The master cylinder decodes as a 1" bore, which should be correct for a factory KH disk/drum system. The larger reservoir does not change line pressure at all, the line pressure change comes only from the bore diameter. The reservoir split is different for a disk/drum master cylinder because each type of brake requires a different amount of fluid volume to activate. So the line pressure out of the master cylinder should be the same for both ports, just backed by a different volume so it doesn't suck air. The combination valve will change the line pressure balance front to rear.
Other than changing the master cylinder and distribution block, did you do anything to the rear brakes at all? Because locking one wheel in the back is only possible with trapped air, a brake line issue, or a brake adjustment issue on that side. The master cylinder and combination valve sends the same pressure all the way back in a single brake line, so, to have one rear brake lock and not the other means the issue is on the rear brake side of the line split at the rear axle.
