Maryland CDL: Air Brakes Practice Test

Not quite. The answer is spring pressure.

Federal safety regulations require buses and trucks to have parking and emergency brakes that don't rely on compressed air or hydraulic fluid to work (because these can leak away). Hence, the parking and emergency brakes rely on mechanical force such as springs.

Not quite. The answer is does all of the above.

The tractor protection valve protects your air brake system by providing the air supply, closing automatically if the air supply drops too low, and closing if you apply the parking brakes on your vehicle.

Not quite. The answer is 35 mph.

On a steep downgrade, continuous application of the service brakes 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. (In this case, with a "safe" speed of 40 mph, you should brake gently until your speed drops to 35 mph.) 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 a yellow lamp on the left side.

A converter dolly with antilock brakes is required to have a yellow ABS lamp on its left side. All converter dollies built since March 1, 1998 have antilock brakes.

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 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 3 psi per minute.

After the initial drop, the maximum acceptable leakage rates with the service brakes on are 3 psi per minute for single vehicles and 4 psi per minute for combination vehicles. If the leakage rate is higher, check for leaks and correct them.

Not quite. The answer is Otherwise, you will have to get a CDL with restrictions.

If you do not pass the air brakes 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 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 vehicle was manufactured after 1998.

Vehicles manufactured after 1998 are required to have anti-lock brakes. They also have a yellow ABS malfunction lamp.

Not quite. The answer is You should drive faster so that you will be able to keep the trailer and tractor straight.

When driving a tractor-trailer combination that has ABS, drive the same way that you always have. Do not drive any differently just because of ABS. Use only the braking necessary to keep control, pay attention when you slow down, and brake the same way regardless of what you're driving. Never speed up unless it is part of the safe driving process already.

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 your vehicle will stay in a straight line.

If you must brake in an emergency, brake in a way that keeps your vehicle in a straight line while still letting you turn if necessary. You can use the "controlled braking" method or (in a vehicle without antilock brakes) the "stab braking" method.

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 spring pressure.

Federal safety regulations require buses and trucks to have parking and emergency brakes that don't work on compressed air or hydraulic fluid (because these can leak away). Hence, the parking and emergency brakes rely on mechanical force, such as springs.

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 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 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 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 reduce drive-axle braking

A tractor's drive-axle brakes are designed to be powerful enough to handle a fully loaded trailer. However, a bobtail tractor doesn't have the weight of a trailer over the drive axles. This increases the chance that the powerful drive-axle brakes will lock the wheels when you brake the tractor. To help prevent this, bobtail proportioning valves significantly reduce drive-axle braking when the tractor is bobtailed.