What type of vehicle are we dealing with? You could have one tail light out and a stuck brake light switch... if you have to refill the fluid constantly, you have a leak somewhere. The level will go down over time with brake pad wear but should never become so low that it would turn the inside warning light on, unless there was exreme wear on the pads. How often and how much are you adding brake fluid?
You definately have a leak somewhere. It could be anywhere in the system but some of the most likely areas are rear wheel cylinders, brake hoses and the master cylinder. If repairing the brake system is beyond your capabiities or if you'd like to find out exactly what's wrong, we would be happy to look at if for you at no charge. We can even give you a free estimate of exactly what it would take to fix it correctly. You can call me at the shop if you want 770-945-4556 ask for Matt.