Leak on ferroli boiler from pump air vent?

I have just bought a repossesed house with the ferroli boiler, which was fully drained including the radiators. As I was filling water in the boiler, it started to drip leak from the vent at the top back of pump. My understanding is that the system was fully working when the previous owner left the property and it was made safe by the repossesing company by draining the system. What shall I do ? Is the pump or airvent gone faulty? Does it require replacing or is there a solution to this?
Thank you for your help.