CDL Air Brakes Using Air Brakes Summary

Controlled braking means applying the brakes as hard as you can without locking the wheels. Keep steering movements very small while doing it.

Wrong choices

• Braking fully, releasing when wheels lock, and braking again: Braking fully and releasing when the wheels lock is stab braking, a different method.
• Pumping the brakes lightly and quickly: Light, quick pumping is not how the manual describes controlled braking.

Stab braking:

1. Apply the brakes all the way.
2. Release them when the wheels lock up.
3. As soon as the wheels start rolling, apply the brakes fully again.

Wrong choices

• Brake as hard as you can without ever locking the wheels: Braking hard without locking the wheels is controlled braking.
• Pull the parking brake knob and steer straight: The parking brakes are not used to stop a moving vehicle in an emergency.

Hydraulic brakes (like on a car) work almost at once. Air brakes need a little time, one-half second or more, for the air to flow through the lines to the brakes. This is called brake lag.

Wrong choices

• About 5 seconds: 5 seconds would be dangerous. Normal brake lag is about half a second.
• No time; they work instantly: Air brakes always have some delay because the air must travel through the lines.

At 55 mph on dry pavement, air brake lag adds about 32 feet to your stopping distance. The vehicle keeps moving during the half second or so it takes the air to reach the brakes.

Wrong choices

• About 3 feet: 3 feet is far too small. At 55 mph the vehicle travels much farther than that in half a second.
• About 300 feet: 300 feet is closer to the whole stopping distance at 55 mph, not just the lag part.

If the low air warning comes on, stop and safely park your vehicle as soon as possible. There might be an air leak. You can make a controlled stop only while enough air remains in the tanks.

Wrong choices

• Keep driving to your delivery, then report it: If you keep driving, the spring brakes may come on suddenly, possibly in a dangerous place.
• Pump the brake pedal to build air back up: Pumping the pedal uses air. It does not build air back up.

Using the service brakes too much makes them overheat, which leads to brake fade (the brakes get weaker and the vehicle is harder to stop). Brakes out of adjustment can also cause fade.

Wrong choices

• Using the engine's braking effect on a downgrade: Engine braking takes the load off the service brakes. It helps prevent fade.
• Driving in a low gear on a downgrade: A low gear lets the engine help hold your speed, so you need the service brakes less.

For a vehicle with air brakes, total stopping distance = perception distance + reaction distance + brake lag distance + effective braking distance. Brake lag is the extra part that comes from the time the air takes to reach the brakes.

Wrong choices

• Perception + reaction + effective braking distance only: This leaves out brake lag distance, which air brakes add.
• Reaction + brake lag distance only: This leaves out perception distance and the actual braking distance.

Once you are in the right low gear:

1. Apply the brakes just hard enough to feel a definite slowdown.
2. When your speed is about 5 mph below your safe speed, release the brakes.
3. When your speed rises back to your safe speed, repeat steps 1 and 2.

Wrong choices

• Hold light, steady pressure on the brakes all the way down: Light, steady braking all the way down heats the brakes constantly and can lead to brake fade.
• Do not touch the service brakes at all on a downgrade: The engine does most of the work, but you still use the service brakes to keep your speed safe.

For a normal stop, push the brake pedal down and control the pressure so the vehicle stops smoothly. With a manual transmission, do not push in the clutch until the engine rpm is down close to idle. Then select a starting gear.

Wrong choices

• As soon as you start braking: Pushing in the clutch right away lets the vehicle coast and you lose the engine's help in slowing down.
• Before you take your foot off the gas: Pushing in the clutch even earlier makes you coast before braking at all.

In newer air brake vehicles, you set the parking brakes with a yellow, diamond-shaped push-pull knob. Pull it out to set the parking brakes and push it in to release them.

Wrong choices

• A red, eight-sided push-pull knob: A red, eight-sided knob is the trailer air supply control on combination vehicles, not the parking brake.
• A blue, round push-pull knob: The parking brake control is not a blue round knob.