My set-up
My set-up is just like Andy describes - I just close the ball valve when the engine is running, I've never had a problem with this setup. Also, I have another Tee in the same line (also isolated with a ball valve) with a line off the Tee that runs into the bilge. If I close the seacock and the ball valve to the foot pump I can start the engine and suck the bilge dry real quick. It's an emergency bilge pump. I haven't had to use it in an emergency but I tried it out with a full bilge and it works great. I used fresh clean water from the hose to test it so as not to gunk up anything in the cooling system, but I wouldn't hesitate to use it if I found a foot or two of water in the cabin. That's actually what prompted me to install it, I found 2 feet of water in the cabin while in some real nasty seas in the middle of the channel between between The Big Island and Maui. The bilge pump eventually caught up with the water, but it got me to thinking "what if it didn't?"