You will likely find that late '31 horizontal tube quite encrusted with sediment. It won't simply flush out. I have a scraper made from a cheap extra long screwdriver with the end 1/8" bent over 90 degrees. I got a mountain of crud out from several late 31 tank pipes with it. I followed with a .38 cal bronze bore brush, a thread chaser, and a thorough flush.
I have counter-bored the inlet hole on several late 31 valves to accept a 3/16" brass tube stub that serves as the base for an inlet screen made from 80 mesh bronze screen rolled 1/4"D x 4 1/2"L. It fits easily into a clean '31 tank tube and is much larger in available surface area than the short fuel valve inlet screens sold by the vendors.