Combi Pressure Variable

Hello I hope someone can help. My CH combi boiler seems to lose pressure when the boiler is running. It is set at 1 bar cold but when it is running (burner on pump running) this falls to about 1/2 bar. If you then turn this off to fill using the fill loop the pressure gauge immediately goes back to close to the 1 bar.
Should I set the pressure with the boiler running? or is it OK Any help appreciated. Thanks
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Thanks for the replies. I am going to have a proper look over the holidays and don't see any problem topping up the expansion vessel. I'll post how it goes afterwards
Thanks again

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