I now have servo-assisted brakes and it is a wonderful thing.
After my local garage dropped the pipe off last night I fitted it and went for a successful test drive.
It looks like the fault was the acid-coroded one-way valve.
Here is a pic of the old pipe :
and a closeup of the damaged valve :
I did some blowing tests on this pipe. From the servo end you could blow but not suck, as it should be, but from the manifold end you could do both as air was escaping from the valve.
Here is the new pipe :
and a closeup of the label if you need to order one for any reason :
Below you can see the new shiny connector on the intake manifold :
and the pipe fitted, with the valve near the Y-piece in the vacuum pipes :
Finally, here is a photo showing where the pipe goes into the servo – it is awkward to see, and awkward to get to :
The servo pipe is coming in from the bottom left, and has a push-fit just left of where you can see “SAAB” in white writing.
I am happy now my brakes work, and at Â£20 this is one of the cheapest problems I have had. Hopefully my new battery will avoid such issues in future.