So...more junkyarding today. First, here is what I found from the MB parts guy.

For old style VCV's, PN:1230700046. (its printed on top of the VCV). He said that: They are supposed to bleed a little bit even when closed. The spec is an average 1 mmHg per second. Max bleed per second is 3 mmHg. Based on testing 10 of these things I have to believe this to be true. I now have one from an '85 TD that falls within this spec. I have 3 others that are bad now on my bench. Here is what I have learned from dissecting them:

- the internals are very simple. a simple spring that breaks a 'seal' is all there is.
- they are prone to moisture getting in and rusting the spring and metal prts out, seizing them or other wise.
- the primary failure is going to be the spring failing somehow. the symptom of this is the lack of ability to bleed vacuum off. On my original one the spring would no longer open the valve, no vac bleed ever. On another one I have the spring seizes at the top position, not pulling the valve back one max bleed has occurred.

Here's how to do a comprehensive test for those reading along:
* this is not an MB official thing, this is my inferred procedure.

Take VCV off IP.

1.) Test with MityVac. Disconnect all vac lines. Attach mityvac to top input. With throttle at min, it should bleed at a rate of 1mmHg/S from 10 to 0 or so. Much more than 2 mmHg/S I'd replace it. I think the rate will naturally be slower the lower the vacuum. From 5-0 should be slower than 10-5 etc. This makes intuitive sense to me.
2.) Feel throttle position input mechanism on VCV. Pushing up you should feel the resistance of a spring. If you fee no resistance it's busted. When you go to full throttle and let go of the throttle input lever the spring should push the lever back towards the min throttle position. If it stays at full it's busted.
3.) Mity Vac again. Pump up at min throttle. While vac is going down, move throttle lever slowly up. At some pint it will bleed out the vacuum. Good.
4.) Take off the two screws and remove the side panel of the VCV. Take a peek on the inside. Rust inside is bad. If it is a junkyard find look for one that still had the vac lines connected to it - this keeps moisture out. Look for general signs of mechanical unhappiness. Youll know it if you see it.

If it passes all of these then you have a good one, as far as I can tell.

Optional: When you have it open, clean it of big gunk that may have accumulated, and put some WD40 on the spring for rust prevention and lubrication. I also sprayed some WD40 on the outside moving parts. Careful not to get it near the valve. This all made my junkyard one a little smoother and good as new, far as i could tell.

If helpful I can post some pics. I plan to put all of this in a document at some point when I hopefully get my tranny working. Hopefully I am now on to the next phase of tranny fixing - I have replaced all of the vac components, so now its time to adjust the cable and modulator. Wouldn't it be nice if after all this the tranny actaully shifts correctly?

-dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car