C5 citeron estate pressure in cooling system

We don't have any info for this car but have it pressure checked to see if the head gasket or intake gasket is leaking. Especially with the coolanloss you have.

Pressure and expansion of coolant is normal the coolant goes into reserve tank and returns to system when cooling off when engine is off. Bad radiator cap can prevent this return. Did you get new cap?

If vehicle does not have radiator cap then it is not a return/overflow tank but an integral expansion tank. Coolant loss is either external leak somewhere or internal and escaping thru exhaust.