[Ross F-1]

I presume you haven't added any gas since you picked it up? So the gas tank level has been slowly dropping?

It's been a l-o-n-g time but I recall a thread on a shoebox Ford that was losing pump action, and it turned out there is a short section of rubber hose in the steel line, sort of hidden in a section of frame in front of the rear axle. It was a little higher than the rest of the line and it was cracked, letting in air when fuel level dropped, weeping gas when the tank was fairly full. I've never owned a shoebox so it's just "something I read on the internet"....