727 Problem

I have a Australian Valiant 318 with 727 torqueflight and the box is a 1978 model. The car had been sitting for twenty five years so I have taken a full restoration and rebuilt the engine and put new seals and a transco kit in the auto. My brother is the auto tech guy as I have only ever had manuals and this is my first auto. When we fill the auto , start it and select reverse it goes in the first time then no more reverse but forward gears work. We have pulled the valve body out twice but it does the same think everytime. Reverse worked once then no more. Do you have any ideas?