I suspect that the “error was air” using an outdated backflush procedure BPL followed. The printed instructions included with my filter (4/08 version) have been updated in the video now shown on their site. My 4/08 manual states:
Backflushing the Filter Cartridge
Always perform backflushing in a controlled area to prevent the loss of small parts.
1. Collect ½ liter of filtered water into a clean MSR recommended container.
2. Remove Inlet Hose and unscrew Pump Inlet from Pump Cylinder. Do not pump
because trapped air will hamper operation. (See Troubleshooting.)
3. Reverse the Large Check Valve and reinstall it inside Pump Inlet.
4. Pull Pump Cylinder to end of stroke and turn until Piston locks.
Use cylinder to unscrew Piston from Filter Cartridge to reach Small Check
5. Reverse the Small Check Valve and reinstall it inside Piston.
6. Thread Pump Cylinder (with Piston and reversed valve) carefully onto the Filter
NOTE: Both tips of Large and Small Check Valves should point toward
the Pump Inlet
7. Thread Pump Inlet onto the Pump Cylinder. Again, do not pump air into pump.
8. Connect container with filtered water to Outlet Adapter (or Outlet Nipple).
If using a hard bottle (Nalgene®), invert container to keep any air from
If using a flexible water system (MSR® Dromeday™ Bag with Quick Connect™ Bottle
Adapter), purge air from the bag before attaching Outlet Adapter.
If using a personal hydration system (Platypus® Big Zip SL™, CamelBak®), remove
Bite Valve, purge air from bag and Drink Tube, and attach Drink Tube directly onto
9. Pump 10+ full strokes of filtered water to flush Filter Cartridge.
Wait for Pump Cylinder to fill with water during each stroke.
When backflushing is completed, disassemble pump and reinstall the Large and Small
Check Valves in their original, forward flow position. Dry pump assembly and then
reassemble. CAUTION: Protect Filter Cartridge from accidental drops, which may
damage the filter.
In contrast to these printed instructions, what I have found to work best (note steps 4, 10, & 13) and what the current MSR instructional video now shows is:
Secondarily, the most recent review has a picture with the caption, "Pushing on (or downwards) with the testing" and the original review has a picture with the caption, "The pumping action"... they also indicate air introduction to the outer surface of the hollow fibers which may also reduce the filter’s effectiveness.
The following 2007 independent research report entitled, “Unstable filtration behavior with submerged hollow fiber membranes” was authored in part by UNESCO Center for Membrane Science and Technology, Chemical Sciences and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia.
It says in part, “…The formation of these stagnant bubbles is attributed to some ‘dry’ points existing on the internal surface of the hollow fiber membrane. It appears that these ‘dry’ points cause significant and unstable local resistances for the permeate flow inside the fiber lumen. For a fixed average flux the high local resistance results in increased suction pressure in the lumen region downstream of the ‘dry point’ and this shifts an additional flux load to this region. The resultant maldistribution of local fluxes caused by abnormal local flow resistances can markedly affect the filtration behavior of the hollow fiber membrane. Interestingly the effect was not observed for all fibers and appears to require local ‘dry point’ existence within the lumen.