There is one other sensor on the brake warning light...
not only could it mean you are low on brake or hydralic fluid (but your brake would likely not feel any different unless you are really low on brake fluid), but it also has a pressure sensor for the hydralic brake boost. In order to check if you have a booster problem first check the fluid levels and top them up (make sure you use the right hydralic fluid, the wrong stuff will kill your seals, it is expensive BTW), then start the car and let it idle for a few minutes, this will let the pressure build. Then start pumping the brake pedal, you should get at least a good 10 pumps before the brake warning turns on. If you don't it could be a number of things, first would be the "bomb" (hydralic pressure accumulator), then the brake booster (mounted to the base of the master cylinder), and then the power steering pump.