On my 1.9 130 I have a small coolant leak, which only needs topping up every 1000 miles or so. When I went to top it up a few days ago I noticed that with the filler cap off when topping up coolant started to run out of the bottom of the bottle quite quickly. As soon as the filler cap was replaced the leak almost stopped. I assume that without having an available air inlet the pressure of the system prevents the coolant from leaking as quickly as it does with the cap off? It appears to be leaking from what feels like the mounting point for a clip on the bottom of the header. Is there something there that is prone to leaking? Is it repairable or do I just get a new tank? How much coolant does the system take?