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Hello, my name is Mark and I will try help you with this.
The first thing you need to do is establish if you have a leak and from where.
This can be done by using a tool to pressurise the system and then looking for signs of water dripping under the vehicle.
If the pressure drops and you cant see any leakage then either the heater matrix is leaking into the cabin and the water is being soaked up by the insulation under the carpet or you have an internal leak in the engine, possibly the headgasket.
Releasing the cap will allow for coolant to rush back into the tank. This is not a fault or an indication of a fault.
The system being under pressure is also normal.