It is certainly not an exact science, however i do believe that it has nothing to do with the breed. As Well bread, avarage, and not so well bread dogs all have quit in the past, being well bread, a little less often ofcourse from proven dogs. Many people have a different idea of a Jacker/cocker for e.g you can have these dogs that are wizzing hares up and down and just put the brakes on while being well on terms with its quarry and there is a dog running its quarry till it has no stamina left, therefore ran itself to a standstill, i would not call this a jack. Just My Opinion