The chemical air drier removes excessive moisture from the bleed air. Dry, clean air is sent to the reservoir through the check valve, air regulator, and relief valve. TWO bleeder valves are installed in the flight and combined system reservoirs. One is found on the air side of the reservoir and the other on the fluid side. The air side valve permits the bleeding of air pressure during system maintenance. It allows the bleeding of any hydraulic fluid seepage past seals to the air side. The fluid side bleeder reduces excessive fluid level and bleeds air from the fluid side. Quick-disconnect fittings in the hydraulic power systems permit easy pump or engine removal without loss of fluid to the system. The fittings connect ground hydraulic test stands for maintenance purposes. The pump disconnects should not be forced together against the back pressure of a pressurized reservoir or system. Forcing disconnects together may result in damaged seals in the male ends of the disconnects. When the disconnects do not slide in smoothly, they should be removed and checked for proper seating of the O-rings. Replace seals if they are damaged. The seal goes on top of the O-ring. When the disconnects are uncoupled, the ends not being used should be properly protected from dirt and other contamination. Use only approved metal closures. EMERGENCY POWER SYSTEMS According to the military specifications, which establish the requirements for aircraft hydraulic systems, all hydraulically operated systems considered essential to flight safety or landing must have provisions for emergency actuation. The specifications further state that these emergency systems may use hydraulic fluid, compressed gas, directed mechanical linkage, or gravity for their actuation. Some aircraft use mechanical linkage or gravity in conjunction with pneumatic pressure for emergency actuation of landing gear and other actuating systems where limited actuation is required. Most other essential hydraulically operated systems have emergency power systems that are powered by a hand pump, electric motor-driven pump, ram-air turbine-driven pump, or a pneumatic compressor. On some aircraft the hand pump is a part of the auxiliary hydraulic system and is not considered as part of the emergency power systems. The hand pump is used for ground operation of the canopy, extensible electronics platform, nose radome opening, and to recharge the brake accumulator. These systems may be operated by aircraft system pressure or, if the aircraft is shutdown, they may be powered by the auxiliary electric motor-driven hydraulic pump or the hand pump. HYDRAULIC COMPONENTS Learning Objective: Identify the various hydraulic system components and recognize t h e  p r o c e d u r e s  r e q u i r e d  f o r  t h e ir maintenance. Various types of hydraulic components makeup a power system. The components discussed in this chapter are representative of those with which you will most likely be working. Values such as pressure, temperature, and instructional tolerances have been given to provide detail in the coverage. When actually performing the maintenance procedures discussed, the exact location and make up of the various hydraulic and pneumatic components will vary with the design of the hydraulic system. You should consult the current applicable technical publication for the latest information on items such as location, pressure, temperature, and tolerances. RESERVOIRS The reservoir is a tank in which an adequate supply of fluid for the system is stored. Fluid flows from the reservoir to the pump, where it is forced through the system and eventually returned to the reservoir. The reservoir not only supplies the operating needs of the system, but it also replenishes fluid lost through leakage. Furthermore, the reservoir serves as an overflow basin for excess fluid forced out of the system by thermal expansion (the increase of fluid volume caused by temperature changes), the accumulators, and by piston and rod displacement. The reservoir also furnishes a place for the fluid to purge itself of air bubbles that may enter the system. Foreign matter picked up in the system may also be separated from the fluid in the reservoir, or as it flows through line filters. 7-5