Not sure what engine you have.

A friend of mine had what looked like a suction problem that drove him crazy for days. Turned out that he had not cleaned the old (literally paper) paper thin gasket off of the face of the water pump when he rebuilt it. So he had 2 gaskets in place. That thin extra piece of paper gasket did not allow the pump to seat as well as it should.

Once he cleaned it up it sucked up the ocean!

Greg