Proportioning valve leak

[Jk918]

I have a proportioning valve in the rear brake line, it is new but has a very faint seepage leak that doesn't drip, but shows on a clean paper towel where the adjustment knob o ring is (yellow). I've tried to tighten as tight as it could, tried new o ring, even tried a flat style rubber washer. Is there a secret to get it to stop, any pointers would be appreciated