My 85's was vacuum controlled from the factory. Always acted quirky, combine that with an anemic 305 and 1550 rpm @ 70 mph, it was worthless. But it did have the "tap" feature you are referring to, only to increase speed though (in 2 mph increments). Most newer cars allowed to decrease speed the same way, but these trucks never did. I disconnected it since I never use cruise control, not even on cross-country trips.