I have an 87 300TD and the transmission has started slipping. I have never worked on a transmission before, so this is all new to me. While Googling around I found that there is a pressure relief valve thing in the engine bay. I looked at it and saw (after driving and the engine still on) a clear hose with red fluid (transmission fluid?) and a lot of air bubbles in th line. Has this valve failed or is there moreto the answer than I have found? Going to check all the rubber hoses this afternoon to make sure they are still good.

Thanks
Joe

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Are you referring to the vacuum amplifier (aka the blue ufo thingy?) It's not the source of your problem. If there's fluid there, it's coming from a failed vacuum modulator on the side of the transmission. These references may help:

Mercedes-Benz Transmission, Steve Brotherton, ImportCar, February 2002

http://jenniskens.livedsl.nl/Technical/Tips/Files/Mercedes%20722.4%20adjustment%20guide.pdf

Thanks MxFrank. Yep, I was looking at the blue thing. Traced it back to the modulatar. Looks like that will be the starting point. Any tips or links on replacing it? I couldn't find any videos.

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A few references I filed away:

PeachPartsWiki: Transmission Vacuum Modulator Adjustment

http://www.benzworld.org/forums/w123-e-ce-d-cd-td/1261744-transmission-flaring-clunking-vcv-diy.html#post1889497

Google below, but links no longer work. I saved the web-page.
"PROPER VACUUM SYSTEM "TUNING" ACHIEVES OPTIMAL SHIFT CHARACTERISTICS AND AN ACCURATE REPAIR"

I think your transmission is the same as in my 1985 CA 300D (722.4xx). The blue saucer is a "vacuum amplifier", which improves shifting, and accounts for intake boost. It began in 1985 cars.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

FYI, the Webarchive ("http://web.archive.org"), sponsored by Library of Congress, has snapshots of the entire web going back 20 years! This article can be retrieved at:

Mercedes-Benz Transmission, Steve Brotherton, ImportCar, February 2002