[Steeveedee]

It is still possible that the detent cable is not adjusted properly. But if it was OK when left to sit, it would be OK now. You can take the modulator off and hook it to vacuum to see if the piston moves. Don't hook it to the truck vacuum with the modulator off, you'll have a mess on your hands. It is possible that after having sat long enough, the valve in the valve body could be stuck. This is about as far as I am willing to go in terms of recommendations, as I am not a transmission expert.