HELP!! - Coolant Overflow filling up and not returning - WTF?

[TODA AU]

Thanks Adrian..

If its lower or higher what does it mean??

Set the pressure to say 15psi & walk away.
15~20mins later, come back & see what the pressure is...
If it has dropped, you have a leak.
If it's the same, all good...
If it has increased, someone is hiding in the dunny playing tricks on you.