pressure relief valve?

presuming the pressure is not constantly above 3.0 bar, then the prv is passing, usually due to a bit of dirt under the valve seat. If you know what you are doing the valve insert can be removed & cleaned, which will generally cure the problem, if not it will need replacing. If access is possible then you can open the valve, by turning the red valve top briefly to open the valve briefly which may clear the debris. Hope this helps.

Ian
January 2009