32-41-00
ALL ú ú 01 Page 5 ú Feb 18/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
4. Alternate Brake Return Valve (Fig. 4)
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A. The alternate brake return valve automatically provides a direct path to the hydraulic system No. 1 return lines when hydraulic system No. 1 is used to apply the brakes, including during gear-retract braking. As both brake source select valves contain an internal restrictor in the return path, this direct path to System No. 1 return minimizes the amount of hydraulic System No. 1 fluid that could enter the hydraulic System No. 2 retrun lines.
B. The alternate brake return valve is on the inboard bulkhead of the right body gear well next to alternate source selector valve 1.
C. The alternate brake return valve has three ports. The control port "P" gets a control pressure from hydraulic system No. 1. The brake return port "BR" gets fluid from the brake return lines. The return port "R" sends fluid back to hydraulic system No. 1.
D. With hydraulic system No. 1 pressurized, the valve opens, allowing flow from "BR" to "R".
E. With pressure removed from hydraulic system No. 1, the valve closes, blocking flow from "BR" to "R".
F. This valve is removed and installed using a procedure in AMM 32-41-25. A test on this valve is done after installation.
5. Hydraulic Brake Pressure Switches (Fig. 3)
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A. An alternate-brake-pressure switch and a system-4-pressure switch are electrically connected in series to operate the BRAKE SOURCE annunciator light on the captain's instrument panel. These switches are also connected to the EICAS Interface Units to cause a BRAKE SOURCE indication on the EICAS and to the brake control unit (M80) for antiskid logic (AMM 32-42-00/001).
B. The alternate-brake-pressure switch is on the ceiling of the left wing gear wheel well. The hydraulic-system-4-pressure switch is on the aft wall of the left wing gear wheel well.
C. The alternate-brake-pressure switch is mechanically connected to the brake pressure lines of the alternate-source-selector valves and the pressure port of the alternate-brake-metering valve. The switch operates at 1600 psi when one of the two alternate-source-selector valves opens to pressurize the alternate brake system. The switch operates back when pressure is removed from the alternate brake system.
D. The hydraulic system 4 pressure switch is mechanically connected to hydraulic system 4 pressure lines. This switch operates when hydraulic system 4 pressure increases above 1600 psi and operates back when pressure decreases below 1400 psi.
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32-41-00
ALL ú ú 01 Page 6 ú Feb 15/99
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
LEFT-WING-GEAR-WHEEL WELL
LEFT-WING-SEE A GEAR-WHEEL-WELL CEILING
SEE B
HYDRAULIC
LINE
PRESSURE SWITCH
UP
HYDRAULIC MANIFOLD
LINE
ALTERNATE BRAKE SYSTEM PRESSURE SWITCH
PRESSURE
SWITCH
A
MANIFOLD
LEFT-WING-GEAR-WHEEL-WELL-AFT BULKHEAD SUPPORT
BRACKET
HYDRAULIC
LINE
HYDRAULIC SYSTEM-4-PRESSURE SWITCH
B
Hydraulic Brake System Pressure Switches Component Location Figure 3
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32-41-00
ALL ú ú 01 Page 7 ú Oct 10/91
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL
E. When pressure is removed from both hydraulic system 4 and the alternate brake system, these switches open an electrical ground to turn on the BRAKE SOURCE annunciator light on the captain's instrument panel and to cause a BRAKE SOURCE indication on EICAS.
F. These pressure switches are removed and installed using procedures in this section. A test of the pressure switches is done after installation and also as a part of the hydraulic brake system test (AMM 32-41-00/501). No other maintenance operations are done on the switches while they are installed on the airplane.
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