Wipers  Seal Materials Large Aircraft Hydraulic Systems O-Ring Installation  Boeing 737 Next Generation Hydraulic System Reservoirs 12-38 • Autopilots •  Thrust reversers •  Nose wheel steering •  Wheel brakes •  Landing gear •  Trailing edge flaps •  Leading edge flaps and slats •  Flight controls The Boeing 737 Next Generation has three 3,000 psi  hydraulic systems: system A, system B, and standby. The  standby system is used if system A and/or B pressure is lost.  The hydraulic systems power the following aircraft systems: The system A, B, and standby reservoirs are located in the  wheel well area. The reservoirs are pressurized by bleed air  through a pressurization module. The standby reservoir is  connected to the system B reservoir for pressurization and  After the removal of all O-rings, the parts that receive new  O-rings have to be cleaned and inspected to make sure that  they are free from all contamination. Each replacement  O-ring should be removed from its sealed package and  inspected for defects, such as blemishes, abrasions, cuts,  or punctures. Although an O-ring may appear perfect at  first glance, slight surface flaws may exist. These flaws are  often capable of preventing satisfactory O-ring performance  under the variable operating pressures of aircraft systems;  therefore, O-rings should be rejected for flaws that affect  their performance. Such flaws are difficult to detect, and  one aircraft manufacturer recommends using a four-power  magnifying glass with adequate lighting to inspect each ring  When removing or installing O-rings, avoid using pointed  or sharp-edged tools that might cause scratching or marring  of hydraulic component surfaces or cause damage to the  O-rings. Special tooling for the installation of O-rings is  available. While using the seal removal and the installation  tools, contact with cylinder walls, piston heads, and related  precision components is not desirable.  Figure 12-62  provides an overview of hydraulic components  in large aircraft. Most seals are made from synthetic materials that are  compatible with the hydraulic fluid used. Seals used for  MIL-H-5606 hydraulic fluid are not compatible with  phosphate ester-based fluids and servicing the hydraulic  system with the wrong fluid could result in leaks and system  malfunctions. Seals for systems that use MIl-H-5606 are  made of neoprene or Buna-N. Seals for phosphate ester-based  fluids are made from butyl rubber or ethylene-propylene  elastomers. Wipers are used to clean and lubricate the exposed portions  of piston shafts. They prevent dirt from entering the system  and help protect the piston shaft against scoring. Wipers may  be either metallic or felt. They are sometimes used together,  a felt wiper installed behind a metallic wiper. A solid copper washer is used for spark plug gaskets where  it is essential to have a noncompressible, yet semisoft gasket.  Cork gaskets can be used as an oil seal between the engine  crankcase and accessories, and where a gasket is required  that is capable of occupying an uneven or varying space  caused by a rough surface or expansion and contraction.  Rubber sheeting can be used where there is a need for a  compressible gasket. It should not be used in any place  where it may come in contact with gasoline or oil because  the rubber deteriorates very rapidly when exposed to these  substances. Gaskets are used in fluid systems around the  end caps of actuating cylinders, valves, and other units. The  gasket generally used for this purpose is in the shape of an  O-ring, similar to O-ring packings. After inspection and prior to installation, immerse the O-ring  in clean hydraulic fluid. During the installation, avoid  rolling and twisting the O-ring to maneuver it into place. If  possible, keep the position of the O-ring’s mold line constant.  When the O-ring installation requires spanning or inserting  through sharply threaded areas, ridges, slots, and edges, use  protective measures, such as O-ring entering sleeves, as  shown in  Figure 12-60A.  After the O-ring is placed in the  cavity provided, gently roll the O-ring with the fingers to  remove any twist that might have occurred during installation.  [Figure 12-61] before it is installed. By rolling the ring on an inspection  cone or dowel, the inner diameter surface can also be  checked for small cracks, particles of foreign material, or  other irregularities that cause leakage or shorten the life of  the O-ring. The slight stretching of the ring when it is rolled  inside out helps to reveal some defects not otherwise visible. asbestos, copper, cork, and rubber. Asbestos sheeting is used  wherever a heat resistant gasket is needed. It is used extensively  for exhaust system gaskets. Most asbestos exhaust gaskets have  a thin sheet of copper edging to prolong their life.