Paul, one last thought on the vapor lock thing.  Very glad you have found that it was the problem.  There is nothing worse than not knowing. 

 

It is quite possible that changing EITHER of the two variables (in-line Facet or vent on the header/sump tank) will make the system work.  Just because you remove one and it works does not mean you have found the root cause.  Earlier I used the "strikes" analogy, and we don't know how many of these it takes to make an "out".

 

  From your background I'm sure you know that it is frequently a combination of things that can cause a failure.  Removing one of them may 'fix' the problem now but add in other variables like fuel vapor pressure, altitude, etc, and the problem could come back.  The goal is to make the system as tolerant of all factors as possible.  Happily, this frequently means making it simple too!

 

One other thought on simplicity.  I have never landed without power (in my rotary powered plane) but I have come close several times in the early days.  The only thing that prevented it was having a fairly simple system and knowing it very well.  This helped diagnose the problem and fix or compensate for it before running out of altitude and ideas.

 

Tracy