I had passingly read about the difference between 1/2 and 3/4 ton routing before playing with mine. Actually, my booster and master did come from a '71 C20 if I recall correctly. It appears that I was thinking a bit in the wrong direction the other day. I looked closer, and the larger reservoir is in the rear on this master cylinder which should mean it is the one meant for the disc brakes. This would indicates that the original line routing was correct. Still, line routing from master to combo valve shouldn't make any difference in the amount of pressure metered out. The combination valve should take care of that as long as the outlet lines are hooked up correctly. And according to this diagram, mine are.
Either way, with the intermediate lines now in the 'incorrect' routing, we bled the brakes again yesterday and I went for a test drive. The rears still lock up first, the pedal was pretty mushy and the front brakes could be heard dragging very lightly all the time. I think I've got a big air bubble in the front circuit because there is a damaged bleeder on the driver side caliper. I'm going to replace it with a spare I've got lying around, bleed again and see if that corrects the braking. If all is good after that, I'll just switch the intermediate lines back (then bleed yet again) and call it a day. If not, I'll be looking into buying a new combination valve I guess.