J/30 Class Association Forums J/30 Forum The Boat, Sails and Equipment Anti siphon valuse

You shouldn't need to close the valve. If you loose prime, you probably have an air leak somewhere between the seacock and the suction to the raw water pump. I had a problem that turned out to be an air leak with the gasket at the top of the raw water strainer. I replaced the gasket and haven't had a problem since. Also make sure you have a good air tight seal for each of the hose fittings. You can have a water tight connection, but still have an air leak.

Try an experiment - run ~ 3 foot hose clamped directly on the raw water pump suction to a bucket of water sitting in front of the engine. The raw water pump should take a suction and self prime when the engine is started. If that works, you definitely have an air leak in your setup. If not, you probably need to change out the impeller, and potentially the back plate on the raw water pump.

The hose in the bucket method is how I winterize my engine, through the seawater strainer after cleaning it out. I can tell right away when I haven't sealed the strainer cover gasket properly.