mespe,
I have not located and/or studied the vacuum diagram for your particular YR/model but my limited experience to date tells me that the various restricted orifices in these beasties are critical to proper vacuum balance and of course this changes with age/miles [ for us humans too!].

I recently obtained all 6 of the different sizes of M-B's in-line orifices.
I mention this because this could be a way to further lower the vacuum range your modulator is seeing. As the tranny ages I think [even though a newbie]... I think many here accept the premise that it is pretty much an art form to adjust the vacuum system/modulator in order to squeeze out a few more miles from these trannies. I just turned 300K on my 1980 300D[na] with a 722.1 and yes it flares too. But I have not given up yet.

Back to your situation:
Q1 - Have you found [with some assurance] the proper vacuum diagram for your car?
Q2 - Does it show any "in-line" orifices, orifices built into "Ts" , or orifices in vacuum "dashpots" or whatever other names they go by?

Not all the vacuum diagrams identify what size orifices are specified and in my case my M-B dealer's parts counter did not even know about or have a way to come up with the P/N(s) for these "cheap" but critical parts! Let know if you want P/N(s) for the in-line orifces! Their mechanic had the PN(s) written down on an old parts envelope... that's where I got my numbers!

Orifices - wherever found, and by whatever name can easily become clogged… and keep in mind that many [if not most] of these cars have an open vent line hanging under the dash.
Sidebar - On this let me relate to ALL that my mechanic showed me how he often installs clear universal fuel filters in the vacuum lines of these car’s engine/tranny vacuum systems.
One on the vent line under the dash to filter out debris from being sucked in!... and
one on the line to the diesel shut-off valve on the side of Injection Pump to give an early visual warning that your shut-off valve diaphragm is leaking oil!

Sam