Nebraska CDL: Air Brakes Practice Test

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 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 Stop and park safely. Continue only after the system has been fixed.

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 (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 3 psi per minute.

One of the tests you must make of the air brake system is a test of the leakage rate without the foot brake applied. Fully charge the air brake system, turn off the engine, and release the parking brake. The air pressure should not drop more than 2 psi per minute for a straight vehicle or 3 psi per minute for a combination vehicle. If it is greater, check for leaks in the system.

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 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 1 inch

The slack adjusters won't move if the parking brakes are on. To check the slack adjusters, park on level ground, chock the wheels, and then release the parking brakes. Use gloves and pull hard on each slack adjuster that you can reach. If a slack adjuster moves more than about one inch where the pushrod attaches to it, it probably needs adjustment.

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 Otherwise, you will have to get a CDL with restrictions.

If you do not pass the air brake knowledge test, you will only be able to get a CDL with a restriction that limits you to driving vehicles without air brakes. So, good luck!

Not quite. The answer is 100 psi.

In a typical air brake system, pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check the manufacturer's specifications for the actual figures.)

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 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 brake normally.

Antilock brakes will automatically keep your wheels from locking so you can brake normally.

Not quite. The answer is 32 feet.

When you depress the brake pedal, it will take perhaps a half second for the air to reach the brakes. At 55 mph, your vehicle will travel about 32 feet in that time before the brakes kick in. This will add to your total stopping distance.

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 It disables the spring brake.

If your vehicle's service brakes fail, the emergency (spring) brakes will activate and stop your vehicle. Then to tow your vehicle for repairs, it will be necessary to disable the spring brakes. A technician can manually compress or "cage" the spring in each spring brake chamber. Obviously, the vehicle will be undrivable at this point because it will have no working brakes at all.

Not quite. The answer is park on level ground, chock the wheels, and release the parking brakes.

The slack adjusters won't move if the parking brakes are on. To check the slack adjusters, park on level ground, chock the wheels, and then release the parking brakes. Use gloves and pull hard on each slack adjuster that you can reach. If a slack adjuster moves more than about one inch where the pushrod attaches to it, it probably needs adjustment.

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 release the spring brakes.

Even after loss of air pressure has caused the spring brakes to come on, you may need to move the vehicle again in an emergency. Some vehicles such as buses have a separate air tank that can be used to release the spring brakes. You can then move the vehicle, after which you can set the spring brakes again. This action is controlled by dual parking control valves: One valve lets air flow from this air tank to release the spring brakes, and the other valve sets them again.

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.