This work presents a control design for an automotive antilock braking systems. Our approach is based on the sliding mode control concept. We formulate the problem of achieving minimum stopping distance as that of extremum searching in a highly uncertain situation when the optimized function is not known analytically but its output values can be observed on-line. For the braking problem the magnitude of the tire/road friction force is a maximized variable. It is considered to be an output of a nonlinear dynamic system which includes the model of mechanical motion and equations of the hydraulic circuit. This setting is complicated by the optimized variable (friction force) not being directly measurable. To overcome this difficulty a sliding mode observer was designed.