For those leaning towards the prop valve, here is the section where the manual clearly states its for metering pressure to the rear brakes to prevent lockup.

I have done this swap several times with great success, I'm starting to think I have a defective master since the rest of the system looks to be up to snuff. Stranger things have happened. I have a lightweight manual master with adapter on my Dart, I'll try swapping it and see what happens.