Your proportioning valve is what would have come on your Roadrunner. I've had plenty of 72-73 Challengers and Cudas with that exact same valve. You can clean it out and rebuild it. I've got one just like it on my 71 Challenger conv. That car will stop on a dime and give you 9 cents change!
Good to hear haha. Do you mind taking a look at my post just before yours where I mention the shuttle valve? I have everything disassembled except for that. I'm not too sure how to remove that shuttle valve.