i had a very similar experience with my former esi ... after radiator overhaul, changed plastic top into copper, and radiator cap with generic one ... the car ran for a few weeks without trouble and then the problem is back ... losing water

i supected the radiator cap, but since it is fairly new, i figured maybe the top rubber seal isn't quite up to specs (it looks thin) so i tried adding another rubber seal from a tire interior rubber (cut with scissors) and it worked!