Maine CDL: Air Brakes Practice Test

Not quite. The answer is brake lag.

When you depress the brake pedal on a vehicle with air brakes, there will be a lag of about a half second before the air flows through the lines to reach the brakes. This will add to your total stopping distance.

Not quite. The answer is the air pressure may drop too low during driving.

The manufacturer's specifications should state how fast the air pressure should build up once the engine has reached normal operating speed. If the air pressure does not build up fast enough during your final air brake check, the air pressure may not replenish quickly enough during normal braking either. Don't drive the vehicle until the cause has been found and corrected.

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 inside to collect at the bottom, the water may freeze and cause brake failure.

Not quite. The answer is 32 feet

55 mph = 81 feet per second. Air brake lag typically lasts about 0.4 seconds. Therefore, the lag distance equals 81 feet per second × 0.4 seconds = 32 feet.

Not quite. The answer is You should drive faster so that you will be able to keep the trailer and tractor straight.

When driving a tractor-trailer combination that has ABS, drive the same way that you always have. Do not drive any differently just because of ABS. Only use the braking necessary to keep control, pay attention when you slow down, and brake the same way regardless of what you're driving. Never speed up unless it is part of the safe driving process already.

Not quite. The answer is Leaky glad hand seals

Glad hands have rubber or urethane seals to form a tight connection and prevent air from escaping. The glad hands are constantly being connected and disconnected, so the seals tend to wear out over time. Leaky seals can cause loss of air pressure.

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 reduce air pressure at the front wheels.

Most modern tractors are equipped with an automatic front axle ratio valve, which supersedes the manual front brake limiting valve in older tractors. In light-to-moderate braking, the ratio valve automatically reduces air pressure (and hence braking force) to the front wheels to reduce the chance of front-wheel lockup (which could result in a skid) on slippery roads.

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 All of the above are true.

A dual air brake system is a system designed for safety and 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 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 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 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 Before driving

Air compressors are either belt driven or (as is more often the case these days) gear driven directly from the engine. Most modern compressors are cooled by the engine cooling system and are lubricated by engine oil. However, some compressors may still be self lubricated or air cooled. If the compressor has its own oil supply, check the oil level before you drive.

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 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 1/4 inch

According to federal regulations on air brakes, the brake linings of non-steering-axle brakes must be at least 1/8 inch thick for disc brakes or at least 1/4 inch thick for drum brakes. The brake linings of steering-axle drum brakes can be thinner.

Not quite. The answer is always keep it on the "normal" setting.

Some older vehicles have a manually controlled front brake limiting valve to reduce air pressure to the front brakes when traveling on slippery roads. However, tests have since shown that front wheel braking is unlikely to cause a skid even on slippery roads. And reducing the air pressure reduces the vehicle's stopping power. Keep the front brake limiting valve on the "normal" setting at all times.

Not quite. The answer is 125

The air compressor will start pumping air into the air tanks at about 100 psi and stop at about 125 psi. This action is controlled by the air compressor governor.

Not quite. The answer is All weather conditions

Front wheel brakes have been shown to be ideal under all weather and driving conditions. Front wheel braking is unlikely to cause a skid even on icy roads.