Iowa CDL: Air Brakes Practice Test

Not quite. The answer is All of the above are true.

A dual air brake system is a system designed for safety and is used on most heavy-duty vehicles. It uses one set of controls. Typically, the "primary" system operates the regular brakes on the rear axle, and the "secondary" system operates the regular brakes on the front axle.

Not quite. The answer is that you are applying to the brakes.

If your vehicle is so equipped, the application pressure gauge will show how much air pressure you are applying to the brakes. If you have to apply more pressure than before to achieve the same braking effect, your brakes are fading (becoming less effective).

Not quite. The answer is 3 psi in one minute.

When testing for an air leak, make sure the system is fully charged, turn off the engine, fully apply the foot (service) brake, and hold the brake for one minute. The air pressure should not drop more than 3 psi for a single vehicle or more than 4 psi for a combination vehicle. If it does, check for an air leak somewhere.

Not quite. The answer is Pressure-protection valve

Various auxiliary air systems, such as air suspension and the driver and passenger air seats, usually get their air from the service tanks of the air braking system. To prevent the tanks from losing air in the event of a leak or failure in an auxiliary air system, a pressure-protection valve is installed between each auxiliary air system and the service tank. If tank pressure drops below a preset level, the pressure-protection valve will close to stop any more air from flowing to the auxiliary air system.

Not quite. The answer is With the engine off, step on and off the brake pedal to lower the air pressure below 60 psi.

The best way to test the low air pressure warning signal is to release the parking brake (all vehicles), release the tractor protection valve (on combination vehicles), and turn off the engine of your vehicle while leaving the electrical power on. Then step on and off the brake pedal. The low air pressure warning signal should come on when the pressure falls below 60 psi. You should never need to get down to 30 psi.

Not quite. The answer is 20 to 45 psi.

An air leak in your brake system can cause your air brakes to fail. If the air pressure drops to the manufacturer's specification (typically 20–45 psi), your spring brakes will come on automatically. You can avoid this by pulling over and checking the air brakes if the low pressure warning light ever comes on.

Not quite. The answer is Stop.

If the low air pressure warning comes on, there may be a leak in the system. Pull over and park your vehicle as soon as possible. If the air pressure drops too low, the spring brakes will come on automatically, which will stop your vehicle – but not necessarily in a safe way. Sudden application of the spring brakes while you're driving might result in a skid.

Not quite. The answer is It won't allow the brakes to cool.

Brakes use friction to convert the kinetic energy of your vehicle into heat. Overuse of the brakes won't allow enough time for the brakes to cool. Excessive heat can cause expansion of the brake drums and chemical changes in the brake lining that will reduce friction. The brakes will fade (become less effective).

Not quite. The answer is a yellow lamp on the left side.

A converter dolly with antilock brakes is required to have a yellow ABS lamp on its left side. All converter dollies built since March 1, 1998 have antilock brakes.

Not quite. The answer is you will still have enough air in the storage tanks to apply the air brakes several times.

The air storage tanks hold enough air for you to apply the air brakes several times.

Not quite. The answer is to drain moisture and oil out of the tanks.

Water and oil from the compressor can collect at the bottoms of the air tanks. If the air tanks are not drained, the water and oil can freeze or coagulate and cause brake failure.

Not quite. The answer is your brakes may fail because of water freezing.

Your air tanks hold a mixture of water and compressor oil. If left in to collect at the bottom, the water may freeze and cause brake failure.

Not quite. The answer is All of the above

Fade and fail are usually due to incorrect use of the brakes that causes excess heat. This may be caused by incorrect brake adjustment, not enough reliance on engine braking, excessive use of the service brakes, or another mechanical factor or human error.

Not quite. The answer is To prevent a buildup of ice in the air brake system

When the air compressor runs, an alcohol evaporator puts alcohol vapor into the air brake system to help prevent water in the lines or valves from freezing. Ice in the lines or valves can cause them to stop working.

Not quite. The answer is one-half

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

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 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 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 Never

Applying the spring brakes and the air brakes at the same time may put too much force on the brakes and can actually damage them.

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 primary system operates the brakes on the rear axle, and the secondary system operates the brakes on the front axle (and possibly one rear axle as well).