Hi everyone,
Thanks for the input. The issue was resolved by replacing the shaft mechanical seal behind the impeller. In theory the impeller unscrews quite easily to access the seal, but I found that 5 years of sap makes pretty good lock-tite. A little heat to the shaft and impeller with a propane torch was enough to crack it loose.

Any other year this would have been a nightmare, but winter will not end in Wisconsin. Season is normally ending about now, but it has been so cold that that I have had next to no sap in the last 10 days and barely enough sap so far to use my RO 3 days total for the season. Should be interesting to see how the season ends