The MB file is called:
'47-720 operation fuel tank positive and negative venting system'
I can't Cut & paste out of a .pdf file so I'll just note a couple of highlights as I am a crappy keyboarder and won't do it all.

"ALL models the tank cap allows pressure out at 100-300 mbars"

-Looks like starting in '81, I think on W126 and W116:

"The vent valve opens at a pressure of 30-50 Mbars
And a vacuum of 1-16 mbars"

As I say I don't have my cd yet to read further. And I really don't think in Mbars but that can't be much pressure or vacuum... so the vent must be plugged eh?

__________________
Thanx,
Alberta Luthier
1983 300CD ('Stinky')155k miles, 2.47 diff, EGR removed, AAZ injectors with 265 nozzles from Sean,and vogtland lowering springs.
1984 300SD ('Old Blue')150k Klicks from Japan originally, came with rear head rests, no sunroof and never had an EGR