A442f fully hydraulic transmission (1 Viewer)

[Squash]

A442f Tranny behind a 1991 hdt.
Low mileage swap, and now no reverse or 4th gear.
Shifts from 1st, 2nd, and 3rd like a dream.
4th is nowhere to be found, it's as if she dropped into Neutral.
Thanks

 

[Squash]

Question
Since reverse and 4th gear do not share valving I'm suspecting "dirt" in the valve body.
Contemplating a Rodney flush, but adding parts cleaner or diesel fuel hoping this clears any passages. Of course the next step is to remove the valve body.
Any thoughts or suggestions!