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PARK BRAKE OFF (Blue - L1540)

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PARK BRAKE ON (Red - L1541).

 (1)  The PARK BRAKE SET light (L1539) comes on when the parking brake is set and the parking brake valve (V23) is closed.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 747-400 MAINTENANCE MANUAL 
UPPER DECK FLOOR 
SEE  A  NO. 1 ENTRY DOOR  PARKING BRAKE LEVER 
PILOT'S CONTROL STAND  ROD ASSEMBLY 
COUNTER SHAFT 
PUSH ROD 
PAWL  TORQUE TUBE ASSEMBLY LOCK PIN 
PAWL 
A
Figure 1 Parking Brake Linkage 

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400 MAINTENANCE MANUAL
EICAS
SEE  A 
SEE  B 
CONTROL STAND A 
SEE  C 
STAND CONTROL  DECAL 
REMOTE BRAKE INDICATOR LIGHT (L1541) P37 PANEL (ON SOME AIRPLANES, THIS PANEL IS LOCATED AFT INSTEAD OF FWD OF NOSE GEAR STRUT)  PARKING BRAKE LEVER 
BRAKE  PARKING  BRAKE 
ON  BRAKE SET  OFF 

PARKING BRAKE  BRAKE OFF
B
SET LIGHT  LIGHT
(L1539) C (L1540)
Parking Brake Indication
Figure 2

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
747-400
MAINTENANCE MANUAL

 (2)  
The PARK BRAKE OFF light (L1540) comes on when both Brake Pedal Position switches (S930 and S931) sense that the brake pedals are not in the applied position.

 (3)  
The PARK BRAKE ON light (L1541) comes on when both brake pressure switches (S129 and S130) sense that hydraulic pressure is present at the brakes.

 E.  
When the parking brake is released the linkage moves to the off position which permits the micro switch to return to off. This passes power form the PARK BRAKE circuit breaker to the open fields of the electric motor in the parking brake module. When the electric motor has cycled to off, power and ground are moved from the Parking-Brake-Valve-Closed relay to Parking-Brake-Valve-Open Relay. When the Parking-Brake-Valve-Open relay is set, a ground signal to the EIUs suppresses a fault indication on the EICAS screen. If the electric motor in the Parking Brake Module fails to cycle to the open position both relays will be relaxed producing no ground signal to the EIUs. This will cause a PARKING BRAKE VALVE indication to come into view on EICAS.

 F.  
If the electric motor on the parking brake module fails to open when the parking brake linkage is released, pressure will build up in the return lines of the antiskid valves causing an antiskid malfunction. For this reason the parking brake valve status is monitored by the antiskid circuits in the Brake Control Unit. When the Parking-Brake-Lever Switch is in the off position and the Parking-Brake-VAlve-Open Relay remains off, a ground is removed from the parking-brake-logic input in the brake control unit. This causes an ANTISKID OFF indication to come into view on EICAS.

 4.  Parking Brake Module and Thermal-Pressure-Relief Valve (Fig. 3)
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 A.  The parking brake module prevents the brake accumulator from discharging through the normal antiskid valves when the parking brake is set. It is installed on the keel beam in the right-body-gear-wheel well and is connected to the hydraulic system return lines from the normal antiskid modules.
 B.  The Parking Brake Module contains an electric motor actuated cartridge valve and a check valve. The motor can set the valve to one of two positions called POS 1 and POS 2. In POS 1 the IN-A port is blocked and the IN-B port is connected to the OUT port. In POS 2 the IN-A port and the IN-B port are connected to the OUT port. The check valve is installed in the OUT port to prevent backflow from the hydraulic system return line.
　
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