When I had the trans. out of the car, I checked the pump...it is good. Also, when I reinstalled the trans., it "sucked up" the new fluid quickly/easily. I don't have a means to actually check the pressure, but I feel like the pump can be ruled out. I think that it is most likely that a valve in the valve-body has "hung open"...mainly the Hi valve.
When I had the trans. out, I found that the lining on the reverse gear lined plates had been coming off. I replaced them. Maybe a small piece of that material found its way into the valve body???
What can I do??? How can I check that for sure???