My 2 cents.
The first Metric 200s ('76-'78) had a 'single stage' servo constructed with one piston. (Some of these larger diameter units have found their way into TR 200-4Rs.) The servo was redesigned for 1979 to contain an inner and outer piston. That design was carried over into the, then new, 200-4R.
Hydramatic's original thought, when the inner piston was added, was that the large piston was slow to react. The smaller, inner piston was supposed to apply the band quicker, with sufficient force to initiate the shift.
In reality, the force of the smaller piston is not adequate and it slides the band until the larger, outer piston takes over. This creates a condition referred to as 'slide-bump'. It doesn't feel good and it's not conducive to band life. By disabling the inner piston, you achieve a cleaner, firmer shift.
The small servo return spring is eliminated to reduce the tendency to flair on the 2-3 shift, when the band release must be precisely timed with direct clutch apply. There is such a great differential between the release side piston surface area and that of the apply side that fluid force has no problem pushing the band off.
