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General
_______
 A.  The reservoir pressurization system controls the reservoir pressurization air supply and puts the air through a filter. The pressurization system releases reservoir pressure if it is more than the permitted limit. The reservoirs are pressurized 40-45 psi to make sure that a positive hydraulic fluid supply is given to the pumps so that cavitation does not occur. The pressurization system components are installed on or near the reservoir in the engine nacelle strut.
 B.  The pressurization system is designed to give the specified pressure to the reservoir in a temperature range of -60°F to +160°F and at an altitude range from sea level to 50,000 feet.
Reservoir Pressurization Module
_______________________________
 A.  Module
 (1)  The reservoir pressurization module is found aft of the reservoir and case drain module in the engine nacelle strut. The module contains these components:
 -air filters
 -bleed valve
 -
two check valves. The module also makes it possible to connect a pneumatic source and test pressure gage for ground maintenance operations.

 B.  
Air Filter

 (1)  
The air filter removes unwanted material from the pressurized air that is supplied to the module. The filter element is a 200 micron, cleanable, wire mesh, nonbypass type.

 C.  
Bleed Valve

 (1)  
The bleed valve is installed in the BLD port of the pressurization module. The valve permits the reservoir pressurization air to bleed as necessary to do system maintenance or to do tests on other units included in the module. The valve is operated manually with the knob on the valve.

 D.  
Check Valves

 (1)  
Two check valves are installed in the pressurization module downstream of the air filter. The valves open to let air go into the reservoir, and close to prevent hydraulic fluid flow into the pneumatic duct.

Reservoir Relief Valve
______________________
 A.  The reservoir relief valve is found on the upper aft side of the hydraulic reservoir. The relief valve is installed to keep reservoir pressure below the permitted limit.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 4. Operation
_________
 A. Functional Description
 (1)  
The pressurization system supplies pressurized air to the hydraulic reservoir at a pressure of 40 to 45 psi. This pressure is kept when the pneumatic system is in operation. If overpressurization of the reservoir occurs, the reservoir relief valve will open at 60 to 65 psi and release the unwanted pressure overboard. The valve will close when the reservoir pressure decreases to approximately 55 psi.

 B.
EICAS Messages

 (1)  
If the pressure in one of the hydraulic reservoirs decreases to 21 psia or less, the HYD RSVR PRESS 1 (2, 3, or 4, as applicable) status message can be viewed on the auxiliary EICAS display. To look at the status message it is necessary to push the status (STAT) key on the Display Select Panel.

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 ALL  ú ú 01 Page 2 ú Oct 10/88
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
 HYDRAULIC RESERVOIR PRESSURIZATION SYSTEM - ADJUSTMENT/TEST
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 1.  General_______
 A.  This procedure has one task which is a test of the pressurization system for the hydraulic reservoir.
 B.  You must open the doors in the trailing edge fairing to get access for this test. On systems 2 and 3, you must also open a strut access panel to get access for this test.
 TASK 29-17-00-705-001
 2.  Hydraulic Reservoir Pressurization System Test______________________________________________
 A.  Standard Tools and Equipment
 (1)  
Pressure gage - 0-100 psig (this gage connects to the port GA on the reservoir pressurization module)
　
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