From hot engine to ambient temperature the system should go into so vacuum and that is how coolant is sucked back from the reserve tank. If the ambient temperature goes up there may be some pressure build up but that should be minimal. On a cold engine with the cap removed when the engine is started the level should remain the same or even get sucked down by the water pump. As the engine warms the level will slowly rise and after some time flow over. If the coolant starts getting pushed up right after starting something is wrong. 

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Dan N
Sent: Wednesday, February 17, 2016 1:02 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Expansion tank under pressure when cold?

I think it depends on how long you let the engine rest after you shut it down. There always some residual pressure in the cooling system and it will come near zero if you wait long enough... I let it sit overnight. My mechanic revs the engine up to bring the level down, add the coolant and put on the cap presto..