Low fuel in the filter

I have been having problems with fuel level in the filter its always low real low where it sucks air when i turn it on and then it stubles and sometimes turns off. Level will always stay low even after 10000 miles of driving. I have changed the fuel check valve at the back of the engine and it made no difference Anyone got any ideas what to look for? It also has developed harder starting where it has to crank for about 5 sec till it catches. After i shut the truck of i have to fill up the filter up so next time i turn it on it doesn't stuble and turn off this way it doesn't suck air. Im at lost so any help will be appreciated.

 

Yes that is the one drop in fuel filter in the glass where it fills up when it gets dirty. Mine never gets full highest i have seen it go was 3 inches of the bottom (TRUCK- 2003 freight liner Columbia with series 60 12.7) So far i have changed fuel line and fuel check valve. What should be looking at next? I should mention after replacing check valve level did raise up to about 80% dirty i then changed the filter and the level problem came back after 3 days of driving with the new filter i can see the bottom of the bowl and i can also see air bubbles. Is it normal to see any air bubbles pushing out? My next thing was to remove and change valves that sit on top of the transmission which are connected to the fuel tanks on both sides?

I was also going to look at the injectors might need new cups? but it never misses under load it will miss if i sit at the stop light for few minutes then when i leave it might miss for a sec or two and the its goes smooth and i am using Baldwin filters

 

The glass bowl gets small air bubbles coming out this could be normal im not sure i really can't remember what it was like when it was all normal. I am planning to take it to the shop for overheads injector cups and injectors inspection as a maintenance i will ask them to look in to it and see if they can figure something out. Till then i will be looking at every valve that is connected to the fuel line like the ones i mention on top of the transmission. I was told by a shop that replaced my fuel check valve that there is another one closer to the bowl and if my fuel check valve back at the engine does not fix it i should check it. Which valve is he talking about he said it has a small ball in it and can get stuck any one know where this is at?