[rickf]

The combustion chamber and crankcase scenario is not the leak I mean.

What I mean is that the manifold is connected to the PCV valve, which is in a valve cover.
The space under teh valve cover is connected to the crankcase (it has to be, or it would not work to vent the crankcase).
So the space under the other valve cover is also connected to the crankcase.
In that other valve cover there is a filter "connected" to atmosphere.

So there is an air flow route thorugh the valve cover filter into the valve cover, through the crankcase, up to the other valve cover, and through the hose to the manifold.
It seems to me that qualifies as a vacuum leak.
But it must be that the flow is sufficiently restricted or otherwise accounted for, because it seems to work fine.

But it also seems to me that valve cover filter could be plugged off, and just let the crancase run with a vacuum. Is that not okay? Is that not the same as the "closed system"?