On Fri, 28 Jan 2011 15:28:58 -0500, lid wrote:

Make sure the point of the cone washer is entering the copper socket
and not catching on a flat surface. The cone nose must compress into
the copper fitting to clamp the tailpiece.

I had a cone washer leaking, so I grabbed one from my plumbing stash.
It leaked also. Apparently there are different sizes?