727 transmission problem

So this is a pretty old trans coupled to a 318 mopar from a 1983 d150, the transmission isn't responding well to throttle and the fluid seems to be vanishing, no leaks no noise. I filled it up cause when I got it it was empty I used atf+4 for transmissions with 75000+. Poped it into gear and..about 5000 rpm gets me going backwards, forward is the same thing but it doesn't matter if I use half throttle or full..I get the same acceleration..torque converter?? Please help.