DIY Doctor UK DIY Forums

I have an old, but reliable, cast iron boiler- pumped system. The problem is that I'm getting air in to the system and want to fit an auto air vent - can anyone advise a) recommended type / model etc and b) correct place to fit ...... highest place in system - got a bleed valve off from my hot water cylinder - do I remove the existing bleed valve and replace with the auto vent ?
.............................. thanks for any help - suggestions welcomed