syco

I know it's a while since you posted this, so you've probably sussed it by now but I recently just had the same problem myself. It might sound really simple but check the rubber seal on the expansion tank cap. I spent three nights trying to get to the bottom of what I thought was an air lock, but it turned out to be that the system wasn't sealed properly, so the water kept backing up and gushing out passed the o-ring which had slipped out of its recess.