I haven't found any info how to best test the vacuum amplifier on a 1985 300D.
I even bought " Mercedesource" booklet on Tuning Diesel Transmissions and it doesn't have anything on testing the amplifier.
On my car, the vacuum modulator tested fine, plenty of info on how test it, but when it's connected to the vacuum amplifier, the transmission starts to slip between 1 and 2nd gear so for now I block vacuum to the amplifier.
Using a vacuum pump on the top middle fitting of the amplifier and blocking the fitting on the top back going to the transmission, I can pump it up to 20 psi of vacuum but it doesn't hold. Thanks for any help.

This info could be helpful:

From Mach 4, Benz World,org:

380SL diesel
Joined Jul 21, 2011
1,275 Posts
#9 • Jan 13, 2014

...now I just have to make sure the vacuum is sustained from the vcv all the way to the UFO...
The quickest test would be to tee into the line going to the vacuum modulator on the transmission from the "blue flying saucer". You should see a high reading, probably 12" at idle and as the throttle is pressed, it should go down to almost 0". I don't know exactly how the vacuum amplifier works, but it obviously takes an additional pressure signal off the ALDA and "adjusts" the raw vacuum coming from the VCV before it goes to the transmission. (Turns out I just happened to do that test on my engine today as part of another project).