Question, does the mower run fine. If so then don't worry about how much fuel is in the fuel filter. A lot of people think there is a problem if the fuel filter isn't full. The truth is most mowers run just fine with very little fuel in the filter. If the fuel pump was before the filter, it would purge the air because of the pressure. But since the pump pulls fuel through the filter it causes a small amount of cavitation within the filter, which traps air in the filter.
Gravity feed fuel systems tend to keep more visible fuel in the filter, because of lack of vacuum/cavitation of the fuel.
