Looking at the picture, it looks like you need Torx sockets, not ordinary sockets.
Thanks for your answer Skippy. Your help is REALLY appreciated.pcaseiro,
My take is that it likely has fine threads and, given it's diameter, will need some persuasion to remove. There is a sealing washer under it if I'm looking at the EPC correctly. It looks like it's #14 Hollow Screw on the attached.
There's another picture suggesting the leaker is the high pressure side.
Any possibility there is residual pressure?
I recall from other posts that you will need to bleed the system well following your work as it's easy to introduce air into the system which is difficult to remove.