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5.4 – Using Air Brakes

5.4.1 – Normal Stops

Push the brake pedal down. Control the pressure so the vehicle comes to a smooth, safe stop. If you have a manual transmission, do not push the clutch in until the engine rpm is down close to idle. When stopped, select a starting gear.

5.4.2 – Braking with Antilock Brakes

When you brake hard on slippery surfaces in a vehicle without ABS, your wheels may lock up. When your steering wheels lock up, you lose steering control. When your other wheels lock up, you may skid, jackknife, or even spin the vehicle.

ABS helps you avoid wheel lockup. The computer senses impending lockup and reduces the braking pressure to a safe level, and you maintain control. You may or may not be able to stop faster with ABS, but you should be able to steer around an obstacle while braking and avoid skids caused by over-braking.

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

When only the tractor has ABS, you should be able to maintain steering control and there is less chance of jackknifing. But, keep your eye on the trailer and let up on the brakes (if you can safely do so) if it begins to swing out.

When only the trailer has ABS, the trailer is less likely to swing out, but if you lose steering control or start a tractor jackknife, let up on the brakes (if you can safely do so) until you gain control.

When you drive a tractor-trailer combination with ABS, you should brake as you always have. In other words:

  • Use only the braking force necessary to stop safely and stay in control.
  • Brake the same way, regardless of whether you have ABS on the tractor, the trailer or both.
  • As you slow down, monitor your tractor and trailer and back off the brakes (if it is safe to do so) to stay in control.

There is only one exception to this procedure: If you always drive a straight truck or combination with working ABS on all axles, in an emergency stop, you can fully apply the brakes.

Without ABS, you still have normal brake functions. Drive and brake as you always have.

Remember: If your ABS malfunctions, you still have regular brakes. Drive normally, but get the system serviced soon.

Multiple-Choice Questions:

Question #282 (1 of 5)

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ABS helps you avoid wheel lockup. How does it work?

  • If your wheels begin to lock up, the computer will reduce the ratio of air pressure to volume, reducing brake pressure to a safe level, and you maintain control.
  • If the hydraulic pressure gets too high, the ABS system will back off of the governor, reducing brake pressure to a safe level, and you maintain control.
  • None of these are correct.
  • The computer senses impending lockup and reduces the braking pressure to a safe level, and you maintain control.
ABS helps you avoid wheel lockup. The computer senses impending lockup and reduces the braking pressure to a safe level, and you maintain control.
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Question #284 (2 of 5)

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Having ABS on only the tractor, only the trailer, or even on only one axle:

  • Still gives you more control over the vehicle, but only if you have bias-ply tires on the drive axles
  • Gives you less braking force than if you had no ABS at all. You must brake more cautiously with partial ABS than you would without ABS at all
  • None of these are correct
  • Still gives you more control over the vehicle during braking. Brake normally.
Having ABS on only the tractor, only the trailer, or even on only one axle still gives you more control over the vehicle during braking. Brake normally.
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Question #283 (3 of 5)

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You may or may not be able to stop faster with ABS, but you should be able to:

  • Steer around an obstacle while braking and avoid skids caused by over-braking.
  • Lock up the wheels, which creates more traction on the roadway
  • Keep the air brake fluid primed for maximum braking force
  • Keep the steer tires from veering too hard and thus avoiding a rollover
You may or may not be able to stop faster with ABS, but you should be able to steer around an obstacle while braking and avoid skids caused by over-braking.
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Question #286 (4 of 5)

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Remember, if your ABS malfunctions:

  • You still have regular brakes. Drive normally, but get the system serviced soon.
  • You still have some braking force, but not enough to stop in an emergency. Get the system serviced soon
  • You still have limited brakes. Drive more slowly, but get the system serviced soon
  • Your braking force has been limited considerably. You must not drive the truck until it is fixed
Remember: If your ABS malfunctions, you still have regular brakes. Drive normally, but get the system serviced soon.
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Question #285 (5 of 5)

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When you drive a tractor-trailer combination with ABS, you should brake as you always have. In other words:

  • All these are correct
  • Brake the same way, regardless of whether you have ABS on the tractor, the trailer or both.
  • As you slow down, monitor your tractor and trailer and back off the brakes (if it is safe to do so) to stay in control.
  • Use only the braking force necessary to stop safely and stay in control.

When you drive a tractor-trailer combination with ABS, you should brake as you always have. In other words:

  • Use only the braking force necessary to stop safely and stay in control.
  • Brake the same way, regardless of whether you have ABS on the tractor, the trailer or both.
  • As you slow down, monitor your tractor and trailer and back off the brakes (if it is safe to do so) to stay in control.
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