Brake Fluid Leaking! What to do?!!?

So yesterday I'm driving and I'm at a stop light, I push the brakes and it goes all the way to the floor. Only the back brakes are stopping the car now. So today I buy some brake fluid, fill it up and it starts dripping on the passenger side about even with the middle of the back door. All the fluid pretty much leaked out. Any idea what this could be?? Or how much this could cost??

Year, make and model??????

If your car is really old, it could be that a brake line rusted through. If it's not that old, maybe a bad fitting or connection. Unless you know how to work on brakes, take it to a qualified shop. You don't want to take chances with something so critical. . .

It's a 1991 Oldsmobile 98
The bottom is somewhat rusty... the exterior is in very good shape though

you probably need to replace the line itself. Each wheel has it's own line from the master cylinder on that year. The things to consider is wether or not the system was drianed when you drove it. If the master cylinder ran dry, you'll need to rebleed all of the lines for safe measure instead of just the left rear. Also, if that car has ABS ( antilock brakes) you'll also have to purge the air out of that as well.