fluid quantity. Gauges incorporated on or in the reservoir may  be a direct reading glass tube-type or a float-type rod that is  visible through a transparent dome. In some cases, the fluid  quantity may also be read in the cockpit through the use of  quantity transmitters. A typical nonpressurized reservoir is  shown in  Figure 12-8.  This reservoir consists of a welded  body and cover assembly clamped together. Gaskets are  incorporated to seal against leakage between assemblies. Nonpressurized reservoirs are slightly pressurized due to  thermal expansion of fluid and the return of fluid to the  reservoir from the main system. This pressure ensures that  there is a positive flow of fluids to the inlet ports of the  hydraulic pumps. Most reservoirs of this type are vented  directly to the atmosphere or cabin with only a check valve and  filter to control the outside air source. The reservoir system includes a pressure and vacuum relief valve. The purpose of  the valve is to maintain a differential pressure range between  the reservoir and cabin. A manual air bleed valve is installed  on top of the reservoir to vent the reservoir. The valve is  connected to the reservoir vent line to allow depressurization  of the reservoir. The valve is actuated prior to servicing the  reservoir to prevent fluid from being blown out of the filler as  the cap is being removed. The manual bleed valve also needs  to be actuated if hydraulic components need to be replaced.  Air-pressurized reservoirs are used in many commercial  transport-type aircraft.  [Figures 12-9  and  12-10] Pressurization of the reservoir is required because the  reservoirs are often located in wheel wells or other non- pressurized areas of the aircraft and at high altitude there is not enough atmospheric pressure to move the fluid to  the pump inlet. Engine bleed air is used to pressurize the  reservoir. The reservoirs are typically cylindrical in shape.  The following components are installed on a typical reservoir: Air-Pressurized Reservoirs Pressurized Reservoirs Reservoirs on aircraft designed for high-altitude flight are  usually pressurized. Pressurizing assures a positive flow of  fluid to the pump at high altitudes when low atmospheric  pressures are encountered. On some aircraft, the reservoir is  pressurized by bleed air taken from the compressor section  of the engine. On others, the reservoir may be pressurized  by hydraulic system pressure. •  Reservoir pressure relief valve—prevents over  pressurization of the reservoir. Valve opens at a preset  value.  Figure 12-9.  Air-pressurized reservoir. Figure 12-8.  Nonpressurized reservoir. 12-10 Glass sight gauge Filler neck, cap, and fastener Connection for emergency system pump Connection for return line Connection for main system pump Connection for vent line or pressurizing line Normal fluid level Finger strainer Baffle Stand- pipe Fin Fin Fin Fin