Interesting, if it were mold it would be fairly easy to remove but that's strange you haven't seen any improvement from your efforts, I would try apc but be carefull with belts as you are not supposed to clean them with chemicals/lots of moisture etc as it can effect the fabric,weave etc. use a George if you have one to suck the moisture out during and after cleaning then leave them fully pulled out till they are bone dry again.
If you can't get that out I would say bite the bullet and get some new ones or put up with the spots.