Philip W. Moore, Jr. wrote:
Howdy folks. I've got a pair of Spinergy Rev-X tubular wheels that require
a valve extender for my Tufo tires. As those who use valve extenders know,
you have to leave the valve open before you attach the extender to the
valve.

My Tufos have removable cores, so I bought Tufo extenders: they allow
the valve to be reinstalled at the accessible end of the extender.

But for me, this results in a loss of pressure in the rear tire.
Have any of you ever dealt with this? If so, how did/do you solve this
problem.

I use teflon plumber's tape on the threaded joints.

It's also possible your valve stem is slightly bent; I've known that to
make the valve's locknut press against the inside of the extender. That
prevents the valve from seating and so air gets out. To fix this I've
either carefully straightened the valve stem, installed it at the
accessible end of the extender or replaced the valve.