Sounds like you have an adjustable proportioning valve. Amazing how helpful photos are when you ask questions. It is not a simple valve, but rather a spring-loaded differential pressure regulator. The knob sets the dP. You shouldn't need to adjust it to bleed the rears. If you press the pedal hard enough, it should open and flow to the rears. I'm not even sure which way the knob works. I assume that as you screw the knob cw, the rear braking gets stronger. That means it opens at a lower pressure, so wouldn't hurt to screw the knob all the way "in".
