Hello
On a 496 exhaust manifold they have a drain assembly on the water supply hose, how does this drain work or operated . please

Just take that blue tire pump attached to the heat exchanger and pump that system with air until it pumps full. It's not for a real hard freeze but it gets most of the water out of the system. Pull the brass ring release when you're done. You'll hear the water pouring out in the bilge.

You can also just pull the bottom hose off of each side just above the spark plugs. Either way will drain the manifolds. It will not clean debris out of them - just water.

You can also hook up a light duty (think 12V cigarette lighter type) air compressor. It empties the majority of the water from the sea-water side of the system; but as mentioned above--do not use this as your only method of winterizing.