Ahoy there AVB, and many thanks across 14,368km (from Brisbane to Tennessee, according to Google...) for the typically-American hospitality of your reply!
Nicely spotted. I actually have the Exploded Parts Diagram here myself, but I didn't bother to look at it because I didn't notice anything like a check-valve anywhere on the tank when I had the machine apart recently (mind you, I wasn't looking for one at the time...)
Well, I guess I'll have to strip the machine down again in order to get to the valve in question.
Many Thanks,
Mike.