I am about to replace the A/C service valve again on my 1998 Mystique, anybody have any tricks for making sure it seals? I checked with soap bubbles after replacing it last summer, and it was OK several times. But then, it leaked later, I could see the yellow tinted oil bubbling out of it

when the A/C quit. And, although it's tight, seems to leak around the core threads. The plastic cap looked OK, got a new one now too. I even have the special tool to tighten it down.