[provocating]

I went through all of this with my 1979. I had the same sealed proportioning valve on mine, could not rebuild it. My brake pedal was going to the floor, one of the rear calipers was stuck even though it looked brand new. I replaced the proportioning valve, replaced that rear caliper and finally after a good bleed I have a full pedal and the brakes stop perfect. Now I routinely pull my emergency brake once a month to keep those rear calipers adjusted.