The issue likely wasn't the fluid, but rather the rubber deteriorating with age. If you'd replaced it with the same type of new fluid it probably would have gone dead as well.
When I was a tech we always replaced the master, slave, and hose as a set. If you didn't, the other components were going to fail soon after. If you don't bleed the brakes with new fluid occasionally (who does?) replacing the old fluid with fresh fluid often causes sudden leaks. Brake/clutch hydraulics are weird and problematic like that.
Hindsight 20/20, I would have left it alone. It was old, but it was working until I attempted to do the fluid replacement.