early 134a systems

[Chuck Storms]

I have both a 1994 90 and a 1995 A6 that need their A/C systems charged, my problem is finding the correct fitting (tool) to connect to the low side service ports. On these cars there is a conventional 134a high side service port but for the low side the service port looks like an old r12 fitting with external threads except it is bigger.  With my last 90 I went to the local parts store and bought a low side female fitting for a retro-fit and tapped it out to fit the m12x1.5 threads on the existing port, pulled the schrader valve out and screwed the new one on top, that worked but I would like to find the "correct" way to do this job. I have access to all the equipment I need at work, I just need to find the correct tool to attach to the port or some sort of service port adapter. Any help would be appreciated, btw the local Audi dealer had no idea what I was talking about.