So the Thermostat was dead....

but I changed it...and there is still some presuure in heater tank ( and bubbles when engine is running (hot)).

How can I check up 100% if the head gasket (or head) is OK? There is no OIL in water, no Water in oil...no white smoke...

And when I turn off the engine (hot) ..water is bubble from engine to heater tank - is that normal?