Hummer 2003 H2 Owner's Manual page 364

Brakes
Brake Fluid
There are only two reasons why the brake fluid level in
the reservoir might go down. The first is that the brake
fluid goes down to an acceptable level during normal
brake lining wear. When new linings are put in, the fluid
level goes back up. The other reason is that fluid is
leaking out of the brake system. If it is, you should
have your brake system fixed, since a leak means that
sooner or later your brakes won't work well, or won't
work at all.
6-30
Your brake master cylinder
reservoir is filled with
-
DOT 3 brake fluid. See
"Engine Compartment
Overview" in the Index for
the location of the reservoir.
So, it isn't a good idea to "top off" your brake fluid.
Adding brake fluid won't correct a leak. If you add fluid
when your linings are worn, then you'll have too much
fluid when you get new brake linings. You should add
(or remove) brake fluid, as necessary, only when work is
done on the brake hydraulic system.
CAUTION:
If you have too much brake fluid, it can spill on
the engine. The fluid will burn if the engine is hot
enough. You or others could be burned, and your
vehicle could be damaged. Add brake fluid only
when work is done on the brake hydraulic system.
See "Checking Brake Fluid" in this section.
Refer to the Maintenance Schedule to determine when to
check your brake fluid. See "Periodic Maintenance
Inspections" in the Index.