Proportioning Valve Keeps Tripping?

[KyleSeal]

1967 C10 - Disc conversion (power 8" booster) - master cylinder is 1" bore, and the proportioning valve was advertised when bought in the kit as a disc/drum setup.

I am having an issue with my proportioning valve.
When I go to bleed the brakes, while using the proportioning valve tool to center the shuttle valve, everything bleeds fine, no leaks and good strong flow. No air coming out anymore, I did plenty of extra pedal strokes to ensure this and had no air that I could see come out.
But when I put the brake switch back in, bleeders all closed, and press the pedal down half way, the red brake light illuminates. I have pulled the switch and confirmed the shuttle valve closes the rear brakes. Recenter, does it again.
I confirmed the rear brake shoes are at their proper adjustment with a brake drum/shoe caliper tool to get the measurement even.
Even after adjusting and all this bleeding, it seems like its either seeing a fault at the rear.

So my question essentially is how do I move forward from here?
From the factory, the front brakes are fed by 1 line off the front of this proportioning valve down to a T fitting split to both sides, and obviously rear is one that goes to a T on the differential.
Someone did say my front feed being at the top fitting (pictured) is an issue, I don't see this as an issue being this should be able to balance it, am I incorrect?