Glenn, thank you for info regarding the W&W group.
Janice, I gather the manual you have is missing the pages that tell about the bobbin case and gib screw. Knowing what I do about bobbin cases and gib screws, I would check all the upper threading and guides before tackling the bobbin area.
I don't have a Wheeler and Wilson, but have a newer Junker & Ruh. It has a W&W 8 bobbin case & holder, but has the "thread check", "tension pulley" and "thread guide and controller" like the W&W 9. From what I've read about the Wheeler and Wilson threading and experienced with the Junker & Ruh each has an adjustment and needs to be threaded and adjusted to be able to sew properly.
Another thread you might want to look at is
Adjusting Tension on Wheeler & Wilson 9 Your "thread guide and controller" might be a little different from the ones pictured in that thread. I have seen a few variations, but they are adjustable.
Good luck on getting it to stop breaking thread. Treadles are fun to sew on.
Janey - Neat people never make the exciting discoveries I do.