The roadster has from memory the same plastic expansion tank from a fiesta as your the tank & it pressurises I think to 7 psig. The cap is tight when hot.
This is a custom tank but with a pressure cap
![[Linked Image]](https://tm-img.com/images/2023/11/08/IMG_9991.jpeg)
And this is mine with the same pressure cap
![[Linked Image]](https://tm-img.com/images/2023/09/29/WhatsApp-Image-2022-11-29-at-12.33.49.jpeg)
I found the level after a while as the rad slowly vents air from the header. If the heater works then I expect the level is OK.