Brake parts (calipers, brake lines, and master cylinder) aren't really good junk-yard parts. It's better to go with rebuilt or new parts.
You need to go on a hunt to find where it's leaking. Like I mentioned before, try pumping the brakes a lot while topping off the master cylinder with fluid. Some leaks need a high volume of flow before they become apparent under the car.
If the problem is a corroded brake line, it may be a good idea to replace all the lines, depending on how bad the rest of the lines look. It's a judgement call....it depends on how you feel about your car, how long you plan to drive it, and what level of "tune" you want to keep it in, etc.
Did you say this car sat for five years in a shed unused?....that's tough on any car. Cars need to be exercised, or they'll stop working. First thing to suffer from storage is the braking system. You may want to do a COMPLETE brake job - rebuild all calipers, flush the brake fluid - once you identify your immediate problem.
95 E320 Cabriolet, 131K