hula ko leaking fuel pump yan OR the line leading from the pump may leak. the fuel leaks back to the tank kaya empty yung fuel lines. when you switch to ON the fuel pump runs and fills the lines kaya ok, pero pag di pa umandar yung pump at start mo agad palyado kasi nga may hangin sa fuel lines. it could just be a small leak (sabi mo nga 12 hours or so na stop) kaya ok pa ang fuel pressure mo.