This is one thing that can be easily overlooked during a rebuild, but did you check to make 100% sure that the reverse servo pin is lined up properly with the reverse band? Since it is back in the car and all together then you will need to remove the intake tubing and possibly the exhaust crossover pipe to gain access to remove the reverse servo cover. This is the one on the top side of the transmission case. As the others mentioned above the lip seal on the servo piston can be of concern, but since you rebuild this then likely the seal was replaced and a new seal, as long as installed properly, will work fine. Also did you inspect the band during the rebuild to ensure that it didnt have any cracks on the lugs on the ends that engage against the anchor pin in the case and the apply pin? If the band has a crack on either end it will not apply and you will have no reverse. I would wager more on the pin being mis-aligned and that could take care of your problem.
