Hi perch, welcome to Outdoorking.
It sounds as if the reverse clutch is being engaged when the pedal is not being pressed. That might mean something wrong with the linkage between the pedal and the clutch thrust bearing between the two large clutches under the seat. Depending what model your Greenfield is, there might be an adjustable pull-rod there, or a supposedly-fixed linkage.
Please post some pictures of the clutches under the seat, showing us some detail of the thrust bearing in the center, so we can see if it is being pushed toward the reverse clutch.
