I had pretty much the same problem as you have. I had checked all the gaskets and still could not find the leak. It was also happening at 7psi and above. I had replaced all my vac hoses & all new clamps. Finnally I made this up and found the leak in about 5 mins.
Remove your intake from the front of the turbo, Attach the hose with valve to the front of the turbo, pump up using compressor. (no more than 10 psi) You should be able to hear the leak.
(Engine off, It will not hold pressure long. Just enough to find the leak)
Parts used to make this are
1 right angled silicone pipe from old stock intercooler pipes (or from any auto store)
1 screw in tyre valve
2 hose clamps
1 plastic pvc end cap (bought from a plumbimg store)