tessagin,
Are you sure 1/4 cup bleach to 5 gal of water is enough? I ask because I have a very bad situation of molding sewing machine cases too.
Grey / blue mold. I've tried white vinegar, and a spray that kills black mold, but the stuff on my cases just keeps coming back. Sadly they are in the basement and that's a mold factory. I'm gradually getting as many machines out of there as I can, but the only place for them is the garage and I don't know if that's any better.
Joe