I called Pirate Jack and asked where his parts were made. He said they came out of China and were copies of the Corvette MC.
I know you need a proportioning valve. If you use an adjustable one, you will need separate residual valves. I think most non adjustable GM type prop. valves have residual checks built in.
A good shop with good US made components will tell you the whole story, I'm sure.IMHO

'50 GMC