ok, to the topic of crank breathing system in a Porsche 928:
In models S4/GT has a combined crank breather system, which has both a Main breather system, and a safety breather system:
A) Main breather system:
At the main breather system gasses from the crankcase are transferred from the crankcase to the lower part of the oil filler neck , and from there via a hose without any safety valve directly to the right cylinder bank (US driver side). In the valve cover an oil separator exists to loose most of the oil, and then via a angled connector piece with a valve (hose) to the intake valve assembly where the remaining gasses are is sucked in and burnt.
B) Safety breather system:
At the Safety breather system gasses from the crankcase are also fed into the foot of the oil filler neck, but from there directly via a connection with a safety valve directly into the intake valve assembly.
Function of this Safety system:
The safety breather system ensures that if for any reason the passage via the normal route is blocked (like buildup of ice) the safety valve opens based on the pressure difference and allows the pressurized gasses to escape to the valve body. The safety valve opens at a pressure difference of 0,08 to 0,1 bar.
Ok this is theory. If this icing of the hoses ever happens remains to be seen.
If the disabling of the safety system by plugging
the hose with a screw or so could ever cause any defects is unclear. With a
disabled safety system there should be no problem in normal circumstances.
Porsche apparently closed this line at some cars to eliminate gasses with oil
escaping via the safety route, bypassing the oil separator, and consuming more
oil than usual. This is just assumption and not good practice to
eliminate oil consumption.
Here is a picture of the later model (GTS) oil breather modification: