backflush neutralizer?

[Barkas]

I have started my oil fired open vented heating system up after the summer 3 month break, but only the upstairs rads are getting hot, Ive had this prob on odd years before and I have resolved it by fitting a mains hose to the feed and then return rad pipes downstairs and pushing airlocks back to the header tank. My system is open vented...has 2 multi speed pumps set normally at LOW but have tried to force air out by turning both to HIGH and failed....and a real fire back boiler connected into the system via a Dunsley Baker Nuetralizer. When the pumps are running I can hear water splashing and running through the nuetralizer housing as if its full of air and i think this is where the problem air is gathering, there seems to be no air bleeder on the top, only a large brass stopper. Is it safe to try and bleed the air from this stopper or fit a pipe downstairs and backflush the air out of this neutralizer...ive backflushed it in the past and its got the heating running. Any advice would be appreciated..Regard