Katherine,

Make sure there is a rubber gasket inside the cap.  I added a drain valve
on mine, wrapped the threads with teflon tape and put it on as tightly as I
could.  No leaks, and it never needs to come off as I can open the valve to
drain.

A short description here:
http://bluehighwayvanagon.blogspot.com/2012/03/westfalia-improvements-part-ii.html

Jonathan

On Tue, Jun 12, 2012 at 10:07 AM, Katherine J. Maas <katherine@kjmaas.com>wrote:

> A few years ago the plastic cap for the water tank drain on my 1987 Westy
> broke . Rather than replacing it, I bought a sink drain plug and put it
> inside the tank. It seals fine, but it�s a lot harder to drain the tank in
> the fall. A problem with the 1987 Westy is that it may take one of two cap
> sizes and the plate on the back seat, which is supposed to give the
> information needed to order the right size, is missing. However, a week ago
> I broke down and ordered one of each of the two replacement caps just to
> get
> this job done. Yesterday I tried both of them. Both appear to screw onto
> the
> drain opening OK, but both of them have a slow leak. Has anyone had an
> experience with this that they were able to resolve and would be willing to
> share with me? Please p-mail if you do!
>
> Thanks,
> Katherine & Vanita
>