New proportioning valve leaking

It is leaking right below the hex nut on top of valve. Factory stuff is always better then anything "new".

Sent an E-mail to in-line tube asking for a new one. Used flair wrenches to tighten it up.

When you say back-up wrench do you mean like tightening the wrenches against each other?

Might try the back off and tighten and back off and tighten method or the one wrench attached to another for maximum stripping.