Chevrolet 2011 Volt Owner's Manual page 392

The brake master cylinder reservoir
is filled with DOT 3 brake fluid as
indicated on the reservoir cap.
See
for the location of the
reservoir.
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.
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.
When the brake fluid falls to a low
level, the brake warning light comes
on. See
.