Need some help w fuel system. Mower shut down on me while cutting a ditch. Incline was mild. Found right side fuel tank empty and I can visually see the right side fuel filter is partially filled w air. Left side tank is completely full & left fuel filter has no air in it. Obviously the engine gulped some air but why would it suck in air from right tank as it went dry? Never had this problem before when either the left or right tank emptied out. Any suggestions?