I am going to place my bet now that its a leaking diaphragm in the turbo vane actuator.
its pretty easy to check - attach a bit of vacuum tube to the actuator (pull off the line that goes from the N75 to the actuator), suck on the tube you connect and see if the actuator moves, and if it holds the vacuum in there.
Even easier if you can tee in a boost/vacuum gauge, that way you can just see whats happening on the gauge.
If its in good shape, it shouldn't leak at ALL.
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