Leaking 727 trans

trans-go sells a "no yo-yo" spool valve thats supposed to stop convertor fluid drainback but I dont know if its compatible with your Reverse valve body. If it drains at rest, the leak is higher than the fluid level at idle or P or wherever your supposed to check it at. Possible a selector shaft seal or filler O-ring. Speedo gear has a few O-rings, not just the obvious one.