[vettestr]

I have seen pressure problems created from incorrect expansion tank installs. You said car is new to you so no history or way to know if this was a purchased problem or not.
In short one way to determine if I am right or way off base is an easy test. With a cold engine only do not try this warm or hot. If the remote expansion tank is plumbed right you can start engine without the cap on it and it should leak nothing for a moment or two. If plumbed wrong the missing cap will puke coolant as soon as you start the engine so be ready to shut it off quick. If all is correct the remote expansion tank only has pressure when coolant is heated and then to about the level of whatever cap you have.
If it pukes coolant we can then get into the plumbing. If it does not leak no sense going through the whole 9 yards.