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The diagram can be somewhat confusing. There's two vacuum operated ventilation flaps in the 993. Resirc flap (item 32) and the fresh air switch-over flap (3c) in the engine bay. The first is a known source for ventilation air flow problems, and the latter selects the air intake depending on if you put the car in reverse or not, and not very critical for ventilation functionality.
I have never looked at any vacuum hoses to the brakes, so I can't comment on that.
I've already eliminated the functions related to the rear blower motor. I'm switching to manually controlled heater/ventilation functions, as well as a non assisted dual master cylinder brake setup. So it looks like I'll be able to completely remove the vacuum lines that go foward.
(With the exception of the fuel tank related lines)
The recirc flap for my HVAC system gets sucked closed by the airflow when I accelerate. I think that the inline check valve (the upper instance of #8 in the diagram) might have failed. Can anyone show me where this check valve is located?
On a related note, there is a black and white plastic valve that connects to the intake manifold, which I think is a check valve. It is located in the center of the photo below. Mine seems to be failed closed, I can't blow air through it in either direction. Which valve in the diagram does this correspond to?