Question about pressure and level in coolant

[KuaiThunder]

Thx for the reply.

I changed both the cap and bleed valve few months back when I have moisture issue which solved the moisture problem.

I can understand the pressure build up in the system but I can't understand why pressure pushed down the coolant level. I suppose coolant is not compressible so it has to go some where. I am thinking maybe I have a fail thermostat so the pressure pushed the valve open.