I know that the cooling system is pressurized, but when my both fans switch on, the pressure drops immediately, keeps low until the fans stop again and then raises in about 30sec - 1minute up to the original level. Even if I run with radiator cap off, the coolant level drops down and then goes up again.

I can't understand why, is there any reason? I think that the fans are not somehow directly connected to the coolant?

 

they are not, but what causes pressure is temperature, so if temperature goes down, so also the pressure

just my 02

 

Just for your info: I found that water expands with temperature. We have about 7 litres of coolant in the engine, and when the engine warms from cold to the operating temperature, the coolant expands to 7.1 liters.