During the 1970s and 1980s Kelsey-Hayes supplied a cast iron combination (pressure differential, proportioning, and metering) brake valve to all of the major domestic automotive manufacturers. Here's an example that I removed from a 1979 Camaro:
Given how broadly this type of valve was used I've spent a considerable amount of time researching the parts needed to rebuild them. Disassembly is described in an autopsy article that I first published in July 2011.
I'm making progress. I now have kits that can be used to rebuild the pressure differential and proportioning components of this combination valve. The metering component is proving to be more of a challenge due to the cost to reproduce a specially-shaped rubber seal and the rubber dust cap. In the mean time, I've discovered that the same assembly was used in a Kelsey-Hayes metering valve used on 1970 and 1971 Mopars, and a reproduction of that valve is available. If you buy the reproduction valve and disassemble it you can use the internal components to complete the restoration of your cast iron combination valve.
I'll continue my research in the hope of finding a less expensive source for the metering valve components. As usual, please feel free to contact with me if you have information to share or would like to learn more.