Thought I would update this thread to the current state. With this virus thing going on Cass is unable to talk on the phone for some reason (I didn't ask why), but he is emailing. And we have been doing a lot of that the last couple of days. He is so great to deal with, his customer support is second to none.
So, we are both learning more about master cylinders and wheel cylinders. Cass has talked to the manufacturers of both and taken parts in his stock apart to get this figured out.
What he learned is the the 15/16", 1 1/32", and 1 1/8" aluminum master cylinders that he sells for disc/drum manual systems do not have any residual pressure valves in them. But the 15/16" iron mc DOES have a pressure valve installed in the port for the rear brakes only, and this is the one I have.
He talked to the somebody from where he gets his wheel cylinder from and was told "A modern master cylinder does not incorporate a residual pressure valve because modern wheel cylinders are designed to keep air from entering the system when not in use". My next question to him was should I be removing the valve in my mc, and he was told no, leave it in.
I was not aware how popular the aluminum master cylinders are. Cass says he sells them 100 to 1 compared to the cast versions.
So long story short, everything I have is supposed to be compatible and we are thinking I have a defective prop valve switch which is shorted to ground and bringing on my dash warning light. He is sending me another valve. This is the third one going in this car as the first one wouldn't seal because of a ding in the valve body. Actually the second one he sent me had a bad surface on the top nut so I had to make one out of 2. These valves are such a pain in the a$$ to replace with the motor, steering, and headers in the car....