I'm going out later today to visually confirm the rear port of the MC is plumbed to the read port of the combining valve. I'm fairly certain it is but I'm going to double check.
Every possible next step hinges on that determination.
The combination valve for disk brakes is not really a valve at all. It is a junction/distribution point with a differential switch. If there was no requirement for the differential switch, it could be replaced with a splice for the rears and a tee for the fronts. Not much to go wrong that can cause a pressure differential inside it. Same for the steel lines. If the line is 800 pounds at the master cylinder and 500 pounds coming out of the "valve", this is a head scratcher.