•  Allow approximately 1 pint (250 ml) of fluid to drain.  This purges any settled particles from the sampling  port. •  Insert a precleaned sample bottle under the fluid stream  and fill, leaving an air space at the top. Withdraw the  bottle and cap immediately. •  Close drain valve. •  Fill out sample identification label supplied in sample  kit, making sure to include customer name, aircraft  type, aircraft tail number, hydraulic system sampled,  and date sampled. Indicate on the sample label under  remarks if this is a routine sample or if it is being taken  due to a suspected problem. •  Service system reservoirs to replace the fluid that was  removed. •  Submit samples for analysis to laboratory.  •  All parts should be connected with care to avoid  stripping metal slivers from threaded areas. All fittings  and lines should be installed and torqued in accordance  with applicable technical instructions. •  All hydraulic servicing equipment should be kept clean  and in good operating condition.  •  Pressure filters—3,000 hours  •  Return Filters—1,500 hours •  Case drain filters—600 hours  •  Maintain all tools and the work area (workbenches  and test equipment) in a clean, dirt-free condition. •  A suitable container should always be provided to  receive the hydraulic liquid that is spilled during  component removal or disassembly procedures. •  Before disconnecting hydraulic lines or fittings, clean  the affected area with dry-cleaning solvent. •  All hydraulic lines and fittings should be capped or  plugged immediately after disconnecting. •  Before assembly of any hydraulic components, wash  all parts in an approved dry-cleaning solvent, i.e.,  Stoddard solvent. •  After cleaning the parts in the dry-cleaning solution,  dry the parts thoroughly and lubricate them with the  recommended preservative or hydraulic liquid before  assembly. Use only clean, lint-free cloths to wipe or  dry the component parts. •  All seals and gaskets should be replaced during the  reassembly procedure. Use only those seals and  gaskets recommended by the manufacturer. 5. Ensure that the reservoir is filled to the full line or  proper service level. 4. Disconnect the test stand and cap the ports. 3. Pump clean, filtered fluid through the system, and  operate all subsystems until no obvious signs of  contamination are found during inspection of the  filters. Dispose of contaminated fluid and filter. Note:  A visual inspection of hydraulic filters is not always  effective. 2. Change the system filters. 1. Connect a ground hydraulic test stand to the inlet and  outlet test ports of the system. Verify that the ground  unit fluid is clean and contains the same fluid as  the aircraft. Filters provide adequate control of the contamination  problem during all normal hydraulic system operations.  Control of the size and amount of contamination entering  the system from any other source is the responsibility of the  people who service and maintain the equipment. Therefore,  precautions should be taken to minimize contamination  during maintenance, repair, and service operations. If the  system becomes contaminated, the filter element should be  removed and cleaned or replaced. As an aid in controlling  contamination, the following maintenance and servicing  procedures should be followed at all times: When inspection of hydraulic filters or hydraulic fluid  evaluation indicates that the fluid is contaminated, flushing  the system may be necessary. This must be done according to  the manufacturer’s instructions; however, a typical procedure  for flushing is as follows: It is very important to check if the fluid in the hydraulic test  stand, or mule, is clean before the flushing operation starts.  A contaminated hydraulic test stand can quickly contaminate  other aircraft if used for ground maintenance operations. 12-5 Contamination, both particulate and chemical, is detrimental  to the performance and life of components in the aircraft  hydraulic system. Contamination enters the system through  normal wear of components, by ingestion through external  seals during servicing, or maintenance, when the system is  opened to replace/repair components, etc. To control the  particulate contamination in the system, filters are installed  in the pressure line, in the return line, and in the pump case  drain line of each system. The filter rating is given in microns  as an indication of the smallest particle size that is filtered  out. The replacement interval of these filters is established  by the manufacturer and is included in the maintenance  manual. In the absence of specific replacement instructions,  a recommended service life of the filter elements is: Contamination Control Hydraulic System Flushing