Louisiana CDL: Air Brakes Practice Test

Not quite. The answer is by increasing and decreasing the air pressure.

The air compressor governor should cut in when a lower air pressure is reached and cut out when a higher air pressure is reached. (Check your manufacturer's specifications for these pressures.) To test this, run the engine at a fast idle to raise the air pressure until the air compressor governor cuts out. Then keep stepping on and off the brake pedal to reduce the air pressure until the air compressor governor cuts in.

Not quite. The answer is so you only have to apply the brakes just hard enough to feel a definite slowdown.

When starting down a downgrade, shift into a low gear and use engine braking to slow your vehicle. Apply the brakes just enough to keep your vehicle near your safe speed.

Not quite. The answer is all of the above.

Slack adjusters are an important part of your air brake system that allows you to adjust the brakes to ensure that they are safe. They are located in different places, depending on the type of brakes that you have.

Not quite. The answer is 8

A CMV has two foundation brakes per axle (one on each side). According to FMCSA, even a lift axle must have brakes when it is in contact with the roadway. Therefore, a truck with 4 axles must have 4 × 2 = 8 foundation brakes, even if one axle is a lift axle.

Not quite. The answer is Typically, one brake system operates the brakes on the front axle, and the other system operates the brakes on the rear axles.

A dual air brake system has two separate air brake systems, each with its own air tanks, hoses, lines, etc. Both systems are controlled by the one brake pedal. Typically, the secondary system operates the brakes on the front axle (and possibly one rear axle as well), and the primary system operates the brakes on the rear axles.

Not quite. The answer is lets the air out of the brake chambers.

When you apply the parking brake control, air is released from the brake chambers, causing the springs to apply the brakes and hold your vehicle in place.

Not quite. The answer is Either the front or rear brakes will not be fully operational.

A dual air brake system has two separate air brake systems, each with its own air tanks, hoses, lines, etc. Typically, one system operates the brakes on the rear axle(s), and the other system operates the brakes on the front axle. If one of the systems loses air pressure, either the front or the rear brakes won't work properly. As a result, your vehicle's braking distance will increase. If this happens, stop your vehicle and get the system fixed. Continued braking would soon cause the spring brakes to come on.

Not quite. The answer is The dashboard air pressure gauges won't drop when it's drained.

It may be difficult to visually identify the supply tank. Instead, remember that your dashboard air pressure gauges monitor the air pressure in the primary and secondary service tanks. (In some vehicles, there are two gauges, one for each service tank; in other vehicles, there is one gauge with two needles.) Draining the service tanks will cause the gauges to show a drop in air pressure in those tanks. The supply tank is the tank that does NOT drop the gauges when it's drained.

Not quite. The answer is release the spring brakes.

Even after loss of air pressure has caused the spring brakes to come on, you may need to move the vehicle again in an emergency. Some vehicles such as buses have a separate air tank that can be used to release the spring brakes. You can then move the vehicle, after which you can set the spring brakes again. This action is controlled by dual parking control valves: One valve lets air flow from this air tank to release the spring brakes, and the other valve sets them again.

Not quite. The answer is Apply the brakes as hard as you can without locking the wheels.

In controlled braking, you apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. This method of emergency braking does not require you to lock the wheels. Hence it is compatible with antilock brakes (ABS).

Not quite. The answer is release the spring brakes.

When main air pressure is lost, the spring brakes come on and stop your vehicle. Some vehicles such as buses have a separate air tank with enough air to release the spring brakes in case the vehicle must be moved temporarily in an emergency. (The tank holds only enough air to do this a few times, however.) You control this action with the dual parking control valve.

Not quite. The answer is either (a) or (b).

If you need to stop quickly, use either the "controlled braking" method or the "stab braking" method. However, if your vehicle has antilock brakes, stab braking won't work because the wheels won't lock. Use controlled braking instead.

Not quite. The answer is whenever you leave the vehicle unattended.

Because air pressure can leak when you're away from your vehicle, it should be set with the parking brake, to keep it from rolling. There is an exception to this rule: if the brakes are too hot or wet, use wheel chocks instead, to avoid brake damage or freezing.

Not quite. The answer is 3,000 pounds.

The most common type of air brake chamber in trucks and trailers is the Type 30. The type number gives the area of the diaphragm. Thus, the diaphragm in a Type 30 brake chamber has an area of 30 square inches. If a force of 100 psi (pounds per square inch) is applied to the diaphragm, the resulting pushrod force will equal 100 × 30 = 3,000 pounds.

Not quite. The answer is 45

In dual air systems with minimum-sized air tanks, the pressure should build from 85 to 100 psi within 45 seconds. (With larger air tanks, it can take longer. Check the manufacturer's specifications for the expected amount of time.) If it takes longer than it should, there may be a problem with your system that may cause you to lose air pressure while you're driving.

Not quite. The answer is one-half

During your walk-around inspection, check the brake drums or brake discs to be sure that no cracks are longer than half the width of the friction area.

Not quite. The answer is It disables the spring brake.

If your vehicle's service brakes fail, the emergency (spring) brakes will activate and stop your vehicle. Then to tow your vehicle for repairs, it will be necessary to disable the spring brakes. A technician can manually compress or "cage" the spring in each spring brake chamber. Obviously, the vehicle will be undrivable at this point because it will have no working brakes at all.

Not quite. The answer is Brake firmly while slowly moving forward on level ground.

To test the service brakes, wait to build up normal air pressure. Then move your vehicle forward slowly (at about 5 mph) and brake hard. Check whether your vehicle stops promptly without pulling to one side or the other.

Not quite. The answer is They are not as strong as service brakes.

Spring brakes start to drag as pressure drops to about 60 psi, and they release as pressure rises to about 60 psi. Yet a full service brake application can be 100 psi. The coil spring in most spring parking brake chambers can exert a force of between 1,500 and 2,000 pounds. With a full service brake application of 100 psi, a typical Type 30 brake chamber can produce a force of 30 × 100 = 3,000 pounds.

Not quite. The answer is Yes, but only if the alcohol evaporator is located downstream from the air dryer.

The air dryer contains a desiccant pack. A desiccant is a chemical agent designed to remove moisture. However, alcohol vapor from the alcohol evaporator (injector) can damage the desiccant. Hence, if an alcohol evaporator is installed, it must be located downstream from the air dryer.