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A 737-300/400/500MAINTENANCE MANUAL

BL 0
VIEW LOOKING AFT
Brake Hydraulic System Component Locations
Figure 1

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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 2.  Alternate Brake Selector Valve (Fig. 2)
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 A.  The alternate brake selector valve blocks system A hydraulic pressure to the alternate brake metering valves when system B pressure is available for normal brake operation. When system B pressure is lost, the valve will open to apply system A pressure to the alternate brake metering valves.
 B.  Under normal operating conditions system B pressure holds the select valve closed preventing system A pressure from entering the alternate braking system.
 C.  Following takeoff when the landing gear is retracted system B pressure continues to hold the selector valve closed, but landing gear retraction pressure is now available and ported through the selector valve to the alternate brake metering valves to stop wheel rotation.
 D.  When system B pressure is lost the valve opens under system A pressure and simultaneously closes the alternate braking system to landing gear retraction pressure. At the same time a pressure sensitive switch in the alternate brake select valve activates when system A pressure to the valve reaches 1900 psi and sends a signal to the antiskid control module to annunciate alternate antiskid failures. Autobrake control is not available to the alternate system.
 3.  Brake Control Linkage (Fig. 3)
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 A.  The brake pedals as part of the rudder pedal assemblies actuate brake control linkage below the control cabin floor. Captain's and first officer's brake pedals are interconnected through linkage below the control cabin floor, to have identical brake operation from both positions. Connected to the control linkage are two horizontal quadrants. The quadrants are connected to the brake metering valves by dual control cables. Movement of either right or left brake pedal will actuate the corresponding right or left main gear brake metering valve and movement of both brake pedals together will apply both main gear brakes.
 4.  Brake Metering Valve (Fig. 4)
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 A.  For each main gear, identical normal and alternate brake metering valves are fitted in an assembly for simultaneous operation. Each left and right metering valve assembly is mounted on the wheel well ceiling at LBL 10 and RBL 10 respectively just forward of the junction of the aft wheel well bulkhead. The assembly consists of a cable quadrant mounted above the ceiling driving a shaft projecting through the wheel well ceiling on which adjustable cranks are mounted 180° apart. The metering slide of each brake metering valve is contacted upon rotation of the crank assembly to meter hydraulic pressure. The normal brake metering valve is rigged to actuate slightly ahead of the alternate valve.
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A 737-300/400/500MAINTENANCE MANUAL
PRESSURE SWITCH
TO BRAKES

VALVE POSITION FOR HYDRAULIC SYSTEM B FAILURE
Alternate Source Selector Valve
Figure 2

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