What I would do would be to fill 'er up, start the motor, and have someone actuate the brake pedal while I had a good look.
The master cylinder on the car really shouldn't be anything, as long as it is easily separated from the booster.
I should mention, do this with the car on jack stands and the wheels off. Then see where there are leaks, except the lines. Visually inspect the lines (engine off, wheels back on, on the ground) for rust and worn spots. DON'T Stick your head under the car on jack stands! (you knew that...)
Rebuilding this system shouldn't be tough, as long as you can get parts! You may need new calipers (if the fluid is seeping past the seals, or the sliders are locked up), rotors, cylinders and drums, pads and shoes. All together, a couple hundred bucks should do, maybe even including the calipers, if you're lucky.