If an inspection of an oil sample drawn from a hydraulic system reveals evidence of water, sludge, or acidity, the system must be drained, then cleaned with prescribed acid-free cleaning fluid (flushing oil), and filled with clean hydraulic oil. A hydraulic system may be drained and cleaned as follows:

1. Remove the permanent filters and wash them in flushing oil. Then use low-pressure air for drying purposes. If the filters have replaceable elements, install new elements.

2. Drain the system of old hydraulic oils as completely as possible.

3. Close all connections, and fill the system with acid-free cleaning fluid.

4. Start andoperate the unit under idling conditions to fill the system thoroughly with cleaning fluid.

5. Secure the unit and allow it to stand idle for the prescribed period (usually about an hour). This period of idleness permits the cleaning fluid to dissolve any sludge.

6. Start and operate the unit with a light load for 3 to 5 minutes, unless otherwise specified. Allow the equipment to stand idle for about 15 minutes, then repeat the whole cleaning process. Do this two or three times. Never operate a hydraulic unit with a full load when it is filled with cleaning fluid. Keep the operating pressure as low as possible.

7. If time permits, allow the system to stand idle for an additional hour following the series of short operating periods.

8. Drain the system of cleaning fluid. Reclean permanent filters or, if necessary, install new replaceable filters. Close the system, and fill it with the proper hydraulic oil.

As the system is filled, strain the hydraulic oil through a fine wire screen of 180 or 200 mesh. If the oil is not clean, run it through a centrifuge. You should provide adequate protection against dust and moisture entering the system. Moisture should be expelled from the oil before it is poured into a system. Oil with noticeable water content should be rejected or centrifuged

When a hydraulic system is being filled, sufficient hydraulic fluid should be used to completely fill the active parts of the mechanism, leaving no air pockets. Air valves should he opened during the filling process, so that air can escape to the oil expansion box. Be sure the valves are closed tightly after the system has been filled. For more information on hydraulic fluid filtration, read NSTM, Chapter 556, Hydraulic Equipment (Power Transmission and Control). For additional information on hydraulic fluids, refer to NSTM, Chapter 262, Lubricating Oils, Greases, and Hydraulic Fluids and Lubricating Systems.