Two years ago I replaced the trans in my 79' chevy plow truck. Bought it from a guy in Spokane, his only business is rebuilding 350 transmissions for many years. It has always worked excellent, very little use due to mild winters lately. After sitting for 3 or 4 months this summer I started off to take some cans to the recycling place, it wouldn't shift out of 1st. Reverse works fine. Figured a vacuum problem, nothing there. Checked kickdown cable, it was ok. Would the vacuum modulator have gone bad? Kind of stumped, any input would be appreciated.