do you have ABS?
if you have no ABS, dont worry. its not a huge deal. There are only three things to look for in that case:
#1: the cross-over brake line from the driver's rear to the passenger's rear (goes up over the diffy.)
#2: the junction-box and rubber line to the caliper.
#3: the bleed-valve.
If you do have ABS, the lines may be run different and I'm not sure of the process.
SO, you've got some great advice here in the thread... get a double-flaring tool and some lines.
Things to buy:
#1) Brake line cutting wheel
#2) brake line double-flaring tool.
#3) Brake line.
#4) a Brake line bending tool.
#5) Brake fluid.
#6) can of brake-clean.
#7) beer for your buddy who will help you bleed the system.
Straight lines are super cheep. Get lots of them so you can practice flaring them and bending them...
Simply remove the bad line and use it as a template. Chances are since the car was parked it rusted through, so it should all be in once piece.
I suggest buying at least one extra stick of line and practice flaring, cutting, flaring, cutting, flaring, cutting. Nothing sucks more than getting your whole line done then messing up the last flare. DOH! >.< LOL!
Once you've got the process down, make your new line. Keep in mind when you remove the old one you may drain the entire system of its fluid.
Once its done, simply install it. If the junction-box or rubber lines are bad, you can buy those replacements new from ford. Or, you can get stainless lines from Maximum Motorsports, Stang Suspension, etc.etc.
Once its all re-assembled, fill the res. on the master cylinder with fluid. Start bleeding the brake system of air by opening the bleeder valves on each caliper. there are also two bleeders on the master cylinder itself, dont forget those...
once you've bled the brakes and they have a solid push to them, start the car, turn the wheel from side to side and continue to push the pedal. if they still feel solid, consider yourself a pro! :-D
Once you've decided that the brakes are completely back in working order, wash the entire area down with tap water (garden hose?)... Brake fluid breaks-down in water. If the fluid stays on the paint, it will flake away the clear coat. So, be sure to SOAK it down...
once it air-drys use the brake-clean on the rotors/calipers to remove any oils from your hands and re-assemble the car entirely.
Tip: Keep the res. on the master cylinder closed until you drain/fill/bleed the system. As I said, brake fluid breaks down with water, and this includes the humidity in the air.