1. Preheat oven to 325 degrees F (165 degrees C). Lightly grease two 9×5 inch loaf pans. In a small bowl, stir together milk, lemon juice and vanilla; let stand for 10 minutes.
2. In a large bowl, mix together 1 1/2 cups brown sugar, oil and egg. Combine the flour, salt and baking soda; stir into sugar mixture alternately with the milk mixture just until combined. Fold in rhubarb and nuts. Pour batter into prepared loaf pans.
3. In a small bowl, combine 1/4 cup brown sugar, cinnamon and butter. Sprinkle this mixture over the unbaked loaves.
4. Bake in preheated oven for 40 minutes, until a toothpick inserted into center of a loaf comes out clean.