[C320-list] Raw water pump question

[Crosby Roper]

I went to change the impeller on the raw water pump last weekend on my 2004 C-320 (hull 1054). As always, I closed the sea water intake under the aft berth. When I removed the cover of the raw water pump, there was a steady flow of water coming from the lower opening to the pump. I thought it would stop at some point but it continued in a quite healthy fashion throughout the process of changing the impeller. This has never happened before. I removed the raw water filter by the sea cock to see if perhaps there was a problem with the seacock itself, but there was no flow of water from the through hull at that site, so I am baffled as to what is causing this. I looked in the yanmar 3YM30 manual and it mentions the possibility of siphoning but I'm not clear exactly where it would siphon from. I checked the anti-siphon loop in aft port lazarette and the duck bill valve appears to be fine. Has anyone else experienced this? Any input is appreciated.

Crosby  Roper VMD
s/v Tethys
San Diego, CA