GMC 2010 T-Series Owner's Manual page 182

Checking Brake Fluid
Apply the brake pedal several times with the ignition off.
Clean one of the reservoir caps and the area around
the cap, and remove it.
What to Add
Use the proper fluid listed in the Maintenance Schedule.
Use new brake fluid from a sealed container only.
Always clean the brake fluid reservoir cap and the area
around the cap before removing it. This helps keep dirt
from entering the reservoir.
5-34
The fluid level should be
even with the bottom ring
of the filler opening. If it is
low, add enough fluid to
fill the reservoir to the
proper level.
WARNING:
With the wrong kind of fluid in the brake hydraulic
system, the brakes might not work well. This could
cause a crash. Always use the proper brake fluid.
Notice:
Using the wrong fluid can badly damage
brake hydraulic system parts. For example,
just a few drops of mineral-based oil, such
as engine oil, in the brake hydraulic system
can damage brake hydraulic system parts so
badly that they will have to be replaced. Do
not let someone put in the wrong kind of
fluid, and do not use DOT-5 silicone brake
fluid.
If brake fluid is spilled on the vehicle's
painted surfaces, the paint finish can be
damaged. Be careful not to spill brake fluid
on the vehicle. If you do, wash it off
immediately. See Washing Your Vehicle on
page 5 72.