Cadillac 2011 STS Owner's Manual page 385

There are only two reasons why the brake fluid level in
the reservoir might go down:
The brake fluid level goes down because of
normal brake lining wear. When new linings are
installed, the fluid level goes back up.
A fluid leak in the brake hydraulic system can also
cause a low fluid level. Have the brake hydraulic
system fixed, since a leak means that sooner or
later the brakes will not work well.
The brake master cylinder
reservoir is filled with DOT
3 brake fluid. See
for
reservoir location and
access.
Do not top off the brake fluid. Adding fluid does not
correct a leak. If fluid is added when the linings are
worn, there will be too much fluid when new brake
linings are installed. Add or remove brake fluid, as
necessary, only when work is done on the brake
hydraulic system.
If too much brake fluid is added, it can spill on the
engine and burn, if the engine is hot enough. You
or others could be burned, and the vehicle could
be damaged. Add brake fluid only when work is
done on the brake hydraulic system.
If the ignition is on and the brake fluid is low, the
CHECK BRAKE FLUID message displays in the Driver
Information Center (DIC). See
.
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