GMC SIERRA 1994 Manual page 171

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As you brake, your computer keeps receiving updates on rear wheel speed
and controls braking pressure accordingly.
Remember: Anti-lock doesn't change the time you need to get your foot up
to 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.
Braking in Emergencies
At some time, nearly every driver gets into a situation that requires hard
braking.
You have the rear-wheel anti-lock braking system. Your front wheels can
stop rolling when you brake very hard. Once they do, the vehicle can't
respond to your steering. Momentum will carry it in whatever direction it
was headed when the front wheels stopped rolling. That could be off the
road, into the very thing you were trying to avoid, or into traffic.
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