2003 Chevrolet Astro Owner's Manual - Page 259

Brakes Brake Fluid DOT3 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. Your brake master cylinder reservoir is filled with DOT-3 brake fluid. See Engine Compadment Overview on page 5-14 for the location of the reservoir. 5-42