I gather this is a new system you are debugging? You didn't use .5 synthetic fluid on a non-compatable system did you? Some 'o' rings swell up with the synthetics.
Another tought is when the vacuum hits the brake pedal and pulls it down just make sure that the master cyl piston isn't being affected - i don't think so from your description but something to check. What'll happen is if the master cyl piston cannot fully return to ist' seat then it'll act like a check valve and the fluid won't return to the bowl - when that happens all the wheels will remain tight, not just one.
A stock proportioning valve is more complex than you think, there is an internal floating balance bar inside it. If one side or the other has no fluid when pedal is depressed that flaoting bar will go to its' limits of travel and cannot return and needs to be reset. I don't recall how to reset it, sorry, this subject came up one time and i didn't catch the whole thing as i am slow and was absorbing that there was more to a proportioning valve than i thought.