Modulator Valve?
 

Does the rubber cover over the modulator adjustment hold the vacuum? When mine is off it does not hold vacuum. Now it's all dried out and split. Tried another off the kids wreck but it does not hold 100% either. Are there new ones available?

You will find a new one at the dealer and it should then hold vacuum. If it does not then the modulator is at fault if the line to the modulator is not faulty.

So the rubber cap must seal for sure? Sure seams like a place for a regular failure.

OSH again
 

Ok not original Benz but the 1/2" rubber nut cap from OSH for 29 cents seals tight. I bought four cause I figure one or two will rollaway into oblivion when I'm making adjustments to the modulator:D

Next question while I'm here. Clockwise in for firmer shifts or the other way around? I need to go softer shifts. I don't think I should be loosing traction in the rain on the 1, 2 shift. I'd hate to be cited for exhibition of speed;)

I need one too but I gotta combine some parts with an order so shipping wont be too much.

Here's one place I found them... havent asked dealer yet...

http://www.***************/search/product.aspx?sid=m1ukl455v14bhu45djxlyc24&makeid=800016@Mercedes&modelid=1193789@300DT&year=1984&cid =@vacuum&gid=7776@Modulator

They are also listed in Fastlane here:http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1XZ0NWULG1XZ0NX7KV&year=1984&make=MB&model=300-SD-002&category=J&part=AT+Modulator+Cap
(For some reason, I have found this part under 126 models but have noticed that it is not under 123 models. I believe it is just a data entry glitch.)

Yes, the modulator cap is enough to seal it off. Mine was the same way, I replaced the cap and it held vacuum for longer than I desired to wait.