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Figure 12-26.-Typical power brake control valve system. When the brake pedals are released, the main system pressure port in the master cylinder is closed off, and fluid is forced out the return port, through the return line to the brake reservoir. The brake reservoir is connected to the main hydraulic system reservoir to assure an adequate supply of fluid to operate the brakes. When the emergency air system is used, air pressure, directed through a separate set of lines, acts on the shuttle valves, blocking off the hydraulic lines and actuating the brakes. POWER BRAKE CONTROL VALVE SYSTEM A power brake control valve system is used on aircraft requiring a large volume of fluid to operate the brakes. As a general rule, this applies to all patrol (VP) and reconnaissance (VR) aircraft, and certain attack (VA) aircraft. Because of the weight and size of the aircraft, large wheels and brakes are required. Larger brakes mean greater fluid displacement and higher pressures. For this reason, independent master cylinder type of systems are not practical on heavy aircraft. A typical power brake control valve system is shown in figure 12-26. In this system, a line is tapped off from the main hydraulic system pressure line. The first unit in this line Figure 12-27.-Power brake control valve (pressure ball check type). is a check valve, which prevents loss of brake system pressure in case of main system failure. The next unit is the accumulator, the main purpose of which is to store a reserve supply of fluid under pressure. When the brakes are applied and pressure drops in the accumulator, more fluid enters from the main system and is trapped by the check valve. The accumulator also acts as a surge chamber for excessive loads imposed upon the brake hydraulic system. Following the accumulator are the pilot’s and copilot’s brake valves. The purpose of a brake valve is to regulate and control the volume and pressure of the fluid that actuates the brake. Four check valves and two one-way restrictors, sometimes referred to as orifice check valves, are installed in the pilot’s and copilot’s brake actuating lines. The check valves allow the flow of fluid in one direction only. The orifice check valves allow unrestricted flow of fluid in one direction, from the pilot’s brake valve; flow in the opposite direction is restricted by an orifice in the poppet. The purpose of the orifice check valves is to help prevent chatter. The next unit in the brake actuating lines is the pressure relief valve. In this particular system, the pressure relief valve is preset to open at 825 psi to discharge fluid into the return line. The valve closes at 760 psi minimum. Each brake actuating line incorporates a shuttle valve for the purpose of isolating the emergency brake 12-32

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