System B reservoir System A reservoir Check valve Hand pump To reservoir pressurization system Filter Balance lines Standby reservoir Pressure fill fitting for hydraulic service cart Suction line to clean container of fluid Fluid quantity indicator Differential pressure indicators Pressure filter Temperature transducer Pressure  transducer Case drain filter Ground service disconnect 12-13 Figure 12-15.  Filter module components. Figure 12-14.  The hydraulic ground serive station on a Boeing 737 provides for hydraulic fluid servicing with a hand pump or via an  external pressure fluid source. All three reservoirs are serviced from the same location.  styles of filters. Their position in the aircraft and design  requirements determine their shape and size. Most filters  used in modern aircraft are of the inline type. The inline filter  assembly is comprised of three basic units: head assembly,  bowl, and element. The head assembly is secured to the  aircraft structure and connecting lines. Within the head,  there is a bypass valve that routes the hydraulic fluid directly  from the inlet to the outlet port if the filter element becomes  clogged with foreign matter. The bowl is the housing that  A filter is a screening or straining device used to clean  the hydraulic fluid, preventing foreign particles and  contaminating substances from remaining in the system.  [Figure 12-15]  If such objectionable material were not  removed, the entire hydraulic system of the aircraft could  fail through the breakdown or malfunctioning of a single  unit of the system. The hydraulic fluid holds in suspension tiny particles of  metal that are deposited during the normal wear of selector  valves, pumps, and other system components. Such minute  particles of metal may damage the units and parts through  which they pass if they are not removed by a filter. Since  tolerances within the hydraulic system components are quite  small, it is apparent that the reliability and efficiency of the  entire system depends upon adequate filtering. Filters may be located within the reservoir, in the pressure  line, in the return line, or in any other location the designer  of the system decides that they are needed to safeguard the  hydraulic system against impurities. Modern design often  uses a filter module that contains several filters and other  components.  [Figure 12-16]  There are many models and  Filters •  Parking brake accumulator pressure reads at least  2,500 psi. •  Thrust reversers are retracted, and