If its any consolation I have had the same fight you are having a few times.
You need a lot of patience, silicone grease and cleanliness. A single hair across the o ring can cause a leak
Watch out for split plastic as already mentioned
Take care that you dont get a nick, cut or chafe on any O ring.
Follow that copper pipe across to the left of the boiler and theres a non return valve there that has a habit of leaking too. This usually settles down after a while but sometimes needs a container underneath to catch drips until it does.
You need a lot of patience, silicone grease and cleanliness. A single hair across the o ring can cause a leak
Watch out for split plastic as already mentioned
Take care that you dont get a nick, cut or chafe on any O ring.
Follow that copper pipe across to the left of the boiler and theres a non return valve there that has a habit of leaking too. This usually settles down after a while but sometimes needs a container underneath to catch drips until it does.