Glenn, when the system cools, air will re-enter the system thru the
radiator cap - it is designed to do that

Glenn Ashmore wrote:
Well, it is brass with a high temp polyethylene float. It has to be mounted
close to vertical to open. I was worried that when the engine cooled and
the pressure dropped it would act as an anti-siphon valve and let air back
into the system but after playing with it that does not seem to be a
problem. I do believe however that after the system is purged it would be
a good idea to close the outlet valve just in case and just open it when I
am changing out the coolant.