I suspect cavity wear. With calipers measure the cavity depth and impeller width. The impeller should be a hair wider than the cavity. If the other way around, that pump is done.
I've been able to resurrect two worn Obedorfers by sanding the pump face (not the cover) on a stationary sander but it's not recommended.
I've been able to resurrect two worn Obedorfers by sanding the pump face (not the cover) on a stationary sander but it's not recommended.
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