When I put in the new impeller I noticed the old gasket showed evidence of a small leak near one of the screws for the covering plate. The plate is clean and smooth, I removed the old gasket and put on a new one, but possibly there's a small flaw there that prevents a perfect seal. This fall I might just replace the entire pump, especially if I don't track down this problem. Thanks again for the thoughts, Max

PS its a Yanmar