Need Help with Brakes!

About 3 weeks ago I noticed a change in braking with my 1994 Camry. The brakes seemed to be getting weaker. Was becoming spongy to the feel and actual braking was affected. I checked the Brake Fluid, and sure enough it was almost bone dry. Filled the tank up and was good to go. Two days later the Brake Light come on and the Brake Fluid was low again.

So I ask, Is the "spongy" brake problem causing the low Brake Fluid? Or the low Brake Fluid causing the "spongy" brakes?

If I'm losing Brake Fluid, where should I begin looking?