The control prssure cable adjustment is in the linkages on top of the valve cover. It is even easier than the modulator.
There is only one cable so finding it is no problem. The cable has a ball socket on its end and is attached to its own activation lever. The adjustment is where the cable housing end is mounted. It is a 14mm wrench size white plastic nut. Turn the adjustment 9 flats (1.5 revolutions) in (clockwise). This will shorten the cable and make the trans shift earlier.
The point of this is to get your part throttle shifts at a greater throttle. In other words you will not be lifting your foot as much to get the shift; you will have to keep your foot in it to get later shifts.
The only reason this works is that it affects 3-4 more than 1-2. You are trying to get a stronger 3-4 shift without affecting 1-2.
You can keep adjusting modulator pressure but I can't tell how hard your other shifts have become.
BTW as I have said before on a late 83 and newer the chances are you have clutch problems with these symtoms. If you have metal in the pan on servicing all these adjustments are temporary as everything changes quickly in the end.
Also you are likely to blow the second gear band servo piston after a while at very high pressures. If your problem isn't filling the pan with metal I would think that shift technique could make this thing last indefinitely. You could hasten a repair by taking the pressure too high.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician