CDL Air Brakes Sample Questions

Questions from each area of the CDL Air Brakes test. Pick an answer to see whether you're right, how everyone else did, and why.

CDL Air Brakes Test exam guide

1 General air brake system nomenclature

To see how much air pressure is in the tanks, you read the:

1. low air pressure warning light.
2. supply pressure gauge.
3. application pressure gauge.

Answer: B, supply pressure gauge.. The supply pressure gauge is connected to the air tank. It shows how much air pressure you have stored for braking. (CDL manual 5.1.9)

On a truck with dual air brakes, the air tanks, hoses, and lines are:

1. separate for each system.
2. part of the primary system.
3. shared by both systems.

Answer: A, separate for each system.. A dual air brake system is really two air systems. Each one has its own tanks, hoses, and lines, so a leak in one half does not drain the other. (CDL manual 5.2)

2 The dangers of contaminated air supply (dirt, moisture, and oil)

You skipped draining the air tanks before a freezing night. Water left in the system can:

1. freeze inside the brake valves and lines.
2. cause the brakes to fail.
3. Both of the above.

Answer: C, Both of the above.. Water left in the tanks can turn to ice in cold weather. Ice can block valves and lines, and then the brakes may fail. Both are real dangers. (CDL manual 5.1.4)

What is the job of an alcohol evaporator in an air brake system?

1. To help stop ice forming in brake valves
2. To keep the compressor oil from thickening
3. To remove water and oil from the tanks

Answer: A, To help stop ice forming in brake valves. The evaporator puts a little alcohol into the air. In cold weather, this lowers the chance of ice in valves and other parts. (CDL manual 5.1.5)

3 Implications of severed or disconnected air lines between the power unit and the trailer(s)

If the tractor protection valve fails, what can a bad leak in an air hose or trailer brake do?

1. It can drain the trailer's air tanks, but the tractor keeps its full pressure.
2. It can lower the trailer's braking power, but the tractor's brakes stay normal.
3. It can drain all the tractor's air and set the tractor's emergency brakes.

Answer: C, It can drain all the tractor's air and set the tractor's emergency brakes.. The tractor protection valve is what keeps a trailer leak from reaching the tractor's air. If it fails, the leak can empty the tractor's tanks, and then the emergency brakes come on (CDL manual 6.5.2).

A large leak in the blue trailer service line usually shows up when you:

1. charge the trailer air tanks and hear air leaking out.
2. press the brake pedal and see the air pressure drop fast.
3. start the engine and see the air pressure build slowly.

Answer: B, press the brake pedal and see the air pressure drop fast.. The service line carries air only while you brake. A big leak in it may go unnoticed until you press the pedal. Then tank pressure falls fast, and if it falls far enough the trailer emergency brakes come on (CDL manual 6.2.8).

4 Implications of low air pressure readings

On a tractor or straight truck, the spring brakes come fully on when air pressure falls to about:

1. 55 to 75 psi.
2. 85 to 100 psi.
3. 20 to 45 psi.

Answer: C, 20 to 45 psi.. On tractors and straight trucks, the spring brakes come fully on somewhere between 20 and 45 psi. Most often this happens between 20 and 30 psi (CDL manual 5.1.14).

When a dual air brake system loses air while you drive, you should expect:

1. normal stops, since the other half of the system does all the braking.
2. longer stops, with limited braking before the spring brakes apply.
3. a sudden hard stop, since the spring brakes come on right away.

Answer: B, longer stops, with limited braking before the spring brakes apply.. In a dual system, the vehicle takes longer to stop as air is lost. You can only brake a little more before the spring brakes come on (CDL manual 5.3.3).

5 Procedures to conduct safe and accurate pre-trip inspections

Before you drive a vehicle with dual air brakes, both systems should have at least:

1. 100 psi.
2. 55 psi.
3. 85 psi.

Answer: A, 100 psi.. Before you move, give the compressor time to bring both the primary and the secondary systems to at least 100 psi. Watch both gauges, or both needles. (CDL manual 5.2)

Why is a faulty air governor a safety problem?

1. It can let tank pressure fall too low for safe braking while driving.
2. It can stop the low air warning light from working.
3. It can make the brakes grab too hard on every stop.

Answer: A, It can let tank pressure fall too low for safe braking while driving.. The governor tells the compressor when to refill the tanks. If it fails, the tanks may not stay full enough for safe braking, so have it fixed (CDL manual 5.3.3).

6 Procedures for conducting en route and post-trip inspections of air-actuated brake systems

If your air brakes are out of adjustment, what happens to your braking power?

1. The spring brakes lose power, and the service brakes work as usual.
2. The service brakes and the spring brakes will both lose stopping power.
3. The service brakes lose power, and the spring brakes hold as usual.

Answer: B, The service brakes and the spring brakes will both lose stopping power.. The spring brakes push on the same brake parts as the service brakes. If those brakes are out of adjustment, both your service brakes and your parking or emergency brakes will be weak. (CDL manual 5.1.14)

To test the low air pressure warning, you shut off the engine and then:

1. turn the key on and pump the brake pedal.
2. turn the key off and pump the brake pedal.
3. start the engine again and pump the brake pedal.

Answer: A, turn the key on and pump the brake pedal.. With the engine off, the compressor can't refill the tanks. With the key on, the warning has power to work. Pumping the pedal bleeds air until the warning comes on, which must happen before 55 psi. (CDL manual 5.3.3)

7 General operating practices and procedures

Why should you never push the brake pedal while the spring brakes are on?

1. Air from the pedal can push the spring brakes off.
2. The spring and air forces together can damage the brakes.
3. The spring brakes can jam on and stay locked later.

Answer: B, The spring and air forces together can damage the brakes.. With the spring brakes on, the springs already push on the brakes. Air from the pedal adds a second force, and both together can damage the brakes. Some systems stop this, but not all do. (CDL manual 5.1.15)

On a long downgrade, what can make your brakes overheat and fade?

1. Using the service brakes too much instead of the engine.
2. Letting the engine brake the truck in a low gear.
3. Both of the above.

Answer: A, Using the service brakes too much instead of the engine.. Brakes turn speed into heat. If you lean on them and do not let the engine help slow the truck, they get too hot and can fade or fail. (CDL manual 5.4.5)