Buick 2003 Century Owner's Manual page 255

Brakes
Brake Fluid
A
Your brake master cylinder reservoir is on the driver's
side of the engine compartment. It is filled with
DOT-3 brake fluid. See Engine Compartment Overview
on page 5- 12.
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.
So, it isn't a good idea to "top o f f 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.
5-38