Combine the flour, baking powder, and baking soda in a large mixing bowl. Add the syrup, milk, banana, raisins, and walnuts. Stir just enough to mix. The batter will be fairly stiff and sticky.
Spoon into a 9 × 5 nonstick loaf pan and bake for 45 minutes. Remove the pan from the oven and place on a rack to cool.