Every so often the tapered cones/plugs need to be lap fit with lapping compound or basically liquid sandpaper. The Spartan Lap Compound is a 320 grit silicon carbide in an oil/grease base. It works very well and is specifically made by Spartan for their seacocks.
You can also buy lapping compound made by Loctite, it's called Clover Compound, in a 320 grit formula but it is actually more expensive than the Spartan stuff.
My only complaint with the Spartan compound is that the oil and silicon carbide separate and it needs to be stirred well with each use.