Reverse always worked fine. The issue started as a short delay when going into gear from neutral or park. You'd move the lever over, and then there'd be about a two-second delay before it finally engaged. Once engaged, it drove fine. Upshift, downshift, overdrive, etc. Ran great, no issues except that delay.
Then, the two seconds eventually turned into five seconds, then ten seconds, then half a minute, then not at all- or maybe once when it was cold, and then never again 'til it cooled off.
But no matter what, reverse always worked.
I'm still not exactly sure what the issue was. I've been trying to follow the test procedures and plumbing to see if I can figure out what went wrong. I think that reverse servo is actually a reverse/first servo- it moves one way, you get first. It moves the other way, you get reverse. And if that's right, it probably moved easier toward the reverse position than into the forward position.
I don't know what originally cracked the planetary- maybe somebody dropped it in reverse at 40mph, or like that head bolt on the 307 I just put together, one of the gears was bad from the factory, and eventually just chunked a piece when day-to-day use worked an exiting crack open. Who knows?