Our W126 '83SD transmission has a leak around the frontmost port (forward of Vac modulator?) on driver's side. What is this port and how do I reseal? Are there O-rings to R&R? More important, what is behind the cover and are there parts under compression or requiring adjustment if I remove this cover?

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I can't answer that, but I gotta ask:
Where exactly is that darn modulator?
How do you access it?

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Think Alternative Energy!
300CD '80 (now gone but not forgotten...)

I'm only guessing that the green assy on the right of my photo is the vac modulator. Based on the description in the mbz.org article "Adjusting the transmission on your 123 or 126 diesel."

The green thing in the picture is the modulator.

If you pull out that plug there should be a crush washer you can replace, or a little rtv on the old one should keep it from leaking any more. Looks to be about the same size as the one that comes for the reseal on the power steering pumps. Dont quote me on that though

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1980 500SE/AMG Euro
1981 500SEL Euro
1982 380SEL
1983 300TD
1983 500SEC/AMG Euro
1984 500SEC
1984 300TD Euro
1986 190E 2.3-16
1986 190E 2.3
1987 300D
1997 C36 AMG
2003 C320T 4matic

past: 1969 280SE 4.5 | 1978 240D | 1978 300D | 1981 300SD | 1981 300SD | 1982 300CD | 1983 300CD | 1983 300SD | 1983 380SEC | 1984 300D | 1984 300D | 1984 300TD | 1984 500SEL | 1984 300SD | 1985 300D | 1986 300E | 1986 560SEL | 1986 560SEL/Carat | 1987 560SEC | 1991 300D 2.5 | 2006 R350

A hack/gorrilla installed this plug/cap. I'm stuck.

I tried to remove the plug and approx 75 ft-Lbf wasn't enough to loosen this fastener/plug. I chickened out on the exerting more torque via a breaker bar or jack etc. Didn't want to bugger the in-hex fitting with an offset lever arm. Decided it would be best to get the opinion of a local mechanics on a lift. Not the same shop that had been servicing this car. Looks as though the plug is seated with no crush washer. Shop was gracious enough to put the car on the lift and add ATF for a test drive. But they really wanted me to go back to the hack who did the prior work rather than get tangled in someone else's mess. Can't say I blame them. These are not transmissions experts so they were not able to answer my question about whether this is just a plug, or if there are springs/mechanisms behind this plug. Before I go back to the gorilla/hack it would be helpful to know: 1) if there is anything behind the plug, 2) if there is any potential for damage such as cracking either due to the installation without a crush washer or due to aggressive methods (more torque, impact wrench, heat) that may be required for removal.

that is the reaction valve, there are some o-rings that can be replaced to stop the leak, rest assured it is tight, i can give you the part numbers if you want. there isn't supposed to be a crush washer there.

here are a couple of pictures, the only things you need to reseal this are the items in blue, they are 2 o-rings and a plactic holder for the rod. item number 110 in the picture is the allen plug where the leak is coming from

010-997-08-45
005-997-80-48
140-277-01-51
order one of each

Thank you for help identifying the B1 cover, and the B! valve seals within as the likely leak source.

Is this a simple DIY R&R? As in: remove cover, slide out valve/piston assy and plastic endcap, install new o-rings wiped with ATP lube, reassy same as disassy in revers order.

Or, is there more to it such as a correct installation sequence to compress springs and engage the valve assy with the band mechanism?

It's an 83 300SD. ~270K, shifts OK with proper ATF level.