This paper presents the design of a model-based supervision and diagnosis system for hydraulic and future electro-hydraulic passenger car braking systems. A state space model of the braking system has been derived, which allows calculation of the individual wheel brake cylinder pressures as well as the amount of brake fluid consumed due to faults in the braking system. Based on this model, a real-time supervision system has been developed, which allows early detection and diagnosis of leakages and air entrapments in the hydraulic subsystem. This supervision system only requires data of sensors which are already available in modern passenger cars.