Heater Control Valve Requirement

I have an under-dash AC knee-knocker box in my two A's. I put a valve in the heater hose of all, using a small vacuum switch to a vac-operated water valve. I tapped the "on" wire inside the knee-knockers. As a half-measure, you could tap the AC clutch wire since already under the hood, but that would only stop flow when the compressor is running. Wasn't a big deal since the AC has a separate cabin fan, but I figured why not do like Chrysler did in my 1969 Dart w/ factory AC.

In one I used a little vac switch from a Porsche ($10 ebay) and in others a vac switch from a 1980's Mopar (PickNPull). The water valves have 1 vac line w/ spring to reverse. One is for a 2000's Ford ($8 ebay) and other a metal generic one ($12 ebay). I did the same in my 1985 M-B 300D since the solenoid water valve ("monovalve") is notoriously problematic, especially with the poor rebuild kits from MTC (India). Later M-B used the same setup I have, and indeed one company packages that as a retrofit (pricey w/ M-B parts). If I recall, my 1969 Dart factory water valve had 2 vacuum hoses, but most today use a spring return.