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Using air brakes: normal stops, ABS, emergency stops and brake lag

Air Brakes4 minManual section 5.4.1–5.4.4

How you press the pedal matters. This lesson covers a normal stop, how to brake with ABS on the tractor, the trailer or both, the two emergency braking methods (controlled and stab braking), and why air brakes add "brake lag" to your stopping distance.

Normal stops

Push the brake pedal down and adjust the pressure so you come to a smooth, safe stop.

With a manual transmission, keep the clutch out until engine rpm has dropped close to idle. Once stopped, pick a starting gear.

Braking with antilock brakes

Without ABS, hard braking on a slippery surface can lock the wheels. If the steering wheels lock, you lose steering control. If other wheels lock, you may skid, jackknife, or even spin the vehicle.

ABS helps you avoid lockup. The computer senses lockup coming, reduces braking pressure to a safe level, and you keep control. You may or may not stop faster, but you should still be able to steer around something in your path while braking, and you avoid skids from braking too hard.

ABS on only the tractor, only the trailer, or even only one axle still gives you more control during braking. Brake normally.

  • Only the tractor has ABS: you should keep steering control, and there is less chance of jackknifing. But watch the trailer, and if it starts to swing out, ease off the brakes (when it is safe to).
  • ABS on the trailer only: the trailer is less likely to swing out. If you lose steering control or the tractor starts to jackknife, ease off the brakes (when safe) until you are back in control.

With an ABS-equipped tractor-trailer, brake as you always have:

  • Use only the braking force needed to stop safely and stay in control.
  • Brake the same way whether ABS is on the tractor, the trailer, or both.
  • As you slow down, watch your tractor and trailer and back off the brakes (if safe) to stay in control.

The one exception: if you always drive a straight truck or combination that has working ABS on every axle, you may brake fully in an emergency stop.

If ABS is missing or not working, you still have normal brakes. Drive and brake as you always have, and get the ABS serviced soon.

Emergency stops: controlled braking and stab braking

If someone suddenly pulls out in front of you, your natural reaction is to hit the brakes. That is fine if there is enough distance and you use the brakes correctly. Brake so the vehicle stays in a straight line and you can still turn if you need to.

Controlled braking: brake as hard as you can while keeping the wheels from locking. Keep steering movements very small. If you need a larger steering change, or if the wheels lock, release the brakes. Re-apply them as soon as you can.

Stab braking: press the brakes all the way. Let off when the wheels lock. Once the wheels are rolling again, brake fully again. After you release, the wheels may need up to 1 second to start rolling. If you brake again before they roll, the vehicle will not straighten out.

Stopping distance and brake lag

Air brakes add one more delay: Brake lag, the time it takes the brakes to work after you push the pedal. Hydraulic brakes (cars and light/medium trucks) work instantly. With air brakes, air needs one half second or more to flow through the lines to the brakes.

So an air brake vehicle's total stopping distance has four parts: Perception distance + Reaction distance + Brake lag distance + Braking distance, added together.

At 55 mph on dry pavement, air brake lag adds about 32 feet. For an average driver with good traction and brakes, total stopping distance at 55 mph is over 450 feet — longer than a football field and a half. Leave plenty of space ahead.

References and scope

The topic reference is listed below. Sources support verification; the lesson itself is here. The current content needs technical and language review before public launch.

Manual reference: 5.4.1–5.4.4

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

Educational questions with explanations. The answer review also works without the interactive quiz.

Preparing your questions…

Review questions with answers

  1. When making a normal stop in a vehicle with a manual transmission, when should you push in the clutch?
    1. As soon as you start braking
    2. When the engine rpm is down close to idle
    3. Before you touch the brake
    4. Never during a stop
    Show answer
    When the engine rpm is down close to idle

    Do not push in the clutch until engine rpm is close to idle. When stopped, select a starting gear.

  2. Without ABS, if your steering wheels lock up during hard braking:
    1. The vehicle stops faster
    2. The trailer swings out
    3. You lose steering control
    4. The spring brakes come on
    Show answer
    You lose steering control

    When steering wheels lock, you lose steering control. Other wheels locking can cause skids, jackknifes or spins.

  3. How does ABS help you during hard braking?
    1. It senses coming lockup and reduces brake pressure to a safe level so you keep control
    2. It adds extra air pressure to all brakes
    3. It applies the spring brakes
    4. It shuts off the engine
    Show answer
    It senses coming lockup and reduces brake pressure to a safe level so you keep control

    The ABS computer senses impending lockup, lowers pressure to a safe level, and you keep control and can steer.

  4. Only the tractor has ABS. While braking, you see the trailer begin to swing out. You should:
    1. Brake harder
    2. Pull the trailer hand valve
    3. Turn sharply toward the trailer
    4. Let up on the brakes, if you can do so safely
    Show answer
    Let up on the brakes, if you can do so safely

    With tractor-only ABS you keep steering, but watch the trailer and ease off the brakes (if safe) if it swings out.

