The vacuum hose on my 9K's PCV system fell apart (brittle) and when I also tested the one way valve, found that it was blowing both ways. (which would account for the intermitent rear main seal oil leak)
I have 2 questions that I am hoping someone can answer for me.

1)Which way should the flow be in the system. From the Rocker (Valve) Cover to the Inlet Manifold, or vice versa?
In other words, should the one way valve prevent pressure flow back from the inlet manifold to the engine, which I think would be the case when the Turbo has spooled up?

2)What is the Internal Diameter of the vacuum hose fitted by the factory?

The one way valve should allow flow from the cam cover to the throttle body but not back the other way as you guessed. It prevents boost pressure getting into the crankcase which normally shows by oil staining around the filler.

The hoses used for replacement are normally 4mm silicon (lasts longer than rubber). However, for the breather one way valve I suggest using the Saab 'Eco' black pipe. Silicon seems to 'sweat' oil in this location. The valves are avialable from Saab parts suppliers on their own or that used in the 9-5 is cheaper than the 9000 one for some reason and it is stronger and comes with some pipe.

Thank AlanG.
I have ordered a genuine oem one way valve in from the USA, as my local supplier is out of stock.
In the interim I have fitted an aftermarket clone of a 9-5 valve and new 4mm silicon tubing, simply to get my 9000 mobile over the Christmas & New Year period.
The Breather/PCV connector plug that fits into the rear of the valve cover has also broken, so I am waiting for one of those to arrive as well. (Boy was the whole system rotten)
The plug should be here early next week.

An update on this.
I have renewed all the hoses, plugs, rubber grommets and the check valve in the PCV system.
The result is that fuel consumption has dropped slightly, the engine idle is slightly smoother and a small oil leak from the rear main seal has stopped.