New proportioning valve leaking

Working on the brakes of my 69 swinger. For some reason i decided to replace all of my hard lines. I bought a new proportioning valve from in line tube and it leaks where the front brakes feed into it from the MC. Not from the line itself, but the fitting that goes between the block and the fitting that feeds into it. It looks like a copper o-ring in there.

Anybody had this problem? How do I fix it?

Thanks