The brake fluid reservoir uses the same warning lamp as the handbrake. It sound like bluesol is talking about the warning lamp for burned-out brakelight bulbs.

That one works from difference in wattage from side-to-side. If one bulb is getting ready to burn out, it might set this off. Or maybe corroded or loose connections one side or the other. Sometimes just replacing ALL bulbs ensures that they all match each other.

Then there's the sensor... It's #24 in the picture, but it's not clear to me exactly where it's located.