Valve Exender Question

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. 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.

Thanks in advance for your help.

Regards,

Philip