Figure 7-23 shows the three phases of pump compensation in a pressure buildup order, starting at low pressure and increasing to full system pressure. As shown in view (A), the yoke control piston is spring loaded to hold the displacement yoke at its maximum displacement angle of 30 degrees. This spring is opposed by the existing system pressure, which acts at all times on the “constant horsepower” piston area; however, the hydraulic force will not be sufficient to move the yoke control piston until the actuating pressure (system pressure) builds up to 2,200 psi. Thus, the cylinder block will be canted to its maximum angle, and the pump will deliver its maximum flow, 8 gpm, when system pressure is less than 2,200 psi. View (B) of figure 7-23 shows how the yoke con- trol piston responds to system pressure fluctuations in the 2,200 to 2,950 psi range. Assuming that system pressure is steadily increasing, the displacement yoke angle will decrease from the 30-degree full displacement angle to approximately 22 degrees, which will produce 6 gp at 2,950 psi. View (C) of figure 7-23 shows how the spring load on the compensator spool is overcome by system pressure in excess of 2,950 psi, and the displaced spool meters pressure to the "cutoff" area of the yoke control piston. This pressure will act with the “constant hp” force on the piston, and with increasing pressure, the piston will move rapidly from the 22-degree displacement angle at 2,950 psi to approximatey 0 degrees at 3,000 (plus 150, minus 0) psi. When full pressure exists, the hydraulic power output will be the minimum required to replace fluid that leaks internally. The gearbox installed between the motor and pump contains lubricating fluid for internal lubrication, a dipstick for checking its fluid level, fill port to replace fluid, drain port to drain fluid during maintenance, and a relief valve to allow excess fluid to be relieved overboard. The pump gearbox is drained and reserviced with clean hydraulic fluid as follows: 1. Remove the magnetic drain plug and catch the fluid in a suitable container. 2. Inspect the magnetic plug for foreign particles that may have accumulated during periods of operation. Particles that look and feel like “fuzz” are considered acceptable; however, particles containing metal “slivers” require pump overhaul. 3. Remove the filler plug, and flush the gearbox with hydraulic fluid. 4. Clean the magnetic plug, install with a new gasket, and lockwire after replacing. 5. Refill gearbox with hydraulic fluid. 6. Install filler plug and dipstick, using new gaskets. Also, the pump pressure line, fitting, and filter screen are removed. The filter screen is cleaned, using Dry-Cleaning Solvent P-D-680, and reinstalled using a new gasket. The pump pressure line is reinstalled and an operational check performed. NOTE: Hydraulic pumps that are not functioning properly can represent a serious source of contamination in an operating hydraulic system.    Hydraulic contamination is discussed in chapter 4 of this TRAMAN. RELIEF VALVES Relief valves are not new to most people; different types of relief valves are used in our homes and automobiles, as well as many other places. Relief valves are pressure limiting or safety devices commonly used to prevent pressure from building up to a point where it might blow seals or burst or damage the container in which it is installed, etc. In aircraft, relief valves are installed within hydraulic systems to relieve excessive pressurized fluid caused from thermal expansion, pressure surges, and the failure of a hydraulic pump’s compensator or other regulating devices. Main System Relief Valves Main system relief valves are designed to operate within certain specific pressure limits and to relieve complete pump output when in the open position. Relief valves are set to open and close at pressures determined by the system in which they are installed. In systems designed to operate at 3,000 psi normal pressure, the relief valve might be set to be completely open at 3,650 psi and reseat at 3,190 psi. These pressure ranges may vary from one aircraft to another. When the relief valve is in the open position, it directs excessive pressurized fluid to the reservoir return line. 7-22