1990 300D, I have a reoccurring problem. The high side AC Schrader valve keep leaking. I have replaced with OEM MB valve core & started leaking again. This has been done 3 times & always starts leaking again. AC stops working after approx 2 months.
I have also used R134,retro fit on top of the R12 valve, now that's also leaking.
Any help would be great.

A good valve cap with a o ring will seal pressure even with a bad core.

What method are you using to identify it is leaking?