I don't get it.  What is wrong with the cooling system remaining pressurized?  Mine does and always has. 

 

Some of the coolant is going to vaporize.  This pressurises the system.  Some of that vapor will never go back into solution so there should be pressure in the system even when cold.  In fact.  A lack of pressure in the system a day after shut down indicates that there is a leak.  The fact that it as able to remain pressurized says that it is not leaking out anywhere or else the pressure would fall.

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Dave Leonard
Turbo Rotary RV-6 N4VY
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Dave my system goes back to zero. Do not follow your logic for my system. Nothing should leave my system during flight and should go back to original state after cool down. I have a Volvo pressure bottle with a 21 psia cap that blows about 24 indicated on the EM2

 

Bernie