Pull the head off to check the diaphragm. I am still thinking that there might be something stuck in there causing leakage. It is a pretty easy fix if that is all it is... Just saying... I also assume you have checked that the prescreen filter is clean? The one pump I had that the diaphragm failed was after a few seasons.

If it is indeed the diaphragm, there are parts out there that you can replace it with. In case you strike out with your seller.