904 Valve Body

That's an interesting comment, got me thinking. If you are talking about the 'inner seal', the smaller one that goes on the inner hub (2" or so diameter), the lip on that seal is supposed to be facing downward. I can't visualize how that seal could get rolled while installing the piston unless the seal is installed backwards with the lip facing up.

When I took it apart there was a section of the seal, about 3/4's of an inch long that was fliped over. I had a trans shop install a new seal and reassemble the piston in the drum. They air tested it with the input shaft and pump, it was good to go.
I think the first shop that rebuilt it did have it flipped when they put it together.