I'm interested in this discussion as I think a gravity type filter might be something I'd want on future trips.

I'm curious about something? I take it your all filling the bag directly from the lake, either scooping the bag full or filling with a pan.

From what I can tell, pollen/sediment is the problem after repeated uses.
Do you use anything to pre-filter the water before adding to bag?
I have a very fine mesh bug net that I use to pre-screen my water before I purify with tablets. I also use womens knee-high nylons to do the same thing. Stick the pot inside and that will pre-filter the pollen/large sediment before it gets to filter and becomes a problem. Just using the fine mesh pre-filter I can't see any suspended particles in my drinking bottles, although I'm sure there is some fine sediment etc.

I think that would go a long way in extending filter output before you had to back-flush.