This "never adjust cables" is a direct quote from my 81 Mercedes service manual. No rational given. My presumption is they want you to first verify parking brake shoes are good and all return springs and adjusters are intact. Assuming thats all ok, then adjusting the shoes should provide a parking brake.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.