Drain and clean the pork bones well. In a large pot, add the pork bones, leeks, and smashed garlic,. Add enough water to the pot to cover the bones. Bring the water to a boil and boil rapidly for 15 minutes, then reduce to a simmer and cover with a lid. Simmer the broth, covered, for at least 5 hours, but preferably 12 hours or up to overnight. The longer you simmer the broth the more flavorful it will be.