i have used muriatic (sp?) acid dilluted with water and applied with a stiff bristle brush. wet the brick before and rinse throuroughly after. be sure to cover yourself completely, including eye protection. sorry, i'm not sure of the ratio of acid:water but i would try it very dilluted first and work your way up if needed. i'm not saying that this is a widely used technique, only that i have used it with success.