Phil, I can't answer your question as to how the original master cylinder was built. I'm learning all of this stuff for the first time myself. I'll take a look into the m.c. that I took out of my truck and see if it perhaps has a built in check valve of some sort.
The recommendation for a 2 lb. residual valve, though, is for disc brake applications. It certainly wouldn't have been part of our original systems.
The one question that I still have regards the adjustable 10 lb. residual valve for the rear drum brakes that is shown in the drum/drum system. In other diagrams showing the plumbing to the rear drum brakes there is an adjustable valve installed separately from the 10 lb. valve, but nothing said about what that valve does or how it's adjusted. I understand that you want the rear brakes to come on a little earlier than the fronts. From the threads I've read, that action apparently happens inside the master cylinder itself. So what is the adjustable valve for?
The valve you are talking about i believe is a proportioning valve. we used them in dirt track cars to get the front to back braking to be either more or less pressure in either direction. In the case of a truck, if you have no load, you could use it to reduce the pressure to the back so they dont lock up, and have more pressure if its loaded. look at them as if its a trailer, because they use the same kind of system on trailer brakes.
