a727 no third gear

Well, if you found an issue in the valve body, I would think you are gonna go back there first! Maybe try a known good one and go from there. I would not be a bit surprised to find you have a fried rear clutch, even if it looked good before. Just because it sounds good does'nt mean theres any frictions left. BTW, where did all the fluid go?? If you are saying it leaked out whilst someone was driving the vehicle, well then there is damage done!