Simon, I've just checked my notes and the samples I have left from the earlier dissections. I actually have a 100 cap that I'd forgotten about. The HowTo does include some detais on it and basically, the design is consistant throughout the pressure range. On that basis, the overpressure venting takes the path into the handwheel and out from under the finger grips. So, the exact function of the 'overflow' pipe is debatable. It simply drains the annular space between the outer neck of the tank and the tapered inner retainer for the valve housing. This space is open to atmosphere when the cap's in place, so any excess pressure venting into that area won't take a path through the pipe. It'll go into the handwheel and out. So,my take at the moment is the 'overflow' pipe is exactly that. It drains any coolant that's spilled from the filler neck when topping up.
TC
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