Why is this in "Engine Performance"?
Three things cause the light to come on.
1. Park brake cable gets stiff/high friction, so the park brake pedal does not FULLY return. Lift up on the park brake pedal, see if the light goes out. This is probably the most-common; at least both of my GMT400s suffer from this. As a result, I don't use the park brake like I should, which then meant that the rear brakes of my K1500 wouldn't properly adjust. A cascade of failures stemming from two root causes: Crappy park brake cables, and crappy rear brake design. I still have the crappy cables, but I scrapped the light-duty rear axle with the light-duty brakes.
2. "Safety switch" in the combination valve is off-center, indicating a failure in either front or rear hydraulic system. GENERALLY, this results in a brake warning light that's "on" when you step on the brakes, but goes off when your foot comes off the brake pedal. Verification of proper braking is essential. You could remove the wire from the safety switch, and see if the light goes out.
3. Wiring fault in the two systems above; some of the wire harness has grounded on a stray bit of metal structure. You'd expect the light to blink as the harness jiggled over bumps.