Could be one of three checkvalves on the vacuum pump....could be the vacuum pump itself. I'm sure you've checked for leaking lines, yes?
As Mistress posted....a post in the diesel discussion, will get you lots of response - one of which will be to use the search function, and browse the archives. (just so you know what to expect)

__________________
It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D