Thanks for the advice, Paul. I don't think air leaks are causing the slow filling of the prefilter as I've been running the engine from a red fuel can with short lines. I think either the prefilter is too clogged to work properly, causing a poor fuel draw, or the position of the tank above the inj. pump is hindering fuel draw. I need to replace the prefilter, I just haven't had the chance to do it yet - forgot to grab one when I replaced the main filter, and it has not been convenient to go to the parts store lately.

The reason I may install a lift pump is to aid in dealing with the thicker biodiesel fuel. I agree with your advice regarding pump placement, but I feel that it is worth sacrificing a bit of line pressure to locate the unit under the hood, where it is easier to maintain and protected from road debris and the elements (Minnesota road salt would, I think, quickly kill it).