Never heard of a residual valve on any 1967 or 1968 Mustang. It does not exist. What are you talking about?
I forget (as a mechanic) and use the nomenclature that gets me the part I am asking the part stores (not a Ford Dealership necessarily).
I call these residual valves. They can be also called proportioning valves.
Next time I'll refrain instead of using the wrong terminology. My bad, hand spanked, I behave now