If I understand your thread this $1000.00 car is an 81. If it is, it has no bowden cable. Its controlled thru metered vacuum. There are vacuum check valves upstream of the modulator that control vacuum to the modulator by leaking off vacuum. There is also a valve on the top of the valve cover that is opened/ closed as the linkage pivots. They are prone to leaking or the little lever on them breaks off. Yes I agree, it sounds like too much vacuum is getting to the trans, not too little. On a car that old it could have many issues, threaded throttle rods misadjusted, ck valves leaking, modulators sticky, etc. Make an offer once you find a mechanic that is well trained on this older car. Not before.

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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.