Im chasing vacuum leaks on my 84 300sd, what is the acceptable leakage at the trans modulator? It leaks immediately

I believe a healthy modulator will leak down in about 30 seconds. Don't apply more than about 15 hg, which around where the VCV maxes out.

how about the vacuum valve on top of the valve cover?

The first thing to check is the little rubber "cap". If it is cracked, dry-rotted or torn then it should be replaced.

http://catalog.peachparts.com/ShopByVehicle.epc?q=1984-Mercedes--Benz-300sd-Transmission&yearid=1984%40%401984&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6208%3AED%7C10000134%3AMBC%7C1510%40%40300SD&catid=241185%40%40Transmission&subcatid=241315@@AT+Modulator+Cap&mode=PD

Another note, where are you checking it from? The flex hose at the modulator is very likely to be suspect as well.

Change the modulator after verifying these are not the problem as it is sort of a PIA to replace as there is an adjustment needed that requires pressure checks at a certain speed.

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Sam

84 300SD 350K+ miles ( Blue Belle )

I attached my vacuum pump directly to the modulator and it leaked down almost immediately

I would replace it, all the upstream components work under the assumption that the vac modulator doesn't leak.

Bear in mind that you will have to check and adjust modulating pressure once you install a new one.

steve a

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Steve A
92 W140 OM603
97 VW Jetta TDI
90 Passat variant TDI 6 speed MT
94 Chevy K1500 6.5TD
05 E320 CDI
+ others

I researched the modulator pressure issue when I replaced mine and learned that the specification may not be totally useful given the age of these old transmissions. So I just installed the modulator with the factory setting, drove awhile, and tweaked it until it was satisfactory. I ended up backing it out a 1/4 turn twice until it shifted well.

The little rubber cap is definitely where I would check first. Replacing it is super simple and may quickly resolve the issue.

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Sam

84 300SD 350K+ miles ( Blue Belle )