This may be due to : 1) A leakage definable by sound similar to an escape of gas. Disconnect the rack feed pipe assembly from the end piece on steering housing. Close the flange openings with a sheet metal plate, inserting a seal plate. a) If the leak persists, it comes from the rotating union which must be repaired or renewed. b) If the leak disappears it came from the hydraulic rack control which must be repaired or renewed. 2) A leak causing inflation of the rubber dust cover and consequently an external leakage of fluid. This