Wilkinson's explanations certainly made sense to me. He also added that most people mistakenly call it a proportioning valve. As pointed out above, his website calls it a Valve Assembly Pressure Control. The valve does not have any connection to the rear brake line and thus lacks the ability to function as a front/rear proportioning valve. All three connections are only front brake lines. Again, Wilkinson explained that the valve's purpose is help reduce chatter on the front brakes when the discs become warped. It may be best to call him for more details.
The two valves that I gutted were quite frozen up. After nearly 50 years they were no longer functional due to corrosion and hardened rubber parts. Thus, as I mentioned earlier, gutting or removing them is appropriate. There is no rebuild kit for them and they tend to be an air trap.