I ran my truck with a 195° thermostat for 25 years with no pressure and never had a hiccup. Even going through death valley with temps only in the high 90's with a GVW of 5600 pounds (yeah it was loaded) if that helps. However I did have a mercury with an overflow tank and somewhere it was leaking air and when it was cooling off it didn't draw the coolant back in. Eventually the overflow overflowed, but there was no water in the engine part. That is when I started actually looking in the radiator rather than the overflow tank. It did this for 10 years, and I never did see a drop of antifreeze anywhere outside of the car. I realize it was a mercury and not a chevy, but that part is the same. I don't run pressure in my truck, as the stuff is old, and it doesn't need it.
