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About a month ago I installed a new control valve, cylinder, and hoses on my '76. After this installation, the power steering was very light. I attributed this to the fact that the replaced parts were 27 years old and probably not working correctly.
I put on about 200 miles since the replacement. The last time I put the car in the garage I still had the very light steering. Fired it up today to go for a spin, and I now have the less sensitive steering back again! I prefer the less sensitive steering, but what could have happened while it was sitting in the garage for a week?
200 miles does seem like a long, long time to work all the air out of the system. But hopefully, that was the cause of the light efforts. I don't have any other explanation.
About the only thing that could cause such would be a binding spring or ball etc. in the unit. I can tell you though, that it has nothing to do with air in the system. It just aint so, Urban Legend. You have too look at the design of the system. As soon as you fire it up the thing purges itself, after a couple of extensions of the cylinder each way that is done too.
It's true that I haven't heard of it taking 200 miles to get all the air out. However, if you start your engine with large slugs of air in the cylinder, valve, etc that air gets transported with the oil into the rotating group in the pump. It immediately gets the air and oil whipped into a milky froth. It takes a long time to get the air in suspension back out of the oil. The oil is compressible when it has entrapped air. Therefore, you get real squirrely feeling steering when you have air and oil mixed.
One other thing, you can have a loose clamp on a return line hose. It can be tight enough that oil will not leak out, but it will actually suck air into the oil that streams past when the engine is on. This also is rare, but it can be a real cause of pump noise with a steady stream of air entering the system.
I too believe your steering returned to normal when air was let out of the system. I replaced the exact same PS components and experienced the same change in my steering. Did you use regular or silicone PS fluid?