Marvin, have you checked fuel prices? Unless you're getting a never to be repeated great fuel price or venturing out on an around the world journey, most would be lucky to carry half full fuel tanks :-) FWIW, I leave the top valves open and the bottom closed. The rationale is to totally eliminate any pressure on the sight glass tube that looks like clear water tubing. Gil When I set these up I. Plug off the upper fitting run the tube up as high above the tank as I can and put a U bend in the top of the tube to vent it with a clip to suport it. One less valve one less opening in the tank and it reads all the way to the top of the tank. Brian Palmetto FL