Before you go to the expense and bother of changing it, try adjusting it. If nothing else it will give you practice as the new one will definitely need adjusting. I would suggest turning the tee handle one turn clockwise.
The modulator uses engine vacuum to sense load and is used to vary shift pressure to suit the load of the moment. It works by placing a vacuum on a diaphram opposed by a spring. The adjustment varies the spring pressure and thus changes the hydraulic pressure in the trans called modulator pressure. This pressure works on the main system pressure regulator as the largest influence.
The proper diagnosis and adjustment of modulators (and thus modulator pressure) would to use a pressure gauge. While driving the pressure would vary greatly with engine load and change smoothly (if not sticking). The proper adjustment is achieved (by the book adjustment that is) by removing the vacuum and holding the rpms to 2000. (It turns out that the pressure is constant just above idle - the 2000 is to be sure the pump has good volume). The measured pressure would then be set to the value from technical data.
We do these measurements every time we rebuild a trans, but seldom afterwards. Afterwards we usually adjust them to feel.
BTW the tee handle is under the rubber cap in the center of the device. It pops off with a small screw driver and the tee handle is locked in the depressions of a castelated rim.
Bosch Master, ASE Master, L1
33 years MB technician