part of the Step 5 walk-around, and the governor and warning tests come in the Step 7 final check.

Answer

The slack adjusters on the s-cam brakes

Answer

The governor's cut-in and cut-out pressures

Answer

The low air pressure warning signal

Question 6

With s-cam brakes, what draws the shoes back off the drum once you let up on the pedal?

Accepted Answer

A spring, as the s-cam rotates back — On release, the cam turns back and a spring draws the shoes away from the drum so the wheels can roll freely again.

Answer

Air pressure pushing the shoes outward

Answer

A power screw backing off

Answer

The turning of the wheel by itself

Question 7

Besides making the drums expand, what does excessive heat do to the brake linings?

Accepted Answer

It causes chemical changes that lower their friction — Brake fade comes from overheating. The heat alters the lining material so it creates less friction, and it also swells the drums.

Answer

It makes them grip harder until they lock

Answer

It turns them into a better heat shield

Answer

It has no effect on the linings themselves

Question 8

Your truck has an alcohol evaporator, and you keep it filled every cold day. Does that let you skip draining the air tanks?

Accepted Answer

No; the tanks still need daily draining to remove water and oil unless they drain automatically — The evaporator only lowers the risk of ice in valves and other parts. Water and oil still collect in the tanks, so daily draining is still required unless the system has automatic drain valves.

Answer

Yes, because the alcohol absorbs all of the water and oil

Answer

Yes, but only during the summer months

Answer

No, but draining once a week is enough

Question 9

As brake drums overheat and expand on a long downgrade, why does braking get weaker?

Accepted Answer

The shoes and linings have to travel farther to reach the drums, so they press with less force — Too much heat causes fade in two ways: it changes the lining material so it has less friction, and it makes drums expand. Expanded drums leave the shoes farther away, so contact force drops.

Answer

The drums shrink and grip the shoes too tightly

Answer

The compressor stops making air

Answer

The ABS shuts off the rear brakes

Question 10

At operating rpm, your dual air truck with standard-size tanks takes about two minutes to climb from 85 to 100 psi. What does that tell you?

Accepted Answer

Pressure is building too slowly, so do not drive until it is fixed — In dual systems the 85-to-100 psi climb should take no more than 45 seconds. A slow build means pressure could fall too low while driving and force an emergency stop.

Answer

That is normal for any dual air system

Answer

The governor needs to cut out sooner

Answer

You should drive gently and it will catch up

Question 11

Some pre-1975 trucks carry a front brake limiting valve worked from the cab. What was it meant to do, and what is the catch?

Accepted Answer

It was meant to reduce front-wheel skids on slick roads, but it actually reduces stopping power — In the 'slippery' position the valve halves the air going to the front brakes. That was supposed to prevent front-wheel skids, but it weakens stopping, and tests show braking skids at the front are unlikely even on ice.

Answer

It was meant to cool the front brakes, but it wears out the linings

Answer

It was meant to add front braking on ice, but it locks the rear wheels

Answer

It was meant to replace ABS, but it works only above 55 mph

Question 12

Which statement about wedge brakes is accurate?

Accepted Answer

They may use one brake chamber or two, and some adjust themselves while others need manual adjustment — In a wedge brake the push rod drives a wedge between the ends of two shoes, forcing them against the drum. Designs vary in the number of chambers and in whether they self-adjust, and s-cam brakes remain the more common design.

Answer

They spread the shoes with an S-shaped cam

Answer

They clamp a rotor between the pads of a caliper

Answer

They are the most common type of foundation brake

Mississippi CDL — Air Brakes: Safe Driving Practices practice test

Study questions with answers

1. Your tractor-trailer has ABS on only one axle. What does the manual say about that?

2. Which statement about automatic front-wheel limiting valves is true?

3. You start a truck with a dual air brake system. The primary gauge reads 105 psi, but the secondary reads 80 psi. Are you ready to drive?

4. Which list puts these air system pressure points in order from LOWEST to HIGHEST, using the typical values in the manual?

5. Of these air brake items, which one belongs in the engine compartment step of the seven-step inspection rather than a later step?

6. With s-cam brakes, what draws the shoes back off the drum once you let up on the pedal?

7. Besides making the drums expand, what does excessive heat do to the brake linings?

8. Your truck has an alcohol evaporator, and you keep it filled every cold day. Does that let you skip draining the air tanks?

9. As brake drums overheat and expand on a long downgrade, why does braking get weaker?

10. At operating rpm, your dual air truck with standard-size tanks takes about two minutes to climb from 85 to 100 psi. What does that tell you?

11. Some pre-1975 trucks carry a front brake limiting valve worked from the cab. What was it meant to do, and what is the catch?

12. Which statement about wedge brakes is accurate?

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