Good call! I was about to jump in with the same suggestion, but you discovered the right info first. A proportioning valve is there to allow the rear brakes to apply slightly ahead of the front to avoid front wheel lockup during slippery condition driving like on ice or wet snow. The "combination valve" that most dual circuit master cylinders have serves several functions, including isolating one end or the other of the system in case of a pressure loss. It's necessary to disable that function if there has been a major fluid loss, like replacement of both rear wheel cylinders, for instance. The valve interprets the total lack of pressure in one end of the system as a catastrophic failure, and isolates that part of the system.
Jerry
