Tennessee CDL: Air Brakes Practice Test

Not quite. The answer is takes longer than hydraulic fluid to flow through the lines.

With hydraulic brakes, hydraulic fluid is always in the system, so the brakes will be applied instantly when you depress the brake pedal. With air brakes, the compressed air must flow through a complex series of lines, valves, etc., to reach and fill the brake chambers before the brakes can begin to slow your vehicle. This "brake lag" can take at least a half second.

Not quite. The answer is Overheating, low air pressure, and not relying on the engine braking effect

Brake fade and failure are usually due to incorrect use of the brakes that causes overheating. This may be caused by incorrect brake adjustment, not enough reliance on engine braking, excessive use of the service brakes, another mechanical factor, or human error. Air brakes will also fail if air pressure drops too low.

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 Never

Never try to apply the brake pedal when the spring brakes are on. The combined force of the springs and the air pressure could damage the brakes. However, the brake systems on some (but not all) vehicles are designed to prevent this from happening.

Not quite. The answer is compressed air enters the brake chambers.

Pressing down on the brake pedal lets air into each brake chamber, forcing the brake pads against the inside of the drum (or the rotor for disc brakes).

Not quite. The answer is Stop.

If the low air pressure warning comes on, there may be a leak in the system. Pull over and park your vehicle as soon as possible. If the air pressure drops too low, the spring brakes will come on automatically, which will stop your vehicle – but not necessarily in a safe way. Sudden application of the spring brakes while you're driving might result in a skid.

Not quite. The answer is 125 psi.

A typical air brake system will be fully charged at 125 psi. Check the manufacturer's specifications for the actual figure for your vehicle. The air compressor should stop pumping air at that point.

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 Do all of the above.

To ensure that your vehicle will be safe when you leave it unattended, apply the parking brakes and chock the wheels. Removing the keys is a simple safety tip for any vehicle, even a passenger car.

Not quite. The answer is Clockwise until the bolt stops turning

Adjusting an automatic slack adjuster is an emergency procedure that you should perform only if your brakes have failed on the road. Most automatic slack adjusters have a hexagonal (six-sided) adjusting bolt. For this type of slack adjuster, first turn the bolt clockwise until it stops turning. If the slack adjuster has a square adjusting bolt, first turn the bolt counterclockwise until it stops turning. Then with either type of bolt, back it off about half a turn in the opposite direction.

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 check the alcohol level

Alcohol vapor from the alcohol evaporator helps keep ice from forming in the lines and valves of the air brake system. Check the alcohol level daily and refill as needed.

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 All vehicles with air brakes currently in operation must have low air pressure warning signals.

All vehicles on the road that have air brakes must have low air pressure warning signals.

Not quite. The answer is It is shaped like the letter "S."

The S-cam really is S-shaped. As it rotates, it forces the brake shoes away from each other and toward the inside of the brake drum.

Not quite. The answer is that you are applying to the brakes.

If your vehicle is so equipped, the application pressure gauge will show how much air pressure you are applying to the brakes. If you have to apply more pressure than before to achieve the same braking effect, your brakes are fading (becoming less effective).

Not quite. The answer is Air pressure

Drivers behind you must be warned whenever you brake. The air brake system does this with an electric switch that works by air pressure. The switch turns on the brake lights when you apply the air brakes.

Not quite. The answer is a supply tank.

An Integrated Solution Air Dryer (AD-IS®) and Reservoir System provides an integrated air dryer, purge reservoir, governor, and a number of the charging valve components in a module. The air dryer is so efficient that a supply ("wet") tank is not needed. Instead, the AD-IS sends air directly to the primary and secondary service ("dry") tanks.

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.