Most COMMON leak location on a car with an R4 compressor (300SD) is the shaft seal, but it is usually slower than yours. The size of yours would indicate a large enough leak to find oil residue or the old soapy water trick should work. Heck, you can probably hear your leak. Look for a hole in the condensor from road debris (rock).

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)