The heater control works on a bi-metal strip from the hot air travelling past it so as the air heats up it closes off the vacuum line with a pin that goes through a rubber  which then reduces the water flow. I pulled mine apart and got it working, it works very well now. Mine was seized so I put a bit of rubber grease on the pin to hopefully preserve the rubber. It was a bit of a fiddly job.

Thanks Ian. Sounds like a very problematic device. I understand now why Ford changed it in 57.
Rono

  Ron Lane,  Meridian, ID