This sounds like you either have air in the system or the pump is failing. I understand that you did not see anything when you took it apart but the fact that you only get assist when the RPMs are up, is an indication the pump is failing and most likely leaking internally.
I included the process to remove, rebuild, and reinstall the pump that you may not need but they are there if you do.
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Saturday, July 4th, 2020 AT 6:06 PM