Jeanette,
I got soured on the rubber band belts decades ago when I used to go with my mom to the fabric store. Seems to me she was always buying those little blister packs that have a belt and a bobbin tire in it. These were used on the HOTHER ( < avatar pic ). I can't remember how many of those things I changed out for her.
Many of those belts that came on vintage machines I've purchased have been cracked, rotted, or gooey and almost immediately break.
So I just don't use them if I can possibly avoid it.
Joe