Chevrolet 1998 Prizm Owner's Manual page 255

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 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.
A
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.
When your brake fluid falls to a low level, your brake
warning light will come on. See "Brake System Warning
Light" in the Index.
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