My central heating system has been working fine until just recently when the pump has started banging and the boiler has started tripping. It has never done this before. When the heating is started up it works fine for about 40 mins and then the pump starts banging for about 3 mins and then it works as normal. After an hour or so the boiler will trip. I have bled all the radiators and it has not cured it. I had a look at the 2 air release valves near to the boiler and one has a red cap loosely fitted on it and the other red cap appears to be missing. Would this missing red cap stop some of the air being able to leave the system and this is leaving an airlock in the system that is causing the pump to bang and the boiler to trip ?