2000 770 volvo low on fuel in the water separator

hi I have a 2000 770 Volvo d12 and running low on fuel in the water separator so beetwen 2-3 oil changes the fuel level doesn't raise and even the engine is running like with less power.as soon as I put like 2-3 inches of fuel in the water separator then prime the hand primer I can hear like a squirting on top of the head around injector 2-3 is that normal? after that the engine runs much nicer for like 300-400 miles and again the fuel disappears from separator and engine runs like having hard time to get fuel to the injectors , like when pushing the gas pedal down it is getting false air(in the water separator cannot see any air bobbles).if u have an advise please helpThanks

 

I don't know have to check cause there is no sticker on the davco,thanks

 

how can I know if its a fuel pro 382 or different one? as I was saying there is no sticker on the separator.

 

yes exactly like this.

 

thanks you.