THANKS Dave, for the lead. Yep, it sure looks like a DEEP rabbit hole. But I get the idea now, actually it can all be somewhat simplified. When I have time I'll take a schematic from an A-series/B-series engine and mark it up to post.
In return, here's the Kb part for the bleed valve....
Kubota p/n 14311-60504 -- about 20 bucks. It may be that you can clean the shaft/tighten the packing nut (I think thar's packing under that thar nut??)
FWIW (forgive if you already know this -- others may not) the parts manual that TOAD has on it's website showing the setup for the M-25, XP, XPA IS NOT correct -- the electrical, etc., is NOT FOR THE A-series engines
(the wiring is actually nearly identical to the B-series engines.)
On the bleed/polishing -- (let me take an alternate approach, not having read all the topics on this.)
Yes, keeping the bleed valve open will theoretically run more fuel thru the primary and on-engine filters. I have no idea the amount (10% more, 50% more??) But to what end?
What that is, is not a "poor wo/man's fuel polishing system," it's a "clog up the filters that you depend on system." I don't see a clear benefit to running more diesel thru the filters we depend on, rather than them filtering only the fuel the engine actually needs? Is it better to catch crap sooner, i.e., before catching the same identical crap in the same identical filter(s) at some later point in time? This goes to my (1) darned old "common sense" approach and (2) avoiding creating more unnecessary failure points.
I suppose an argument could be made that there's a benefit to getting crap out of the tank sooner rather than later, even if one needs to change filters more often (unless due to that the engine coughs while one is trying to navigate a breakwall in 40 kts w/ 6' following seas, in the dark)? I (JTSO) think it's a weak argument -- and if being proactive re: the fuel quality is the goal, it's not THAT difficult to install another true polishing filter.