brake lines? master cyl? or both?

Look up in the dash where the master cylinder comes through the firewall and you will probably see the rubber boot wet with brake fluid. Then pull up the carpet under the driver's floor and you may find it soaked with all of that brake fluid you have been adding. (If so, you need to remove the carpet and backing and get the fluid out; it will absorb water and rust out your floorboards.)

Also, look at the firewall on the outside and see if there is brake fluid running down it from the MC.

If all is dry, then look under the car to find where a line is open.

But I bet your MC is leaking. They don't cost all that much, and if old at all, then I'd replace just on principle. Get a new one; sounds like today's rebuilds are hit and miss.

You won't go wrong with Inline Tube products, IMO.