any experience with vacuum assist brakes

[Nicholas Lawrence]

I just installed a vacuum assist on my '90 TQW and am getting no brake
assist.  I have vacuum at the port on the booster and I believe I can hear a
leak coming from the the booster.  Plugging the line to the booster does
change engine idle which I think further points to leak.
Other than removing all parts from donor car I don't know any history.
Parts came from '83 5K.    Do theses boosters have a history of leaking with
age?  Any point in taking it apart?  TIA
Nick