2.  Brake Control Linkage
A.  The rudder pedals (brake pedals) actuate brake control linkage below the control cabin floor (Fig. 2). 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.

Brake Hydraulic System Component Locations  591 
32-41-0  Figure 1 (Sheet 1)  Feb 15/80 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

531  Brake Hydraulic System Component Locations 
Feb 15/71  Figure 1 (Sheet 2)  32-41-0 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

3.  Brake Metering Valve
A.  For each main landing gear, a dual brake metering valve is installed on the brake metering valve control quadrant housing assembly located near the top outboard forward corner of each main wheel well (Fig. 1). The brake-metering valve housing assembly contains a crank, a link assembly, an auto brake piston, a brake metering valve slide, and a brake input shaft. The brake input shaft projects completely through the valve. The brake input shaft's upper end mates with the control quadrant shaft and, on the lower end, a crank arm is attached to actuate a piston of the brake valve feel augmentation actuator assembly (Detail A, Fig. 2). Attached to each valve are seven hydraulic lines: two each pressure, brake, return, and one automatic braking (Fig. 3). Valve ports are opened or closed by operating two circular grooved valve slides from the brake-input shaft through the link assembly. The link assembly will displace both valve slides together or only one slide if the other will not move for any reason.
B.  When the brake pedals are depressed, through the mechanical linkage and cables, an inward movement is imparted to the metering valve slide. As the slide moves in, the return port is closed and the pressure port opens to direct hydraulic pressure to the brakes (Fig. 3). Brake hydraulic pressure through a passage in the valve piston enters the compensating chamber. The pressure in the compensating chamber acts on the end of the piston creating a return force tending to close the valve. This return force varies with the intensity of braking pressure and provides a feel at the pedals. The desired braking effect with antiskid system OFF is obtained by depressing the pedals a greater or lesser distance. Cable stretch and variation of pedal position permit the valve piston to move back until both pressure and return ports are closed. At this point braking effect remains constant. When the pedals are released the pressure in the compensating chamber and the return spring move the valve slide out and the return line opens. As the return line opens, pressure in the brake line falls, the brakes are released and return force on the valve slide is relieved. The desired braking effect with antiskid system on is obtained by depressing the brake pedals to actuate the main wheel brakes as needed based on required stopping distance and holding steady pressure. The antiskid system will modulate pilots metered brake pressure by electrically controlling the antiskid control valve to maintain maximum braking pressure and efficiency over all types of runway braking conditions.
C.  Automatic wheel snubbing for the main gear on gear retraction is provided by a small diameter piston operating in a cylinder attached to the metering valve (Fig. 3). The cylinder is connected to the landing gear retract hydraulic line. When the landing gear control handle is placed to UP position, hydraulic pressure is directed to the automatic brake piston and the piston extends. Because one end of the piston rests on the link assembly, extension of the slide opens the metering valve and applies brakes; therefore stopping spinning wheels before the landing gear retracts.
522 
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Brake Control Linkage  500 
32-41-0  Figure 2 (Sheet 1)  Nov 15/67 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

4.  Brake Valve Feel Augmentation Actuator
A.  The brake valve feel augmentation actuator increases brake pedal resistance at low braking pressure without increasing brake pedal resistance at maximum braking pressure.
　
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