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The seals are probably harder than chinese arithmetic after sitting for that long. Check the fluid level while at operating temp. (t-stat open).Try screwing in the screw in the modulator a few turns to increase the trans pressure and see if that doesn't do it. Remember to count the number of turns you screw it out so that you can put it back where it was. If that doesn't do it you probably need an overhaul. Old seals tend to get hard from sitting after prolonged hot/cold cycling. O/H on a C6 is pretty easy.