Power steering problem

I bought a clean '87 LeSabre for a work car with a very noisy power steering pump. I put on a used one, and it was noisy as well, as I
would expect until I raised the front wheels off the ground and turned the steering wheel all the way left, then right, etc -- about 40 times, after which about 90% of the noise went away. I didn't drive it after that, and then I let someone else drive it for a week while theirs was in the shop. I got in it last night and the power steering pump is as noisy as before, and the fluid looked foamy. Why would that be? Do you sometimes need to bleed the air out of them several times? Or will this likely continue happening because the pump is not any better than the one I took off?
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