Porsche 2012 911 GT3 CUP Technical Manual page 17

3.4.4. Filling the cooling system
A quick-release coupling is integrated in the engine bay in the area of the
expansion tank. The water system can be filled and bled quickly and
safely via this coupling – after an engine change for example.
Quick release coupling
When the water system has reached its operating temperature,
the expansion tank cap should under no circumstances be opened
and the pressure released. Since the operating temperature has
already been achieved the water pressure cannot be built up
again. This system pressure is necessary to increase the coolant
boiling point.
14