Kentucky CDL: Air Brakes Practice Test

Not quite. The answer is a supply pressure gauge.

Every vehicle with air brakes has a supply pressure gauge, which shows how much air pressure is in the air tanks. Some vehicles also have an application pressure gauge, which shows how much air pressure you are applying to the brakes. Older vehicles also had a manual front brake limiting valve, which allowed the driver to cut front brake air pressure in half as a way of preventing front-wheel skids. However, tests have shown that full front-wheel braking is safe.

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 forces the brake shoes against the inside of the brake drum.

When the S-cam rotates, it forces the brake shoes apart and against the inside of the brake drum. Friction will slow the rotation of the wheel.

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 More than 400 feet

The stopping distance equals the sum of the perception distance, reaction distance, brake lag distance, and braking distance. Under ideal conditions, the average driver of a bus or large truck equipped with air brakes and traveling at 55 mph would require about 451 feet to come to a complete stop. The length of a football field including the two end zones is 360 feet.

Not quite. The answer is a Class A CDL without an air brakes restriction.

To legally pull a trailer whose gross vehicle weight rating (GCWR) exceeds 10,000 pounds, you'll have to obtain a Class A CDL. If you don't pass a knowledge test on air brakes, your CDL will include a restriction limiting you to driving only vehicles without air brakes.

Not quite. The answer is 60

The low air pressure warning light must come on before the air pressure falls below 60 psi. On some vehicles, the warning light will come on at a higher air pressure. Check the manufacturer's specifications.

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 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 you have to press the brake pedal harder than usual to control your speed.

If your brakes begin to fade, you'll notice that you have to depress the brake pedal harder to achieve the same braking effect as before. If your vehicle is so equipped, check the application pressure gauge to see just how much pedal effort you are employing. If the brakes continue to fade, you may not be able to slow down or stop at all.

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 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 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 Park on level ground. Chock the wheels. Release the parking brake when you have enough air pressure. Shut off the engine. Repeatedly press and release the brake pedal.

To test whether the spring brakes come on, release the parking brake (all vehicles), release the tractor protection valve control (on combination vehicles), and turn off the engine while leaving the electrical power on to power the instrument panel. Then step on and off the brake pedal. The low air pressure warning signal should come on when the pressure has fallen below 55 psi. Continue stepping on and off the brake pedal until you reach about 20–45 psi, at which point the parking brake valve should close or pop out.

Not quite. The answer is The heavier the vehicle or the faster it's traveling, the more heat the brakes have to absorb to stop it.

Brakes use friction to convert the kinetic energy of your vehicle into heat. A vehicle's kinetic energy is directly proportional to its weight and directly proportional to the square of its speed. If you increase your speed, your brakes will have to absorb much more heat to bring your vehicle to a stop.

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 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 It depends on the adjustment of the service brakes.

The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted properly, neither the service brakes nor the emergency or parking brakes will work right.

Not quite. The answer is parts of the service brakes and parking brakes

Spring brakes use powerful springs to apply the brakes. Normally, air pressure in the brake chambers holds back the springs. Should air pressure drop too low because of a leak, the springs will apply the brakes and stop the vehicle. To apply the parking brakes, you release air from the brake chambers manually.

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.