Vermont CDL: Air Brakes Practice Test

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 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 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 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 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 1 inch

During the walk-around inspection, pull hard on each slack adjuster that you can reach. If it moves more than an inch, it probably needs adjustment.

Not quite. The answer is Whenever your vehicle is parked

You should always use your parking brake except when the brakes are very hot or they are wet and at risk of freezing.

Not quite. The answer is 5 mph slower.

On a steep downgrade, don't keep applying your brakes continuously because this might cause them to fade (become less effective). Instead, apply your brakes sparingly as a supplement to engine braking. Select a "safe" speed. Let your vehicle reach this speed. Brake gently until you are going 5 mph slower. Then let your speed increase back to the "safe" speed. Keep varying your speed up and down this way. This technique is known as snub braking or snubbing.

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 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 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 In the first tank to which the air compressor pumps air

A safety relief valve is installed in the first tank that the air compressor pumps air to. This tank is known as the supply or "wet" tank. The safety valve protects the tank and the rest of the system from too much pressure. The valve is usually set to open at 150 psi.

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 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 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 10

A CMV has two foundation brakes per axle (one on each side). A typical 18-wheeler has five axles: the steering axle, the two tandem drive axles, and the two tandem semi-trailer axles. Therefore, there are 5 × 2 = 10 foundation brakes on the vehicle. Note: According to FMCSA, even a lift axle must have brakes when it is in contact with the roadway.

Not quite. The answer is let the air out of the brake chambers.

The parking brake control allows you to evacuate air from the brake chambers. With the air gone, the spring brakes will come on automatically and hold your vehicle in place.

Not quite. The answer is Between the air compressor and the supply ("wet") tank

The supply ("wet") tank tends to get most of the moisture because moisture condenses there as the hot air from the compressor cools. To help alleviate this situation, the air dryer dries the air from the compressor before it enters the supply tank. Note: Today's air dryer technology is so effective that a "wet" tank may not be needed anymore. In a modern Air Dryer Integrated System (ADIS), air from the air dryer goes directly into the primary and secondary service ("dry") tanks.

Not quite. The answer is They are not as strong as service brakes.

Spring brakes start to drag as pressure drops to about 60 psi, and they release as pressure rises to about 60 psi. Yet a full service brake application can be 100 psi. The coil spring in most spring parking brake chambers can exert a force of between 1,500 and 2,000 pounds. With a full service brake application of 100 psi, a typical Type 30 brake chamber can produce a force of 30 × 100 = 3,000 pounds.

Not quite. The answer is It won't allow the brakes to cool.

Brakes use friction to convert the kinetic energy of your vehicle into heat. Overuse of the brakes won't allow enough time for the brakes to cool. Excessive heat can cause expansion of the brake drums and chemical changes in the brake lining that will reduce friction. The brakes will fade (become less effective).