THERMOSTAT FILLER

[billwill]

If that was true then coolant would be spewing from it whenever pressure exists in the system and that is not the case. That cap has the inner and outer seals, just no valve to allow the inner seal to be released

There are TWO valves at the bottom of our usual cap. The bigger one is controlled highish pressure controlled by the big spring, the return valve is a small disk in the centre of that big valve working in the opposite direction at a very low pressure difference.