Check the o ring in the cap, if water gets past it water comes out a little hole in the side of the expansion tank (a pin hole in a square bit near the bracket which holds it on)
Ihad this few weeks ago and luckily my neighbour had a vento which he lent me the cap off,then i just pushed the seal back in mine.
If you pour water down the edge of where the cap goes,sort of down the side of the thread youll see water come out.
Think a lot of people replace these for the later blue caps.
