No response in brake section, sorry. Quick question!

The valve you're asking about is called a Hold-Off Valve, and it does exactly what you describe.
I converted my 69 Dart SBP disc to LBP disc brakes from a 73 Dart, and used the original prop valve and hold off valve from the 69.
I would recommend it. It ensures stability in a panic stop situation. If it wasn't a good idea, the factory would never have installed it.

George