Abstract: A brake warning system for vehicles with pneumatic air brakes monitors various factors to provide an indication of a pending or actual brake failure condition. According to one embodiment, vehicle brakes are tested by determining a delay time between pressure changes in a control brake line from a treadle valve and the response of the brake. According to another embodiment, brake performance is measured by monitoring applied brake line pressure versus known brake performance values. Measurements are taken with the brakes in a known good condition, during a "training mode," and data is then compared, on a continuous basis, with data acquired by ongoing measurements. The invention provides an on-board, real-time, assessment of vehicle brake performance, through measurement of a relatively small number of parameters.