Add the remaining 8 cups of water to the slow cooker along with the onion, carrot, celery, parsley, thyme, bay leaf and peppercorns. Cover and cook on high until the broth is very flavorful, about 6 hours. Remove the beef shin pieces and reserve for a later use. Strain the broth into a large bowl and skim the fat from the surface. Season the broth with salt. Let cool, then cover and refrigerate.