Should the transmission modulator on a 92 300d 2.5 (green one) hold a vacuum? When I disconnect the vacuum line on top of the injection pump (white hard line) and apply vacuum, it bleeds down within 5 seconds.

i believe it is.
but i have an 82

Yes, if you apply vacuum to the transmission modulator, it should hold that for some time. Assuming your mityvac is connected well and tight to the line, the line has no cracks in it, and the rubber connector at the modulator is new, tight, and isn't cracked, the vaccum should stay put for longer then that. The modulator is basically a diaphragm, and if it leaks vac when the connections are all verified to be tight and good, that usually means you have a tear in the diaphragm. If the hole gets big enough, it'll suck trans fluid into the line. If you pull the rubber connector off the modulator nipple and trans fluid drips out, the modulator is toast.

Be careful not to apply too much vacuum to the modulator. I don't believe it gets full system vac. I think it's more in the range of 10" or so.

__________________
'84 190D 2.2 5MT (Red/Palomino) Current car. Love it!
'85 190D 2.2 Auto *Cali* (Blue/Blue) *sold*
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System vacuum is moderated by the VCV and the vacuum amplifier. It can be as high as 15, as low as 0.

i replaced most of the vacuum lines in the engine compartment. found one of the lines beside the airbox had been unconnected. when i was test driving the car with the guy i bought it from, he said it the blower motor didn't work nor did the vent selector and it shifted hard as hell. when i reconnected that vacuum line the vent selector started working, and the blower motor was the strip fuse.

the transmission felt like at certain throttle positions it couldn't decide what gear it wanted to be in. when it shifted to second, it seemed to do so too soon, then felt like it regretted it's decision to shift and tried to down shift back to first.

i adjusted the modulator by turning it clockwise and then test driving, then turning it some more then test driving. helped a whole lot, you cant even really feel it shift now. maybe even too soft.

today i will get under there bc i want to epoxy some cracks/tears in the belly pan and connect my mighty vac right to the modulator.

just curious, are they difficult to change?

update:
i got under the car this morning and put 5" of vacuum right on the modulator. it bled down within 4 seconds. clearly it's bad. I called mercedes and the ordered me one for $32. I thought the guy was looking at the wrong part, seeing as how they are more expensive online (mercedes source). the other thing is he said it was black, and mine on the car is green. he searched for it buy my vin, so i guess it's the right one.

How does one change this item?