T&T: Water System Mystery Answered, Not Solved

[B_Hales]

  <Snip> Ron, don't forget to check for "mud-daubers" (wasps) in the water tank vent
line.  Normally, this is the problem when the water from the tank is
restricted.  Water flows fine until the vacuum develops and then it slows.<Snip>
    
  Removing the filler cap will tell you if it is blocked or not. No filler cap, no vacuum. It happened to me once. 
    
  Regards, 
    
  Brett