I part out cars and sell the parts online. I look for cars that have an issue and can be bought cheap, but have parts that sell well on Ebay. I came across a running 2006 Cadillac STS with a Northstar V8 w/112k miles. Was a northern car with some underside rust, and was being sold cheap because of having no brakes due to supposed rust. The brake lines are fairly rusted, but I can't find the spot of the leak, but believe it's gonna be a good idea to replace them all anyways rather than just trying to fix one that may be leaking. 

So I was initially going to harvest this car for parts, but kind of like it and am now leaning more towards fixing and keeping it as it has good tires with alot of tread, a new battery, looks like it's in pretty good condition and actually looks quite nice. There is some underside rust, but it appears to be less than or equal to what you'd see on just about any other northern car. This would be my first brake job. Anyone have advice on making sure I diagnose this right? The pedal goes to the floor and the only brakes it has are the e brake and when it's in park. 

The guy I bought it from never had a mechanic really look at it and just had someone tell him it's not really worth fixing. He suspected the leak was in the back passenger area as that is where most of the rust is. I added what I thought was about 8 ounces of brake fluid to the master cylinder and it's topped off. When I got in the car, the brake pedal had a little bit of pressure at first, but that went away and it sinks again. I was expecting the fluid to leak out somewhere, but didn't see any leaks and saw the cylinder still has fluid in it. Is this because there is a leak in one of the lines and the lack of pressure in the whole system is what is keeping the fluid in the master cylinder? 

Just want to make sure I propely diagnose all the faults and am not still having an issue if I replace the brake lines all throughout. I know alot of people shape the new lines to match the old, but down in Florida, there are some salvage yards with brake lines I could pull that don't have rust and from initial looking at the flow of the lines, it's possible I could replace some without much cutting and joining as I think with moving a few things out of the way like the fuel filter, it may be possible to fit the lines where they should go. 

Mainly looking for advice on how you'd go about fixing this or confirming where the exact leak is at. 

Thank you.