To wrap up and share what I've learned since I first posted this...many systems already have UV tracer in them so it is worth snooping around with a UV light in the dark. Also the easiest thing to look for is signs of oil leaks from the AC system. They often tell you were the leak is. The tricky one is the evaporator cause it's buried in there. look at the evaporator drains under the car for hints. Also it is worth noting if that particular make and model have historical evap leak problems or not. a knowledgeable tech specialising in that car or in AC systems could shed light on that.

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1972 450SL
1982 300D Turbo