z wrote:
[...]

Well, as of this AM after a day with the cap on, there's more drool
than ever on the floor, so it's got to be coming up through the
packing around the stem, despite my having tightened the packing nut
as much as I dared. Either that, or the crummy nylon valve is cracked
or something. Hard to see the water on the white semitranslucent
plastic.


AFAIK, there is no way to repair those valves. Make one like shown
on this page:
http://preview.tinyurl.com/2esavf
and replace it. If you have to replace the water heater later, then
remove the one you make and put it on your new heater when you
install it.
--
Grandpa