fuel flow rate depends a great deal on generator load. and even at full load it is slow. IIRC at 1/4 load 1 gallon last 8 hours. Thats about 13 ounce/hr. and any length of hose could hold several to 4 or 5 ounces. so fuel flow will be slow. even at full load it is 4hrs/gallon. I had clear line on one setup and even at moderate load it took 1/2 hour or longer for the generator to use enough fuel to fill a 3 ft fuel line.
as for vacuum being measurable. if the vacuum gets very high at all the generator will stall. see what happens when you forget to turn the cap to "on"