Remember that boost jets do not vent, they simply restrict flow. The stock boost solenoids were responsible for bleeding boost to atmosphere, thereby limiting the signal reaching the wastegates. My guess is the brass fitting takes the place of the solenoid vent, and the MBC acts as an adjustable boost jet. Unless the MBC has a vented port, the system is sealed and turning the knob has no effect. I would have expected the port to be on the valve body, but then again, I'm just guessing.
