TCM 850-1 Transmission Problem, it moves very slow and produced low power

transmission problem and produces low power to unit, we check the transmission and replaced transmission control valve. after testing we found out that it does have a hissing sound from transmission. We also replaced seal rings and outer & inner disc of transmission first and second gear, also forward/reverse clutches. Glad if you can give us your recommendation to our problem.
Details: Model TCM 850-1
Serial # S10-00317
Engine Nissan PD6-027177
Regards,
Johriel Panganiban