Recently replaced my vacuum pod on my 300d for the defrost vent as well all the other diaphragms in the other pods. I also purchased one new switch over valve just in case. The vendor also told me that since the car was an '84, it would be safe to just replace all five as he did, since there was no way new of to test the valves.
At $30 dollars a pop, i don't think so. Last time i checked, my personal labor rate was still pretty cheap. The new valve and pod came in, so replaced the pod immediately. No worky, worky for the defrost vents. Screwing around with the new valve, if figured a battery would trigger the valve to operate. It did. Holding the battery to the terminals caused the vavle to trigger. You can hear it and feel it. So i tested the my valves in the car to find that two were non-functional. I did not have to remove the valves, only the electrical connections.
I am not criticizing the vendor, as he was very kind and helpful, maybe he just did not know about this.
Hope this helps someone out as this forum has helped me.
Email me at [email protected] if i can help you more.

Thanks everyone,
airedale2