You may want to check out this post.
http://www.peachparts.com/shopforum/showthread.php?t=254000&highlight=tenor+man
There is a ton of information in these forums of the dreaded reverse failure in the 722 line of transmissions.
This one has 170,000 miles on it...
I believe that the problem is caused by the spring retainer (it's the part that is in three pieces and broken pins). It it held in place under tension by a small (1mm) diameter wire clip (not shown in the photos) around the shaft.
The spring retainer is made from cast metal (looks like what we used to call "pot metal").
I believe the the spring retainer suffers from fatigue and slides past the metal ring, popping all of the springs out and letting the B3 (reverse) piston ride against the clutches and then wearing out the 4 friction disks...
In short, the transmission must come out...sorry.
The Tenor Man