  5. Only the trailer has ABS. What is true?
    1. The tractor cannot jackknife
    2. The trailer is less likely to swing out, but if steering control is lost or the tractor starts to jackknife, ease off the brakes (if safe)
    3. You should always brake as hard as possible
    4. ABS on the trailer gives no benefit
    Show answer
    The trailer is less likely to swing out, but if steering control is lost or the tractor starts to jackknife, ease off the brakes (if safe)

    Trailer-only ABS keeps the trailer from swinging out, but ease off the brakes if the tractor loses steering or starts to jackknife.

  6. You drive a tractor-trailer that has ABS. How should you brake?
    1. Pump the brakes rapidly
    2. Always slam the brakes fully
    3. Brake as you always have, using only the force needed to stop safely and stay in control
    4. Use only the trailer hand valve
    Show answer
    Brake as you always have, using only the force needed to stop safely and stay in control

    With ABS, brake as you always have: only the force needed, the same way whatever unit has ABS, and back off if needed.

  7. When is it acceptable to fully apply the brakes in an emergency stop?
    1. When you always drive a straight truck or combination that has working ABS on every axle
    2. When only the tractor has ABS
    3. When the road is icy and there is no ABS
    4. Whenever the ABS lamp is on
    Show answer
    When you always drive a straight truck or combination that has working ABS on every axle

    The one exception: with working ABS on all axles, you can fully apply the brakes in an emergency stop.

  8. True or False: If your ABS is not working, you still have normal brake functions.
    1. True
    2. False
    Show answer
    True

    True. Without ABS you still have regular brakes. Drive normally and get the system serviced soon.

  9. Controlled braking means:
    1. Applying the brakes fully, releasing when wheels lock, and re-applying
    2. Using only the parking brake
    3. Pumping the brakes quickly
    4. Applying the brakes as hard as you can without locking the wheels, with very small steering movements
    Show answer
    Applying the brakes as hard as you can without locking the wheels, with very small steering movements

    Controlled braking: as hard as possible without lockup, small steering moves; release if wheels lock or you must steer more.

  10. With controlled braking, what should you do if the wheels lock or you need to make a larger steering adjustment?
    1. Brake harder
    2. Release the brakes, then re-apply as soon as you can
    3. Pull the parking brake
    4. Shift to neutral
    Show answer
    Release the brakes, then re-apply as soon as you can

    Release the brakes when the wheels lock or you need more steering, then re-apply as soon as possible.

  11. In stab braking, after you release the brakes, it can take up to how long for the wheels to start rolling?
    1. 5 seconds
    2. 3 seconds
    3. 1 second
    4. 1/10 second
    Show answer
    1 second

    It can take up to one second. If you re-apply before the wheels roll, the vehicle will not straighten out.

  12. Stab braking means:
    1. Apply the brakes all the way, release when wheels lock, and apply fully again as soon as the wheels roll
    2. Apply light pressure for a long time
    3. Use only the trailer brakes
    4. Brake hard without ever releasing
    Show answer
    Apply the brakes all the way, release when wheels lock, and apply fully again as soon as the wheels roll

    Stab braking: full application, release at lockup, re-apply fully once the wheels are rolling.

  13. What is "brake lag" in an air brake system?
    1. The time it takes the compressor to refill the tanks
    2. Wear on the brake linings
    3. The distance you travel while seeing a hazard
    4. The time it takes the brakes to work after the pedal is pushed
    Show answer
    The time it takes the brakes to work after the pedal is pushed

    Brake lag is the delay while air flows through the lines after you push the pedal: one half second or more.

  14. About how long does it take air to flow through the lines to the brakes after you push the pedal?
    1. Instantly
    2. One half second or more
    3. 3 seconds
    4. 5 seconds
    Show answer
    One half second or more

    Hydraulic brakes work instantly; air brakes need one half second or more.

  15. Total stopping distance for a vehicle with air brakes is made up of:
    1. Perception distance + braking distance
    2. Reaction distance + braking distance + brake fade
    3. Perception distance + reaction distance + brake lag distance + braking distance
    4. Following distance + braking distance
    Show answer
    Perception distance + reaction distance + brake lag distance + braking distance

    Air brakes add brake lag distance, so the total has four parts.

  16. At 55 mph on dry pavement, about how much distance does air brake lag add?
    1. 32 feet
    2. 6 feet
    3. 100 feet
    4. 450 feet
    Show answer
    32 feet

    Brake lag adds about 32 feet at 55 mph; total stopping distance is over 450 feet.

  17. At 55 mph, with good traction and brakes, an average driver's total stopping distance in an air brake vehicle is:
    1. About 100 feet
    2. About 200 feet
    3. About 300 feet
    4. Over 450 feet
    Show answer
    Over 450 feet

    At 55 mph the total stopping distance is over 450 feet.

  18. Your truck has ABS on only one axle. Compared with no ABS at all, what is true?
    1. It gives no benefit unless every axle has ABS
    2. It lets you brake as hard as you want in any emergency
    3. It still gives you more control while braking
    4. It makes the regular brakes stop working
    Show answer
    It still gives you more control while braking

    Even partial ABS (tractor only, trailer only, or a single axle) still gives you more control during braking. Keep braking normally.