We have a 1982 Gleaner M2 for the last 2 to 3 years has been getting slower and slower and I am looking for a little advice on where to start. It has a Dynapower Hydro transmission. When the rear wheel assist is engaged the combine goes even slower. Did not know if it could be the hydraulic pump or the transmission? The hydraulic pump was installed (not new) about 4 to 5 years ago from a salvage yard. Any help would be appreciated.