Agreed, although I think that was aimed at me:
The only pressure generated in an NPG+ cooling system is due to the expansion of liquid, never by the formation of vapor. Consequently the build-up of pressure in a system using NPG+ is typically in the range of just 2 to 7 psig and gasket seals, hose connections, and even the radiator core, are under less stress and operate safer and longer.
If you have no pressure in the Mark VIII cooling system the expansion tank will siphon the low side upon cool down. If you have no cap or a hole in it you will build 0 psi. Duh.
I agree with the fact that until it boils the pressure will be low but its not a magic liquid that will not go above 7psi
so why not just leave the 15psi cap on its not like it will hurt anything if anything more pressure will increase the boiling point.
Also you want the cooling system under pressure since not all of the fluid is at the same level there are hot spots in the engine where a small amount of liquid is super hot and can form a small vapor pocket and actually insulate the heat into the engine higher pressure will help prevent this