The technical condition of the braking system of a vehicle admitted to traffic on public roads cannot raise any objections. Regardless of the intended use and structural solution, the braking systems can be divided into braking mechanisms and mechanisms and systems activating braking. The technical diagnostics include the assessment of technical suitability of the mechanisms activating braking and the determination of the braking system's performance at the test and measurement stand. For a diagnostician who checks the technical condition of a vehicle, this is a basic check in the field of road use safety. One of the symptoms of the technical failure of the vehicle braking system is the lack of tightness of the hydraulic circuit. In the process of exploitation, taking into account the safety of technical facilities, the factor that influences the development of diagnostics involves primarily the minimization of threats to human health and life, threats to the biological and technical environment. Guided by these assumptions, the purpose of the conducted research was to develop a method for diagnosing leakage of the hydraulic circuit for the needs of designing a braking system diagnosing unit. The publication presents the original mathematical model based on Boolean functions, which is a part of the mathematical analyses.