There are two things that will turn the dash brake warning light on.
First is low fluid in the master cylinder resivoir. There is a sensor that reads the level. Even if the fluid level is full, a bad sensor can make the light come on (although this is rare). The fluid can be low enough to set the light without any leaks. As the brakes wear, fluid migrates to the larger volume of the calipers and wheel cylinders from the caliper pistons or wheel cylinder cups moving out with the brake lining wear. This can be an indication that you should inspect your brakes to see how they are wearing.
Second is the park brake release. If the park brake is on, the same light on the dash comes on. If the park brake is not fully released, the light can stay on. If the park brake light switch is defective, the light can stay on. It is rare for the switch to fail, but is is common to find that you have not fully released the park brake.
