Hi @simple7 ,
Don't know your machine at all but was wondering if this link may be of interest to you. "O" rings are usually meant to stop leaking from connections in water systems. ;-)
Here is a link to a supplier if this is the problem. It is not a recommendation to use them.
I have found the service manual for your machine type (I think).
On p.2-9 it shows a block diagram of the water system for an Incanto S-Class De-Luxe. It shows the relief valve being connected at the pump not the boiler, so unless it is physically mounted on the boiler inlet and not on the pump outlet I'm wondering what valve you are referring to.
Going quickly through the manual it appears that the HWS valve controls the water and steam dispensing function. It also states that if there is no steam coming out of the wand it may be blocked and needs to be cleared using a needle.
Hopefully you may be able to take it further with the help of the manual.