727 2nd to 3rd flare up

My 904 trans had the same issue as yours. After I determined it wasn't a drum seal problem, turned out it was a timing problem.
A member here said to remove one of the springs in the front servo. Now the 2-3 shifts great, not as firm as the 1-2, but no flare.
My throttle pressure linkage did the same thing as your's too. I ended up making the intermediate rod longer and it solved that problem.