GMC 1999 Sierra 1500 Pickup Owner's Manual page 195

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As you brake, your computer keeps receiving updates on rear wheel speed
and controls braking pressure accordingly.
CAUTION
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A
Anti-lock doesn't change the time you need to get your foot up to
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the brake pedal. If you get too close to the vehicle in front of you,
you won't have time to apply your brakes if that vehicle suddenly
slows or stops. Always leave enough room up ahead to stop, even
though you have anti-lock brakes.
To Use Anti-Lock:
Use rear wheel anti-lock like regular brakes. You may feel the brakes vibrate,
or you may notice some noise outside your vehicle, but this is normal. Let
anti-lock work for you, but remember: Your front wheels can still stop rolling.
If that happens, release enough pressure
on
the brakes to get the wheels
rolling again so that you can steer.
With the four-wheel drive option you won't have anti-lock braking when you
shift into four-wheel drive. But you will have regular braking. When you shift
back into two-wheel drive, you will have anti-lock again.
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