District of Columbia CDL: Air Brakes Practice Test

Not quite. The answer is compressed air

Air brakes use compressed air to make the brakes work.

Not quite. The answer is emergency

The emergency brake system utilizes parts of the service brakes and the parking brakes in case your regular brakes fail and you need to come to a stop.

Not quite. The answer is Between the compressor and the first air tank

An air dryer may be installed in the compressor discharge line between the compressor and the first air tank. Its purpose is to remove water vapor and contaminants from compressor air before the air enters the first tank. The first tank has usually been the supply or "wet" tank, so called because moisture accumulated in that tank before the air flowed from there to the service tanks. However, today's air dryer technology is so effective that a "wet" tank may not be needed anymore. In a modern Air Dryer Integrated System (AD-IS®), the air compressor governor and the air dryer are combined into a single unit. Air from the air dryer goes directly into the primary and secondary service ("dry") tanks. There is no supply ("wet") tank.

Not quite. The answer is Clockwise until the bolt stops turning

Adjusting an automatic slack adjuster is an emergency procedure that you should perform only if your brakes have failed on the road. Most automatic slack adjusters have a hexagonal (six-sided) adjusting bolt. For this type of slack adjuster, first turn the bolt clockwise until it stops turning. If the slack adjuster has a square adjusting bolt, first turn the bolt counterclockwise until it stops turning. Then with either type of bolt, back it off about half a turn in the opposite direction.

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 the brakes when you depress the brake pedal during normal driving.

Air brakes are actually three braking systems in one: the emergency brake, the parking brake, and the service brake. As these terms 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 S-cam drum brake.

Foundation brakes are present on each wheel. In North America as of this writing, S-cam drum brakes are much more common than wedge or disc brakes.

Not quite. The answer is 3 psi for single vehicles and 4 psi for combination vehicles.

After the initial drop, the maximum acceptable leakage rates per minute are 3 psi for single vehicles and 4 psi for combination vehicles. If the leakage rate is higher, check for leaks and correct them.

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

Oil and water can start to collect at the bottom of an air tank, which can lead to brake failure. Unless your vehicle automatically drains the air tanks, you should drain them at the end of each working day so the moisture and oil will not freeze or coagulate and cause your brakes to fail.

Not quite. The answer is Park on level ground, turn off the parking brakes, and pull hard on each slack adjuster.

To check the slack adjusters, try to pull on them manually while wearing gloves. If there is an inch or more of "give," then there is an issue and they should be fixed.

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 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 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 compressed air enters the brake chambers.

Pressing down on the brake pedal lets air into each brake chamber, forcing the brake pads against the inside of the drum (or the rotor for disc brakes).

Not quite. The answer is it may lead to sudden emergency braking in a single-circuit air system.

The low pressure warning signal should come on before the air pressure falls below 60 psi. If it fails to do so, the air pressure may keep falling without your knowing it. In a single-circuit air system, your spring (emergency) brakes may come on suddenly. In a dual air system, either the front or the rear brakes won't work properly, the braking distance of your vehicle will increase, and continued braking will soon cause the spring brakes to come on.

Not quite. The answer is Use gloves and pull hard on each slack adjuster you can reach.

To check the slack adjusters, try to pull on them manually while wearing gloves. If there is an inch or more of "give," there is an issue and they should be fixed.

Not quite. The answer is Bring the vehicle to a safe stop and continue only after the system is fixed.

A dual air brake system has two separate air brake systems. Typically, the primary system operates the brakes on the rear axle(s), and the secondary system operates the brakes on the front axle (and possibly one rear axle). If one of these two systems loses air pressure (causing the low air pressure warning), 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.

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 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 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.