The idle screw is leaking past the loose running (finger tight) threads. I had thought there may be an internal O ring inside to act as a seal for this - but there is not - least not on mine.
I don't think you can seal the system for this loss, and consequently can't hold a static test pressure.
Unless someone has experience to the contrary.
Alan

Don't have experience to the contrary but it makes no sense Porsche or any engine builder would go the the trouble to assure all air go through a filter to keep dirt, dust, sand etc. out of the engine but allow dirt, dust, sand etc. to be inhaled through a vent to atmosphere.
Does the screw seal when turned all the way in? Add an o-ring seal?
Is this leak the only remaining leak? If so, it's got to be miniscule and the clock gage should be very close to reading true system pressure.

Next time I have the manifold out, I'm goingt to look at whether a seal can be fitted - but there seems no provision for one currently. At 14 psi boost, it is leaking about 10-15 l/min. Probably small in comparison to the cu ft the engine would suck at full boost - but a definite stream of gas and hissing.
The boost gauge is reading OK now. This is the only leak I can find, but have no idea how to rectify at moment. But it is much better than when I first set out looking for leaks.
Alan