My .02
First not understanding the hoping explanation/discription, not ever heard that term.
If it were air in the system, why is it only in reverse?
Valve body maybe, some spring or piston loose not holding position/sealing in reverse. Most likely could be replaced with just dropping the pan.
One thing for sure waited to long, not driven it enough, warranty long gone. Is the drive shaft 2 piece, something going on there?
Hoping, as in going in and out of neutral, shifter linkage loose, not totally at home when reverse is selected?
Anyway back to the valve body, if my GUESS is somewhat close. Some magic goop, trans repair in a bottle from our friends at autozombe.
Lucient goop could offer a quick troubleshoot, it is hyped as a seal and general lubricant and expander. Dump some of that in, if the problem changes, you may need to look closer at the function of the valve. That book shown is a good reference, used the same.
As I think and not a auto trans guy to be clear, that valve assembly is common and a key component to all.
BTW have a TCi & B&M, would not buy a TCI again, its ok but just that ok.
Pep