No idea what the issue is, but i'd at least start with a flush and see how much crud comes out. I've had multiple rear brake balance problems in the past and the specs of dirt in the proportioning valves that caused it were pretty small. Doesn't sound like you have that valve, but i'm just mentioning it as it build up caused lots of headaches. So any build up could be contributing. Someone else will have had this issue before and comment i'm sure. But with old shit, and lack of fluid changes, build up occurs and can cause issues. Just something to start with anyway. I think the cars I had issues with were split diagonally for braking. Current old cars have a tee on the diff to split the single line from front. If no crud in the line to the rear and no build up in the rear tee (could be worth whipping off and cleaning out). Then yeah I guess something is up with master as that is the only other part in the system.