1996 Chevy Corsica Dot-3 Fluid Needs Refilling bi-weekly

My girlfriend's '96 corsica has a leak somewhere with the braking system. We have to refill the dot-3 brake fluid every 2-3 weeks. It never completely leaks (or she would have no braking ability, right?). However, today I had to pour just about half a bottle of brake fluid in to top it off to normal! Last time I filled up was only a month before. I've never driven her car for a certain reason, but I ask her questions like "do the brakes feel loose and unresponsive?" "Are they plushy and really soft and go further to the floor than normal", etc. And she says she's never noticed anything different, except for one day she had a fully loaded car+trunk drive through the mountains and her brakes seriously weren't working at all (not sure if she wasn't putting it in a lower gear and brakes over-heating in addition to the leak). Sometimes the red "brake fluid" light comes on when she is on the freeway curving left or right on an over-pass. So what part(s) need replacement and where does the problem lie? Is it the whole master brake cylinder? Cap? Lines? Brake pads or calipers or whatever the heck? Is this something I can buy the part(s) online cheap for and bring to a mechanic to service?