KH disk front 83/4 rear, prop valve and master cylinder brake line question

The proportioning valve doesn't begin to play a role until line pressure is up around 300 psi. or something like that.
Its in the pamplets I linked to above.
They also explain the concept and why its used so I'm not going to repeat it here. Whom am I anyway? Some guy on a keyboard. Take it direct from Chrysler Tech.