The purpose of the diagnostic tests is to specify the technical condition of an object or process at a moment deemed as important. This specification is necessary in order to compare the actual momentary condition with the model condition, make a judgement on the applicability or inapplicability of the technical object. The publication includes the technical and structural characteristics of the hydraulic braking system (HSUH). The mathematical model of the system's leakage diagnostics process was developed on the basis of the laws of logic and the set theory, including: front axis brake master cylinder chamber (KP_HP), right hydraulic sub-circuit (outside the brake master cylinder) of front brakes PP_HP), left hydraulic sub-circuit (outside the brake master cylinder) of front brakes (LP_HP), rear axis brake master cylinder chamber(KP_HT), right hydraulic sub-circuit (outside the brake master cylinder) of rear brakes (PP_HT), left hydraulic sub-circuit (outside the brake master cylinder) of rear brakes (LP_HT).