Preheat oven to 325 degrees F (165 degrees C). Lightly grease two 9x5 inch loaf pans. In a small bowl, stir together milk, lemon juice and vanilla; let stand for 10 minutes.
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.
In a small bowl, combine 1/4 cup brown sugar, cinnamon and butter. Sprinkle this mixture over the unbaked loaves.
Bake in preheated oven for 40 minutes, until a toothpick inserted into center of a loaf comes out clean.