Potterton Netaheat 10/16 Electronic Boiler?

When switching for ch/dhw boiler tries to fire up but intermittently fails, I can hear like a sloshing, there is on top a fitting which looks like an air vent. From the top of the fitting prob very old looks like a 22mm cap with a lock nut going to a hex nut with holes on each edge, going into a fitting attached to a 22mm pipe which is about 4" above a T connection. The top nut just turns and when the lock nut is lowered the top nut pushes down slightly but no air escapes, is it better to replace this with another air vent which is automatic, and would the system need to be drained. Please help its getting cold.