For the chicken stock: Place the chicken bones, garlic, celery, carrots, onions, parsley, salt, peppercorns, cloves and bay leaves in a 10-quart or larger pressure cooker. Pour in about 16 cups water to just cover ingredients by 1 inch. Add more water if necessary but do not fill more than two-thirds of the pot. Cover and lock the lid. Bring to pressure according to manufacturer's instructions. (If the cooker has two levels of pressure, cook on the lowest.) Adjust the heat to maintain pressure, and cook for 45 minutes.
Let the pressure subside by itself (natural method), or if in a rush, run cold water over the lid for a few minutes.
Strain the liquid through a colander lined with layers of cheesecloth. Discard the solids.
Cool uncovered and refrigerate overnight until the fat floats and solidifies. Remove the fat and save a small amount for matzo balls.
Storage: Refrigerate up to 4 days. Important to bring to a rolling boil for at least 1 minute before using. Freeze up to 6 months.
For the matzo balls: In a mixing bowl, whisk together the seltzer, schmaltz, salt, pepper and eggs. Then in a separate bowl, add the baking soda to the matzo meal and mix together. Next, add the matzo meal mixture into the egg mixture while stirring to a loose consistency. It will look watery, like a loose porridge. If it becomes too thick, either don't add the full amount of matzo meal or add a bit more seltzer. Cover and refrigerate for 1 hour.
Fill a large pot with water and salt and bring to a simmer. With wet hands, roll out balls and gently drop to the water. Simmer until tender, light and fluffy in the center, 45 to 60 minutes. (To test centers, insert a toothpick, which should slide easily all the way through.) Allow to cool in the water undisturbed. Refrigerate in the cooking liquid or transfer to chicken soup.