The leak could be v. small. It could open up as the pipe gets hotter.
Pressure loss could be v. slow.
The problem with boiler readings is that they will vary with the temperature of the hot water. So its hard to check.
All that said- if your problems have arisen only since you put down a new...
I have been experiencing falling pressure with a Sealed System Boiler
set up. I have my central heating zoned. I have no pressure loss when heating upstairs only- but lose pressure when heating the ground floor only. I have added Firnox leak repair and this has had some effect- slowing down the...
Have you considered that you might just have a leak? If you are on the 4th floor you might ask the 3rd floor if they can see signs of a leak from above.
I am experiencing a similar problem- I have added Firnox leak sealant to my system ( this is akin to radweld for cars). My pressure is still...