I recently modified my potable water drain on my '91 Westfalia
camper.  The original drain cap was missing when I bought my vanagon.
I was able to buy a used one although I think the threads on the
drain spout were stripped and thus I couldn't make it screw on.  As
an alternative that has proved to be a much better fit is the
following mod:

I bought an ABS tap at my local Home Depot and a two part epoxy from
Loctite (E30CL(29329)  -  Loctite E30CL 50ml Epoxy cartridge).  This
epoxy has excellent bonding characteristics for polyethylene (water
tank spout) which was the only questionable medium that I was dealing
with for adhesives.  Before gluing on, I had to sand the spout a
slight amount to make the new tap fit snugly.

Here is a link to a picture of my install:  http://
s1003.photobucket.com/albums/af155/onsight2003/?
action=view&current=DSCN6762.jpg

Now to drain my water I simply turn the tap and the water flows out
nicely (with no danger of losing the cap).

Trevor Reynolds

'91 GL 2Wheel drive, AT, Westy "SEN".
http://westfalia-91restore.blogspot.com/