GMC TopKick 2007 Owner's Manual page 221

The heavier the rain, the harder it is to see.
Even if your windshield wiper blades are in good
shape, a heavy rain can make it harder to see
road signs and traffic signals, pavement markings,
the edge of the road, and even people walking.
It is wise to keep your wiping equipment in good
shape and keep your windshield washer fluid
reservoir filled with washer fluid. Replace
your windshield wiper inserts when they show
signs of streaking or missing areas on the
windshield, or when strips of rubber start to
separate from the inserts.
CAUTION:
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Wet brakes can cause accidents. They
may not work as well in a quick stop and
may cause pulling to one side. You could
lose control of the vehicle.
After driving through a large puddle of
water or a vehicle wash, apply your brake
pedal lightly until your brakes work
normally.
Driving too fast through large water puddles or
even going through some vehicle washes
can cause problems, too. The water may affect
your brakes. Try to avoid puddles. But if you
cannot, try to slow down before you hit them.
Hydroplaning
Hydroplaning is dangerous. So much water can
build up under your tires that they can actually ride
on the water. This can happen if the road is wet
enough and you are going fast enough. When your
vehicle is hydroplaning, it has little or no contact
with the road.
Hydroplaning does not happen often. But it can
if your tires do not have much tread or if the
pressure in one or more is low. It can happen if a
lot of water is standing on the road. If you can
see reflections from trees, telephone poles,
or other vehicles, and raindrops dimple the water's
surface, there could be hydroplaning.
Hydroplaning usually happens at higher speeds.
There just is not a hard and fast rule about
hydroplaning. The best advice is to slow down
when it is raining.
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