If you can hear air releasing when you open the cap in the morning there is no pressure loss so it would appear the cooling system is fine. A pressure test is done to determine leaks in the system both internally and externally.
As for the drop in level when engine warm, the thermostat opens up and allows coolant to circulate around the engine and heater system etc.Thats why you should always check level cold.
The initial level drop could be that there was an airlock after the rad was changed.
I would be surprised if you had a problem.
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