An E39 I guess has 4 wheel ABS Module, so you're replacing the line from there?
You should be ok, if you make sure the module is always full of fluid and that removing the pipe doesn't allow the reservoir to run below the low mark.
Again though, for peace of mind, might as well bleed them all, besides, if a line is corroded, could probably do with flushing out all the lines and putting new fluid through.