Old hairbrush tends to get dirty easily because of the tangled hair. It is a ton of works and consumes a lots of time to remove all the hairs that stick in the brush with your hands. In this case, use shampoo. It will remove all the dirt and hair oil around the brush. Dissolve some shampoo in the water until it makes some bubble.And then put your brush into that bubble water. After that takes out your brush from the water and rinses off with the clean running water. If you use the hair drier, it dries really quickly.