GMC 2006 Sierra - Manual (Page 460)

So, it is not a good idea to top off your brake fluid. Adding
brake fluid will not correct a leak. If you add fluid when
your linings are worn, then you will 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 Scheduled Maintenance
(Gasoline Engine) on page 6-4.
Checking Brake Fluid
You can check the brake fluid without taking off the cap.
Just look at the brake fluid
reservoir. The fluid level
should be above MIN.
If it is not, have your brake
system checked to see
if there is a leak.
After work is done on the brake hydraulic system,
make sure the level is above the MIN but not over
the MAX mark.
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