The traditional form of wheel brake consisting of a block of friction material (which could be cast iron, wood or nowadays a composition material) hung from a lever and being pressed against the wheel tread by air pressure (in the air brake) or atmospheric pressure in the case of the vacuum brake. Now the preferred system is a disc brake which, by replacing the tread brake, removes one of the causes of wear on wheel treads, although it also removes the scrubbing action which tended to reduce the risk of wheel slide.