Got the proportioning/combination valve back from a Performance Years website member, 70RAIII (Bill Oxley) who did a really nice job rebuilding it. He indicated that it was all stuck inside, so it wasn't really functioning as anything other than a T-fitting between the front and back fluid routes. Now it's actually working as designed, which is a good thing since if I recall correctly, one of the critical roles of this valve is to internally shift and block the internal passages if one circuit springs a leak and loses fluid. That way you still have braking power for the remaining circuit.
Another thing I noticed is that the purple DOT 5 silicone brake fluid turns yellow after a few years. I'd imagine that means it's time to replace it all, which I did. I flushed the entire system and replaced it with new DOT 5 silicone brake fluid, about four bottles worth, to get all the old stuff out and completely flush the system of the four year old fluid.
The newly refurbished proportioning/combination valve, just back from Bill Oxley:
And after the installation and the brake bleed-a-thon:
