Low pressure Impeller boost pressure High pressure Filtered pressure C$_{1}$ control pressure C$_{2}$ control pressure Case pressure Cylinder block High pressure  port High pressure  port Case drain check valve Case drain check valve In-line housing Control flow filter Yoke position  feedback Compensator valve Low pressure port Low pressure port Impeller Impeller Impeller cover Peripheral impeller Cylinder block Bushing Shaft seal S.A. Actuator piston Piston Yoke Adapter Actuator piston Piston shoe Drive shaft Common foot Bent axis housing Impeller plate subassemblies Blocking valve Green System Pump Yellow System Pump Figure 12-57.  Power transfer unit. Seals are divided into three main classes: packings, gaskets,  and wipers. A seal may consist of more than one component,  such as an O-ring and a backup ring, or possibly an O-ring  and two backup rings. Hydraulic seals used internally on a  sliding or moving assembly are normally called packings.  [Figure 12-58]  Hydraulic seals used between nonmoving  fittings and bosses are normally called gaskets. V-ring packings (AN6225) are one-way seals and are always  installed with the open end of the V facing the pressure.  V-ring packings must have a male and female adapter to  hold them in the proper position after installation. It is also  necessary to torque the seal retainer to the value specified  by the manufacturer of the component being serviced, or the  seal may not give satisfactory service.  Most packings and gaskets used in aircraft are manufactured  in the form of O-rings. An O-ring is circular in shape,  U-ring packings (AN6226) and U-cup packings are used in  brake assemblies and brake master cylinders. The U-ring  and U-cup seals pressure in only one direction; therefore,  the lip of the packings must face toward the pressure. U-ring  packings are primarily low-pressure packings to be used with  pressures of less than 1,000 psi. 12-36 O-Rings V-Ring Packings U-Ring  Figure 12-58.  Packings. U-ring O-ring V-ring U-cup male Female