This happens to me often as well.  I have found that the mechanical pump can
still pump fuel.  Try cranking the engine over a few times, then retry the boost
pump.  Eventually it pulls the bubble through.  Once it starts,  watch your fuel
pressure and perform a pretty healthy run-up to be sure it has cleared the
system.  It only happens when parked hot for a fair amount of time, like while
your eating lunch.. I always open my oil door to vent the hot air when parked. 
This helps.  I have not found where this bubble forms, or how to stop it. 


Tom McNerney
www.N54SG.com