1998 Chevrolet Astro Owner's Manual - Page 294

There are only two reasons why the brake fluid levelin 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 liningsare put in, the fluid level goes back up.The other reason is that fluid is leaking out of the brake system. Ifit 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'tcorrect a leak. If you add fluid when your liningsare worn, then you'll havetoo 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. If you we too much brake fluid, it can spill on the engine. The fluid burn if the engine hot will is 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. Refer to the MaintenanceSchedule to determine whento check your brake fluid.See "Periodic Maintenance Inspections'' in the Index. 6-32