South Carolina CDL: Air Brakes Practice Test

Not quite. The answer is 150 psi

The safety valve, which protects the system from excessive pressure, is usually set to open when the system reaches 150 psi.

Not quite. The answer is At the end of each working day

Unless your vehicle automatically drains the air tanks, you should drain them at the end of each working day so that the moisture and oil will not freeze or coagulate and cause your brakes to fail.

Not quite. The answer is The brakes could fail.

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

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 park on level ground, chock the wheels, and release the parking brakes.

Chock the wheels so you can release the parking brakes safely. Then put on gloves and pull hard on each slack adjuster you can reach. If it moves more than about one inch, it needs to be adjusted.

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 vehicles with air brakes.

A low air pressure warning signal is required on vehicles with air brakes. The warning is usually a red light. A buzzer may also come on. Another type of warning is the “wig wag.” This device drops a mechanical arm into your view when the pressure in the system drops between the dangerous levels.

Not quite. The answer is 2 psi per minute.

To test the static air leakage rate, turn off the engine, release the parking brake (i.e., push the parking brake control knob in), and see how fast the air pressure drops. The air pressure should drop less than 2 psi in one minute for a single vehicle and less than 3 psi in one minute for a combination vehicle.

Not quite. The answer is brake drum.

In North America, the most common type of foundation brake is the S-cam drum brake. The brake shoes and linings press against the inside of the brake drum, which causes friction to slow the wheel.

Not quite. The answer is controls the air pressure applied to the service brakes.

The brake pedal controls the amount of air pressure applied to the service brakes. The harder the pedal is pushed, the more air pressure is applied to the brakes.

Not quite. The answer is 2 psi per minute.

Perform the first test of air leakage with the engine off and the brakes released. The air-loss rate should be less than 3 psi per minute for a combination vehicle and less than 2 psi per minute for a bus or straight truck. After you've tested this initial drop, now test air leakage with the engine off and the foot brake fully applied. This time, the air-loss rate should be less than 4 psi per minute for a combination vehicle and less than 3 psi per minute for a bus or straight truck.

Not quite. The answer is It is shaped like the letter "S."

The S-cam really is S-shaped. As it rotates, it forces the brake shoes away from each other and toward the inside of the brake drum.

Not quite. The answer is expansion of the brake drums.

Overusing the service brakes can cause overheating and expansion of the brake drums. The brake shoes will have to move farther to contact the brake drums, making braking harder or eventually impossible (brake fade).

Not quite. The answer is brake firmly while slowly moving forward.

To test the service brakes, wait for 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 service

Air brakes are really three braking systems in one: the emergency brake, the parking brake, and the service brake. As these names suggest, when you depress the brake pedal in normal driving, it's the service brake system that applies the brakes.

Not quite. The answer is press the brake pedal as hard as you can, release the brakes when the wheels lock, and apply the brakes again when the wheels start to roll.

To stop with stab braking, brake hard, release the brakes once the wheels lock, and then brake hard once the wheels start to roll again. (Note that this method won't work if your vehicle has antilock brakes, because the wheels won't lock. Use controlled braking instead.)

Not quite. The answer is All of the above

A modern Air Dryer Integrated System (AD-IS) combines the governor, air dryer, and safety valve into one system. Compressed air from the air compressor goes to the AD-IS, which removes moisture and contaminants from the air. The clean, dry air then goes to fill the primary and secondary service tanks.

Not quite. The answer is a loss of brake air pressure.

Each time you release the brakes, some compressed air leaves the system and must be replenished by the air compressor. If you keep pressing and releasing the brake pedal, air may leave the system faster than the air compressor can replenish it. The air pressure may drop to the point that the brakes won't work.

Not quite. The answer is turns on the brake lights when you brake.

The stop light switch is an electric switch triggered by air pressure. When you apply the service brakes, the stop light switch turns on the brake lights.

Not quite. The answer is Only if it's not working properly

Once properly installed and lubricated, an automatic slack adjuster should not need manual adjustment. If it does, it may be malfunctioning or there may be a problem with the foundation brake itself. Find the problem and correct it.