The only leak I've had so far since new on my '93 190E was at the expansion valve. I replaced the O-rings, pulled a vacuum, charged with R-12 and it was good to go.
I found the leak with my trusty old propane freon leak detector. You fire it up and when the hose detects a leak the flame turns green.

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible