Pressure in Coolant Header Tank - Problem?

[ph67]

I've noticed over the last month or so that the water level in the header tank is varying, sometimes below the middle of the tank and sometimes above - if I open it when it's cold (and not been driven for a day or two) there is pressure in there and the water level rises as it escapes...it didn't used to do this so I don't think this is right as all my previous cars never had this. I also suspect that the level has gone down and there may be a leak somewhere - has anyone else had this? if so what was the problem please? this will at least give me a few ideas as to where to start looking. thanks

[Celes]

How much pressure is there when the engine is HOT?

[ph67]

About the same or a small amount more than when it's cold - there's not a lot of difference.

[tony-1977]

What you describe sounds like the start of head gasket problems. Especially with pressure being there after not driving for a day or two