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I also thought about a bad shaft, but you say it's a whole new pump. (however, here we need to get a little more technical - was the pump housing that was replaced or the pump housing and motor?) If it's the original motor still, you may have a warped shaft. The other thing that comes to mind is that all the seal assemblies were installed backwards - if the original 'pro' installed it backwards, anyone who went to replace it would have put it in oriented the same way.

What would help us all clarify this issue is to see pictures of the seal assembly and seal plate, from next time you break the thing open to replace it.

While this is certainly a 'head scratcher', it is probably a quickly fixed issue, once we can figure out what's causing the problem. The more info we have, the sooner we'll be able to identify and help you correct this.