?pressure relieving valve dribbling

[alban]

Hi
There is a black plastic cylinder about 4" tall (with an unscrewable red cap on) stuck on the end of a section of pipe coming from the HW cylinder central heating pipe. It seems to be dribbling a small amount of water. Is this a PRV? If so should it be open or closed (it can be turned) and should it be dribbling slightly (prob not)? I have put a new pump in and wondered if that was the cause? I think I 'played' with this valve in the vain hope it might solve the boiler problem I have asked about here and now I can't remember its previous position. A brief explanation would be helpful if anyone has the time!
Derr....?

 

[Gasman1015]

Not a pressure valve but an automatic air vent, cap should be open to allow air out, some drips can be expected but continuos drips indicate a faulty unit.