Possible Causes

WARNING: This page is about a different car, the 1999 Buick Regal and 1999 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.

Any leaks in hydraulic brake system (such as brake pipes, wheel cylinders, calipers, or master cylinder).
Brake fluid level indicator switch malfunction.
Brake fluid level indicator switch line (CKT209) short to ground.