GMC T-Series 2007 Owner's Manual page 230

Four-Wheel Disc Brakes
(Hydraulic Only)
Your vehicle has four-wheel disc brakes.
Some driving conditions or climates can cause a
brake squeal when the brakes are first applied
or lightly applied. This does not mean something
is wrong with the brakes.
Brake linings should always be replaced as
complete axle sets.
Replacing Brake System Parts
The braking system on a vehicle is complex.
Its many parts have to be of top quality and work
well together if the vehicle is to have really good
braking. Your vehicle was designed and tested with
top-quality brake parts. When you replace parts of
the braking system — for example, when the brake
linings wear down and you need new ones put
in — be sure you get new approved replacement
parts. If you do not, the brakes might not work
properly. For example, if someone puts in brake
linings that are wrong for your vehicle, the
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balance between the front and rear brakes can
change — for the worse. The braking performance
you have come to expect can change in many other
ways if someone puts in the wrong replacement
brake parts.
Air Brake Systems
If your vehicle has air brakes, it is important to
get rid of moisture in the system. Moisture
will damage the system if it is not removed daily.
There are two ways to do this. One is automatic
through the air brake vehicle's air dryer with
intregal automatic moisture ejector that purges
air from the system through a self-contained
reservoir. The other way to drain moisture from
the air brake system is to manually activate drain
valves at each reservoir. Drain the air reservoirs
occasionally to be sure the air dryer is working
properly.
Drain them at full system pressure. To be sure
you are at full pressure, check the air pressure
gage. It should read at least 100 psi (692 kPa).