Pitch is in nature fairly brittle when dried (especially as it gets REALLY DRY). That is why when using it for canoes, we add grease (rendered bear, deer, raccoon). Try adding a little of this too it. When we make a new batch, we generally test it by dipping a strip of birch bark into the pitch pot and then dunking the bark into a kettle of cold water. Then, bend the bark and see if it cracks, or if it is TOO pliable.