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POWER BRAKE CONTROL VALVE SYSTEM
Brake Debooster Cylinder

Aviation Structural Mechanic (H&S) 3&2 - How airplanes are built and how to maintain them
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Figure12-28.-Power brake control valve (sliding spool type). system from the normal brake system. When brake the brakes are applied. The seal is held in place by a actuating pressure enters the shuttle valve, the shuttle is automatically moved to the opposite end of the valve. This action closes off the inoperative brake system actuating line. Fluid returning from the brakes travels back into the system to which the shuttle was last open. Power Brake Control Valve (Pressure Ball Check Type) A power brake control valve of the pressure ball check type is shown in figure 12-27. The valve is designed to release and regulate main system pressure to the brakes and to relieve thermal expansion when the brakes are not being used. The main parts of the valve are the housing, piston assembly, and tuning fork. The housing contains three chambers and three ports. They are the pressure inlet, brake, and return ports. The piston assembly is made up of a piston head, piston shaft, pilot pin, and cross pin. The piston head separates the brake and return chambers. A cup seal prevents fluid from escaping to the return chamber when retainer and piston return spring. The piston head has a hole drilled through its center for the flow of fluid to the return port. This hole is opened and closed by the pilot pin. The pilot pin also opens the pressure port. The flange of the pilot pin and the hole in the piston head are lapped together. The piston shaft connects the piston head with the tuning fork. The shaft is slotted, and the cross pin prevents it from turning. The tuning fork connects the brake pedal linkage with the control valve. It swivels on the housing and limits the maximum pressure directed to the brake. The upper arm of the tuning fork is a bar spring that bends from the point of the fulcrum when hydraulic pressure overcomes toe force. Power Brake Control Valve (Sliding Spool Type) A sliding spool-type power brake control valve is shown in figure 12-28. This valve consists basically of a sleeve and a spool installed in a housing. The spool moves inside the sleeve, opening or closing either the 12-33







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