I have one of these which has worked well for years. Some time ago just about everything except for the pump and heat exchanger was replaced. Today I noticed the boiler needed topping up and I did so to around 1 bar. I then found the automatic air vent was dripping. I took it off and tried cleaning the float and crud out. I screwed it back in and repressurised the system. It still leaked! I have a new one but note that there is no washer with it and I can see that the old one had a recess specifically for a rubber washer which was inset.. The new one came with no washer and has no similar recess for such a rubber washer. Should I use PTFE tape instead of a washer or both?