On my last trip I almost overheated again. When I checked, the overflow bottle I have fitted was almost full. And putting that back into the expansion bottle brought the level back up. So I don't have a leak anymore, it's being dumped via the pressure cap on the expansion bottle.
So either the pressure cap is faulty, the seal between the cap and the bottle is dodgy, or the cooling system is over pressurising. The cap is 20psi, so I don't think it is the third option. As a first step I will buy a new cap at 22psi and see if that helps. If it doesn't help then I will have to replace the expansion bottle. I do have a spare, but swapping them is not a trivial task.
Watch this space.
