Brake help! Exhausted ideas...

[Ride The Snake!]

Hello everyone and thank you for taking the time to read this.

Brake issues on my '70 GMC. Manual 4 wheel drums, dual-chamber m/c.

Have had multiple issues with the brakes, have replaced everything that came up, and I'm still stymied.

Hard pedal, almost zero front brake action, when it does grab a little the front right grabs more. Cannot get a good bleed from the front brakes.

SECOND New mastery cylinder, properly bench bled (first one I did I might have messed up during wheel bleed by bottoming it out, but I never got the front brakes to really work with it, either). New wheel cylinders all around. New shoes all around. Front brake soft line sections replaced.

My understanding is the "combination valve" below the m/c, even if failed, will not impede the flow of fluid. But that is the last component (other than the hard lines) that I haven't replaced, along with the soft line that runs over the differential.

Do I need to consider a possible clog in the hard lines to the front? What else?