Hi Wayne and I am unaware of any overflow/vent pipe, it being an air lock system which is something I have never come across till getting the C8. I have tried just about everything to get the truck to run without the coolant overflowing after twenty minutes driving and the only thing left would be to strip the water pump to see if the drive turbo is intact, I also don't have a thermostat fitted which would I suppose make a bit more pressure in the system but in Queensland the need for a thermostat isn't there. Let you know how I go with the road run.
Paul
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