Proportioning valves come with a release button on them; at least the factory versions do/did/might have. For bleeding a completely new system. that release button has to be depressed and kept down. That allows the proportioning valve to "free flow" meaning that it isn't trying to maintain the residual pressure in the rear circuit that it is in the system to do. In that free flow mode, the brake fluid can move more easilly and readily out into the circuits.
Once you get the air and frustration out of the system, and the brake pedal seems to be consistent for feel and action, then release the button and the valve will go back to keeping line pressure resident in the rear circuit
With disc front/drum rear, the final test is with the engine running and vacuum on the brake booster. For myself I have taken the extra steps of replacing both the vacuum hose from the intake as well as the one way valve that plugs into the booster body on several occasions. Both are cheap and neither tends to want to tell you that it is about to betray you.
Nick
