If a bitch "throws the towel in" she should "never" be bred from no matter how good the bloodlines are,
If the line she is from, are good digging dog's, over generation's, they have got that way by "culling" sub-standard stock and certainly not breeding from it
and let's face it, if the only bitch left out of the bloodline is a "quitter" to breed from then the line is lost anyway