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I've been working on this for the last four days, and it took over 120 cycles (what the manual says could be "several" but I lost count) of back-and-forth rotation to get it to where there were no more air bubbles and the pump was finally quiet again. Determined to fix this once and for all, I had gotten an OEM pump off a junkyard Z34 and installed it, then proceeded with the bleeding procedure in cycles of 10 at a time. I've replaced the pump at least three times before over the years, and have tried the cold-weather fluid as well. Nothing has ever worked and most things just made it worse, because the problem all along has been pump cavitation from air in the system that I never knew the right way to bleed it out.

So I know it has been a while, but in case this is still a problem for anyone and for future reference, I found that I have been doing the bleeding procedure incorrectly - and most of the YouTube instructional videos are too. I found the proper procedure in the FSM and I'm posting it here. BLEEDING THE POWER STEERING SYSTEM After replacing the fluid or servicing the power steering hydraulic system, you must bleed air from the system. Air in the system prevents an accurate fluid level reading, causes pump cavitation noise and over time could damage the pump. To bleed the power steering system, proceed as follows: 1. Begin with the engine off, front wheels off the ground. and wheels turned all the way to the left. 2. Add either standard power steering fluid or optional cold climate fluid, to the "FULL COLD" mark on the fluid level indicator. Refer to MAINTENANCE AND LUBRICATION (SECTION OB) for information on power steering fluids. 3. Bleed the system by turning the wheels side to side without hitting the stops. ! Important This may require turning the wheels from side to side several times. Keep the fluid level at the "FULL COLD" mark. Fluid with air in it has a light tan appearance. This air must be eliminated from the fluid before normal steering action can be obtained. Start the engine. With the engine idling, recheck the fluid level. If necessary, add fluid to bring the level to the "FULL COLD" mark. 5. Return the wheels to the center position. Lower front wheels to the ground. Continue running the engine for two or three minutes. 6. Test the vehicle to be sure the steering functions normally and is free from noise. ! Important Inspect for fluid leakage at connection points along the power steering system. 7. Recheck the fluid level as described in Steps 3 and 4 except that the fluid level should now be up to the "FULL HOT" mark after the system has stabilized at its normal operating temperature.