I finally got around to working on my bike. Since the tank is off the bike, started with cleaning out the tank. It's been sitting for awhile with a full tank of vinegar. Man, a load of rust chunks came out, including some grey gritty/sand-like chunks. there's still some sloshing around, but less so each time I rinse and shake. Water/rust mix just pours out of the main fuel hole, but nothing is coming from the reserve line. My understanding is that, with the petcock off, whatever is in the tank should come out of both holes. So, assuming it's just a bit of rust/junk blocking up the reserve pipe, then the next cleaning should take care of it.