Connecticut CDL: Air Brakes Practice Test

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).

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 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 the brakes to fade.

Excessive heat caused by overuse of the brakes can expand the brake drums away from the brake pads or shoes and affect the brake linings, reducing friction. The brakes will fade (lose their effectiveness). On a long or steep downgrade, rely on engine braking more than the service brakes.

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 service brake valve and the parking brake valve.

A spring parking brake can exert a force of up to 2,000 pounds, and a Type 30 service brake chamber can exert a force of 3,000 pounds or more. If both forces were applied at the same time (which is called compounding the service and parking brakes), the combined force could damage parts of the braking system. To prevent this, most modern commercial vehicles employ an anti-compounding valve. One input port of this valve is connected to the service brake valve (foot valve), and another input port is connected to the parking brake valve (yellow knob). Suppose you set the parking brakes by pulling out the yellow knob. That valve will send air to the anti-compounding valve. If you also press the brake pedal, the foot valve will also send air to the anti-compounding valve. Then the anti-compounding valve will partially release the parking brakes by the same amount of force that the foot valve has applied.

Not quite. The answer is All of the above are correct.

On both tractor-trailers and single vehicles, to test whether the spring brakes will come on automatically, use the same method you use for testing the low air pressure warning signal: Step on and off the brake pedal, this time until you reach an even lower psi reading and the parking brake valve closes or pops out.

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 (b) Spring brake priority

Instead of providing air pressure to both the service brakes and the spring brakes at the same time, trailer brake subsystems give priority to one or the other. Trailers with spring brake priority supply compressed air to the spring brakes (to release them) before the service tanks have filled. Trailers with service brake priority fill the service tanks before supplying air to the spring brakes. When coupling your tractor to a trailer with service brake priority, you must wait until the service tanks are full before you can release the trailer's parking brakes. Trailers more than 25 years old were more likely to implement service brake priority. Today's modern trailers are more likely to implement spring brake priority. Ask your employer or a qualified person whether your trailer uses spring brake priority or service brake priority.

Not quite. The answer is It is exhausted from the system.

Each time you release the brakes, some compressed air leaves the system and must be replenished by the air compressor.

Not quite. The answer is controls the air pressure to operate the brakes.

When you press the brake pedal in a CMV equipped with air brakes, the foot valve underneath the pedal opens to allow compressed air to flow to the brake chambers. The brake chambers exert mechanical force to operate the foundation brakes.

Not quite. The answer is glad hands

As part of your vehicle inspection test, if your vehicle is equipped with air brakes and has a trailer, you will inspect the air connections between the truck or tractor and the trailer. These connections are made with hose couplers (commonly called "glad hands" because they vaguely resemble shaking hands). These glad hands have rubber seals to form a tight connection and prevent air from escaping. Make sure that the glad hands are locked in place and free of damage or air leaks.

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 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 either (a) or (b).

If you need to stop quickly, use either the "controlled braking" method or the "stab braking" method. However, if your vehicle has antilock brakes, stab braking won't work because the wheels won't lock. Use controlled braking instead.

Not quite. The answer is the emergency brake system.

The system that uses parts of the service and parking brake systems to stop the vehicle in a brake system failure is called the emergency brake system.

Not quite. The answer is Never

It used to be thought that applying the steering axle brakes on a slippery surface might cause a front-wheel skid. However, more recent tests have shown that front-wheel braking is good under all driving conditions. Front-wheel skids from braking are unlikely even on ice. Therefore, always have front-wheel braking available. If you're driving an old truck equipped with a front brake limiting valve, leave the control set at "normal" to have normal stopping power. The front-wheel brakes provide about 12% of the total stopping power of a loaded combination vehicle.

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 60 - 55 psi.

The low air pressure warning light must come on before the air pressure falls below 60 - 55 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 when air is pumped into the air storage tanks.

The air compressor runs at all times while the engine is running. To keep the air pressure from eventually rising too high, the air compressor governor controls when the compressed air from the air compressor should flow into the air reservoirs. When air pressure drops to a certain level (typically 100 psi), the governor "cuts in" and routes the compressed air into the storage tanks. When air pressure rises to a certain level (typically 125 psi), the governor "cuts out" and routes the compressed air out of the system, into the atmosphere.