First check the vacuum connections at the 5-way vacuum distribution close to the vacuum booster. It a Mityvac is available, check the modulator membrane at the transmission.

Next check the cable assembly at the cable assembly at the side of the intake. If the cable at the turning point to the tranny is too loose, the tranny will change gear too quickly and stay in the highest gear while driving. It feels very soft. If it is too tight, the shifting point is delayed to higher speeds, also invoking harsh shifts.

Then the next subject is modulation pressure.
modulating pressure M1 and M2
modulating pressure must control the main pressure; it can be measured and adjusted.
two modulating pressures differing from each other are used for control.

MODULATING Pressure M1 on control valve (69) is load dependent by the way of intake vacuum.

Modulating pressure M2
on control valve (5) is road speed dependent (governor pressure).
Modulation pressure M2 is amplified in the lower speed range.

In transmission A 28 the modulating pressures do not have influence on up shifts and downshift points!

You can adjust the M1 modulation pressure by turning the adjuster key under the rubber cover on the side of the transmission. Check the rubber and vacuum line while you're at it.

1. remove rubber cover from vacuum control unit.

2. pull the retainer out slightly and adjust the setting screw in the vacuum-control-unit with the retainer.

Note: one full right turn of the setting screw increases the pressure by approx. 0.2 bar.
so that one turning from one catch to another catch changes the modulation pressure by approx. 0.07 bar This gives firmer and later shifts due to the change of the pressure. The correct pressure should be 61-64.5 PSI (4.2-4.45 bar)

clockwise: pressure is increased
anti-clockwise: pressure is decreased