Complex sentence: After a long period of inbreeding and culling, obvious genetic problems were worked out of the breed (they are as healthy as any today), but a new problem worked its way in--today's dogs are sometimes too big for truly tight underground work.
Split into simpler sentences: After a long period of inbreeding and culling, obvious genetic problems were worked out of the breed. They are as healthy as any today, but today's dogs are sometimes too big for truly tight underground work.