Dissolve yeast in 1/4 cup warm water in a large bowl; stir in 1/4 cup brown sugar, and let stand 5 minutes.
Combine remaining warm water, all-purpose flour, and salt; stir into yeast mixture. Cover loosely with plastic wrap or cheesecloth, and let stand in a warm place (85°), free from drafts, 1 1/2 hours.
Stir batter well; let rest 5 minutes. Combine boiling water, butter, and 1/3 cup brown sugar, stirring until butter is melted. Add butter mixture to batter; stir well. Add 4 cups whole wheat flour; stir until mixture is smooth. Stir in enough remaining whole wheat flour to form a moderately stiff dough.
Turn dough out onto a lightly floured surface; knead 8 minutes or until smooth and elastic. Place in a greased bowl, turning to grease top. Cover and let rise in a warm place, 1 hour or until doubled in bulk.
Punch dough down; turn out onto a lightly floured surface. Divide into 3 equal portions, shaping each into a smooth ball. Cover and let rest 10 minutes. Shape each portion into a loaf; place in 3 greased 9- x 5- x 3-inch loaf pans. Cover and repeat rising procedure 40 minutes or until doubled in bulk. Bake at 375° for 45 minutes or until loaves sound hollow when tapped. Remove bread from pans immediately; cool on wire racks. Brush with melted butter.