The hydraulic system comprises a casing (10) in which there are disposed hydraulic apparatus (12), a shaft (14) engaged in the casing, and a sealing device (16) between the shaft and the casing. The sealing device comprises first and second sealing elements (20, 22) disposed in succession on the shaft while forming between them a discharge chamber (24) connected to a drain (26), the first sealing element being situated between the discharge chamber and the internal space (11) of the casing. Between the discharge chamber (24) and the internal space (11) of the casing, the hydraulic system has a communication duct (28) in which a calibrated jet nozzle (30) is disposed.