Jim,
Unfortunately we did not have time to conduct a post mortem on the distribution block. We got a late start and quit about 11 last night as I had a 4 AM morning today. I would like to conduct a post mortem just to satisfy my curiosity and keep anyone else listening from experiencing similar grief.

All,
My theory…
Remember the beginning of this trauma. We had bled the brake system after finding an air leak and gained a hard pedal. (no fluid; just air being drawn into the system from a loose rear hard line) We then decided to re-bleed the front system as a precaution. That’s when the trouble started. I suspect as soon as we opened the front bleeder screw the pressure differential between the front and rear systems caused the spool in the distribution block to shift. Why the spool never returned to the center position through all of the subsequent bleeding/MC changing/tail chasing is a mystery. Likely stuck in 38 years of crud. Why we were unsuccessful at bleeding the system with the spool off center (assuming this theory is correct) is also a mystery to me. We continued to push a good amount of fluid out of all four wheel cylinders so it didn’t appear that the shifted spool restricted the system flow.

Given all of the above information my dilemma is:
• We appeared to have good fluid flow from all four wheel cylinders;
• If the system was indeed closed I think we still should have been able to gain a hard pedal even if the spool was shifted off center but not off center far enough to restrict system flow.

I still feel we had an air leak somewhere. Obviously, the original distribution block sealed as the system was functional prior to the differential being removed for rebuilding. The problem didn’t appear to be any of the hard line connections at the original distribution block as we had a hard pedal after finding the loose rear hard line. So…what happened when we went to re-bleed the front system? I wonder if it is possible that after the spool shifted we gained an air leak around the switch. I don’t know; just a thought. I’m not sure why, how or even if this could happen as I’m not familiar with the exact construction of these switches nor the distribution blocks. And I’ve never heard of a switch failing – either electrically or sealing to atmosphere. Certainly an air leak at the switch on the distribution block, assuming the system could be properly bled with a shifted spool, would explain the issue.

Unfortunately, hypothesis above is conjecture on my part. What we do know is replacement of the original distribution block resolved the problem. We don’t know why.