DOT 5 silicone doesn't absorb water like regular glycol fluid so it should maintain its boiling point forever, which is ~500 F. If water did get in, it would settle at the lowest point, which is unlikely to be the hot spot. If it was, the water could boil to cause spongy vapor. But don't drip water in since it can cause rust. Same issue with your power steering reservoir. DOT 5 is actually a rating and there are glycol fluids which now meet it. They are termed 5.1 to distinguish from silicone. Being glycols, they can absorb water, which lowers boiling point and causes rust.