proportioning valve

I've never found anything like what you speak of. The only place to get originals is in the junkyard. What's the problem with the one you have?

Yes new ones are available. I just purchased an AC Delco combination valve for 4 wheel discs, the disc drum are also available.
As said they rarely go bad, what problem are you having? Send a PM if interested in a good used one.

I have in 25+ years of fixing cars only ever ran into one bad prop valve (they are actually a combination valve). That was the cast iron valve I had on this Nova when I bought it. I replaced it with a used brass valve I got from Philip.

These guys sell new valves.

http://inlinetube.com/

I know there are other places too because I've seen them. I'd see what Philip has and if that doesn't work out check my link. I'll bet a Google search will turn some up too. Post back up if you still have trouble. I like a good scavenger hunt