Quote:
Originally Posted by Kev75
Is the hiss pressure releasing or air being sucked in to a slightly lower pressure in the header tank?
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The pressure valve system in the cap also contains a hair spring vacuum relief valve that allows air into the tank if required during cool down. Of course air will only be drawn in if any air or coolant was emitted during the engine run. Here's a detailed drawing of the pressure cap in which the vacuum valve can be seen at the top of the central unit.
TC