That's different then. Whatever it builds up to it should hold that when you shut the engine off. If it does that eliminates a leak as the suspect. If there's no leaks I would suspect a weak pump, especially if it does the same thing at the other nipple. If the vacuum build much more quickly at the other nipple then it is entirely likely the nipple in question could be clogged.

One the of oft-mentioned online articles about setting the transmission vacuum gives a drill size for reaming the orifice in the nipple when it gets clogged. Of course I don't remember if they are both the same size or which site the article was on. You could always try poking at it with a paper clip.

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'82 300D - Light Ivory, 2nd Owner (Back in the wind April 2013!)

'95 E300D - White, grey interior. (Suffering from stuck/broken glow plugs)

Deuteronomy 22:4-
"Thou shalt not see thy brother's ass or his ox fall down by the way, and hide thyself from them: thou shalt surely help him to lift them up again."