Preheat the oven to 350°F and grease an 8x4-inch loaf pan.

In a medium-sized bowl, combine the flour, baking powder, baking soda, and salt. Mix well.

In another medium-sized bowl, use a mixer to cream together the margarine and sugar until light and fluffy. Add the mashed bananas, eggs, and milk and mix well.

Add half of the dry ingredients to the wet ingredients, mixing until the flour is just incorporated. Then, add the remaining flour and the chopped pecans, mixing until just combined. Do not overmix.

Pour the batter into the prepared loaf pan and top with the chopped walnuts. Bake for 55 to 60 minutes or until a toothpick inserted near the center comes out clean.