John Deere 450c Transmission issue

I have a John Deere 450c dozer that doesn't seem to have any power moving forward or backwards. It has enough power to move itself but that's about it but very slow. I read where there is a hydrolic valve that controls the oil flow to the transmission that could be bypassing or something. It said the valve may be stuck or needs replaced ?? Does anyone know where this is located ? I just got the dozer and can't figure out why it won't move very fast ? I pulled the transmission filters and tthey were clean ?? Looking for advice

John Deere 450c Transmission issue