This used to be a very common problem on the earlier models. The channel plate gasket is blown out and when this happens - main line fluid pressure will spray straight up into the vent and leak outside the trans. This requires lowering down the transmission, removing the side cover, pump, valve body, and channel plate to replace the gaskets. These gaskets have had several revisions but by 99-00 there really wasnt a problem with this anymore. Since your transmission is already rebuilt either the bolts werent torqued properly, sealing surfaces were dirty or not flat, or some type of sealant or adhesive was used on the gasket which causes inconsistant torquing and often will blow out with age.
