The master cylinder requires you to block off the fluid out connections with your fingers when you bench bleed them. You can't just route the output back into the reservoir. Did you do that?
I did, both on the bench and on the runner. Before I swapped the MC, the rears would only dribble out brake fluid. Now with the new MC, more fluid is definitely being pushed out. Going to try to bleed the entire system again tonight and double check the drums being adjusted as a last shot effort.

