1980 300sd tranny vacuum questions, broken mightyvac?

[dave1980_300sd]

Hi everyone,

This is my first post... I've been lurking for a while and have read a lot of the archives, but I'm having trouble finding the answers to a couple of specific questions on my 1980 w116 300sd (seems like a bit of a rarity around here).

My transmission flares quite a bit and is just generally not quite right, so I bought a mightyvac and followed the instructions for checking the vacuum going to the modulator (using primarily info from Brian Carlton's posts). The line going to the tranny doesn't hold vacuum for long, so I'm assuming I need to get under the car and replace the vacuum cap on the modulator. However, I'm not sure what the vacuum should be at the VCV when I put the mightvac in place of the transmission line. Right now it reads about 17" at idle. This seems high, as I read in an archived post that it should be around 8." However, I'm not sure 8" is correct for a 116. Does anyone know what the correct vacuum should be here on the w116 so I can adjust the VCV properly?

Also, I'm wondering if I may have gotten a mightyvac that was bad out of the box. Are they supposed to hold vacuum when you release the handle? Mine does not... the second you release the handle, the vacuum is released. It seems like it should hold vacuum, right? I think I'll return it and get another one just in case, but from what I've read around here it seems like it should be able to build up vacuum with consecutive pumps, which right now it cannot do.

thanks for any help you guys can provide... glad to be aboard the forum. it's already helped me a lot.

- dave