Boost control is done by the link using a mac valve.
If there was a leaky diaphragm wouldn't i hear it leaking out the port on the mac valve when i pressurized the intercooler pipping. Same with a leak in the vacuum/pressure hose except i would hear it leaking from the hose.
In the high gears boost shoots up the 15psi then slowly creeps to 18psi. Normal?