W126 Freon leak
 

I put a compressor in my 1989 420 SEL last July and converted to R134 by replacing the drier and oil. This spring it won't hold a charge and the sniffer tells me it's leaking in the evaporator box. What's the usual leak, a bad joint, bad O-ring or bad expansion valve? I looked at the pictures of the parts on Fastlane and don't know what I'd pick. I'd like to have the likely parts on hand when I open it up. Any repeat experiences?