1996 Mazda b3000 No Breaks, started soft pedal to no pedal

I have a Mazda b3000 1996 78,700 miles. Yesterday I noticed after driving that the pedal was very soft but I still had breaks.Today I went to leave and the pedal was soft down to the floor, little break, and the "brake" liht came on. I checked the break fluid oil, but there are two reservoirs that have caps stating DOT3 fluid. One is up to what looks to be the strainer tip. The other reservoir is oblong and is low but not out...well below the fill. But there is fluid in both yet no breaks, though the fluid levels appear to be low. Is this a break line problem? What would make the break light come on "low fluid" situation or could a bad break line cause this?
Many thanks!