With pipe threads, if they're not tight enough, they'll leak thru the threads. A copper washer might help seal it, but the threads might be more prone to strip out, as they wouldn't be fully engaged. A new fitting like George suggested would be the first thing to do. It's possible the threads in the housing are expanded too much to fit correctly even with a new fitting. Another thing to try would be additional wraps of teflon tape, being very careful not to get any over the end of the fitting.
