[wilkin250r]

At this point, I wouldn't trust too many "assumptions". I'm not 100% sure how the proportioning valve works, but I know a thing or two about troubleshooting. You've gone through all the likely scenarios, it's time to start testing unlikely ones.

A fautly proportioning valve does indeed seem like the logical culprit. You have essentially eliminated blocked lines and frozen calipers, and you have replaced the master cylinder. The prop valve is now the only place left I can think that would allow pressure to build in one circuit of your brakes, but not the other.