CDL Air Brakes — All Questions

Why does an empty truck sometimes take longer to stop than a fully loaded one?

• a.With less weight, the tires have less traction and the brakes can lock more easily, reducing control✓
• b.Empty trucks always use only the trailer brakes
• c.Empty trucks have weaker compressors
• d.Empty trucks have no spring brakes

An empty vehicle has less weight pressing the tires to the road, so the tires have less traction and the wheels can lock up more easily during hard braking. Locked wheels skid and lengthen the stop, so an empty truck can actually require a longer distance to stop under some conditions. Brake smoothly and avoid locking the wheels regardless of load.

The total stopping distance for an air brake vehicle includes which of the following?

• a.Only braking distance
• b.Perception distance, reaction distance, brake lag distance, and braking distance✓
• c.Only perception distance
• d.Perception and reaction distance only

For air brakes, total stopping distance is made up of perception distance, reaction distance, brake lag distance, and braking distance. Air brakes have an added brake lag because it takes time for the air to travel through the lines and apply the brakes. This makes the total stopping distance longer than for hydraulic brakes.

What is brake lag in an air brake system?

• a.The time it takes to build air pressure at startup
• b.The distance the vehicle travels before you see a hazard
• c.The time the compressor takes to cut out
• d.The short delay before the brakes work after you press the pedal, because air must travel through the lines✓

Brake lag is the time it takes, after you push the brake pedal, for the air to flow through the lines and actually apply the brakes. On air brakes this adds roughly half a second of travel that hydraulic brakes do not have. Drivers must account for this extra distance when stopping.

What is brake fade?

• a.The brakes slowly releasing while parked
• b.The loss of air pressure overnight
• c.The lag before air reaches the brakes
• d.A reduction in braking power from overheated, overused brakes✓

Brake fade happens when brakes are used so much that they overheat and lose stopping power. Excessive heat causes the drums to expand and the linings to become less effective, so the brakes do not grip as well. Fade is a major risk on long or steep downgrades.

What is the most important thing to do to control your speed on a long, steep downgrade?

• a.Use a low gear so the engine helps hold the vehicle back✓
• b.Pump the brakes continuously as fast as possible
• c.Ride the brakes lightly the whole way down
• d.Shift into neutral and coast

The most important factor on a long downgrade is to select a low gear before starting down, so engine braking helps control your speed. The proper braking technique is then used only to supplement the low gear. Being in the right gear keeps the brakes from overheating and fading.

What is the proper braking technique (snub braking) on a long downgrade?

• a.Never touch the brakes at all
• b.Hold steady light pressure the entire way down
• c.Coast in neutral and brake only at the bottom
• d.Apply the brakes firmly to slow about 5 mph below your safe speed, then release, and repeat✓

With the vehicle in a low gear, apply the brakes firmly enough to feel a definite slowdown, then release when your speed has dropped about 5 mph below your safe speed. Once speed climbs back to the safe speed, repeat the process. This snub-braking method keeps the brakes from overheating compared with light, constant pressure.

On a vehicle with air brakes, when is the proper time to release the brakes during snub braking on a downgrade?

• a.Only when the vehicle comes to a complete stop
• b.When the air pressure warning comes on
• c.As soon as you touch the brake pedal
• d.When your speed has been reduced to about 5 mph below your safe speed✓

During snub braking, you apply the brakes to slow the vehicle and then release them when you have slowed to about 5 mph below your safe speed. This lets the brakes cool between applications. Continuous light braking, by contrast, keeps heat building and can lead to fade.

Compared to a car, how does the stopping distance of a heavy vehicle with air brakes change as weight and speed increase?

• a.It is unaffected by speed
• b.It stays the same regardless of weight
• c.It increases significantly, requiring much greater distance to stop✓
• d.It becomes shorter because of the heavier weight

Heavier vehicles and higher speeds both greatly increase the distance needed to stop. A loaded truck traveling at highway speed needs far more room than a car. Drivers must increase following distance and plan stops well ahead.

