The factory shop manual has a lot of information including a description of how a proportioning valve works. This is the 1967 Mustang / Cougar / Fairlane / Falcon/ Mercury Intermediate manual but there is a similar section in any year Ford product shop manual. So look at yours.
Or use Google or any other search engine.
I’m no expert so educate me. How about posting some references,articles,videos etc... so that I may get a better understanding
Dan