1994 Chevrolet Astro Owner's Manual - Page 250

Brakes Brake Master Cylinder Your brake master cylinder is here. It is filled with DOT-3 brake fluid. There are only two reasons why the brake fluid level in your master cylinder might go down. The first is that the brake fluid goes downto 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, won't work at or all. So, it isn't a good idea to "top off' your brake fluid. Adding brakefluid 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 done on the brake is hydraulic system. Refer to the Maintenance Scheduleto determine whento check your brake fluid. See "Periodic Maintenance Inspections" in the Index. 6-32