The light is activated by one of two switches: the parking-brake pedal switch and the combination valve switch. Both are grounding switches. The pedal switch is obvious; the valve switch will only activate if there is a large difference in system pressure between the front & rear hydraulic circuits due to a leak. It does not indicate fluid level itself.
First thing to check is the pedal switch to see if the pedal is fully depressing it. The pedal being fully disengaged and locked in the up position is no guarantee as the switch can dislodge or wear out. Remove the wiring from the switch and don't let it contact anything. If the light goes out, replace the switch.
If it's not the pedal switch, then check the valve switch after you verify that the brakes are functioning correctly at all four corners. Don't just push the pedal and/or look for leaks. Is the system properly bled with all four brake cylinders engaging? If not, the lamp did its job. If so, then check the valve itself. First, check for continuity between the switch terminal and the combination valve's body. No? You're done here and can proceed with wire tracing. If you do have continuity, reconnect the switch. After verifying a good connection, see if there's a manual "reset button" on the valve (it's actually a protruding stem on the metering valve itself, usually with a rubber boot over it). It will be on the end and may not be easy to depress, plus it doesn't move very far. If that does not turn the light off, have someone put light pressure on the pedal and push it again (it's going to be harder this time, you're fighting brake pressure). If that doesn't work or if you don't have the "hold off" button, try a few light taps on each end with a hammer--don't smack the hell out of it. The grooved piston that actuates the switch is spring-loaded on both ends; if it's stuck you may dislodge it back to the "off" position.
If neither of the above solves the problem, it's time to break out the multimeter and start chasing down a short circuit. Get a service manual for your car or see if you can find the wiring diagrams online. They're a great help at this point.
