If it is just tied, I would replace the back myself. If it's a treasure it just isn't worth the heart ache of a bleed. Once you have it re-tied or quilted though, I might treat the washing like I wash antique quilts. In a bathtub with tepid water and gentle soap on top of a sheet. (the sheet is use to lift the wet quilt out of the tub) Gently agaitate by hand, drain the tub and refill with fresh water, agaitate and then drain out the water again. Some old materials cannot withstand the agitation in a regular washer.