If your air brakes fail and pressure drops, what safety feature will eventually stop the vehicle?

• a.The alcohol evaporator
• b.The spring (emergency/parking) brakes applying automatically✓
• c.The trailer hand valve
• d.The service brake pedal alone

If air pressure drops too low, the spring brakes apply automatically because there is no longer enough air to hold the springs back. This is designed to stop the vehicle when the air system fails. However, the driver should react early to a low-pressure warning rather than wait for automatic application.

What should you do first if the low air pressure warning comes on while driving?

• a.Pump the brake pedal rapidly
• b.Brake to a stop safely and get off the road as soon as possible✓
• c.Ignore it until the brakes stop working
• d.Speed up to reach your destination faster

If the low air pressure warning comes on, you should stop and safely park the vehicle as soon as possible because a total loss of air is possible. Continuing to drive risks the spring brakes applying suddenly or the brakes failing. The cause must be found and fixed before continuing.

When stopping normally with a vehicle equipped with air brakes, how should you apply the brakes?

• a.Apply steady, controlled pressure without letting the wheels lock✓
• b.Slam the pedal to the floor immediately
• c.Use only the trailer hand valve
• d.Pump the pedal rapidly the entire stop

For a normal stop, push the brake pedal down with controlled, steady pressure to slow smoothly without locking the wheels. Locked wheels can cause a skid and loss of control, especially on vehicles without anti-lock brakes. Smooth, firm application gives the best control.

Why should you not fan (pump) the brakes on and off on a long downgrade with standard air brakes?

• a.It automatically applies the spring brakes
• b.It makes the brakes too cold
• c.It can use up the air supply faster than the compressor can replace it, reducing braking✓
• d.It speeds up the vehicle

Repeatedly pumping the brakes on and off can use air faster than the compressor can rebuild it, lowering tank pressure. If pressure drops too far, braking is reduced and the spring brakes may apply. Proper technique is to use a low gear and steady snub braking instead.

What is the effect of anti-lock braking systems (ABS) on air brake vehicles?

• a.They help keep the wheels from locking up during hard braking so you can maintain steering control✓
• b.They shorten the vehicle's normal stopping distance dramatically
• c.They replace the need for the parking brake
• d.They eliminate the need to check air pressure

ABS helps prevent the wheels from locking during hard or emergency braking, which helps you keep control and steer. ABS does not necessarily shorten stopping distance or let you drive faster; it is a system to help you retain control. You still brake normally, and the ABS works only when needed.

To make sure you can stop in time, how should you adjust following distance with air brakes at higher speeds?

• a.Follow closely to draft the vehicle ahead
• b.Reduce following distance to save time
• c.Increase following distance because stopping takes longer at higher speeds and heavier weights✓
• d.Keep the same distance as a passenger car

Because heavy vehicles need much more distance to stop, and brake lag adds even more, you should increase your following distance. The faster you go and the heavier you are, the more room you need. Leaving extra space gives you time to react and stop safely.

Before starting down a long grade, when should you select the proper gear?

• a.At the bottom of the hill
• b.Only after the brakes start to fade
• c.Never; leave it in the highest gear
• d.Before starting down, because it is difficult and dangerous to downshift once already descending fast✓

You should select a safe, low gear before you start down a grade. Trying to downshift after you have already gained speed can be difficult or impossible and can leave you without engine braking. Being in the right gear before descending is critical to controlling speed.

What is a proper way to test the trailer service brakes while stopped or moving slowly?

• a.Drain the trailer air tank
• b.Move at about 5 mph and apply the trailer hand valve to confirm the trailer brakes engage✓
• c.Set the parking brake and rev the engine
• d.Slam the tractor foot brake to the floor

To test the trailer service brakes, move the vehicle slowly (about 5 mph) and apply the trailer hand valve. You should feel the brakes come on, which confirms the trailer brakes and their air connection are working. This should be done as part of your pre-trip checks, not used for normal stopping.