While doing some other work, I have found that the cooling system on my 328 doesn't seem to pressurize. The normal pressure build-up seems to be being vented by the small disc valve that is on the cap to allow coolant to return to the system. Since there is no overflow tank on these engines, it SEEMS to me that the proper cap should EITHER have no such disc at all OR the little disc should be lightly spring loaded closed as opposed to hanging open. If it's open, I don't see how the system can pressurize with a normal, slow increase in coolant volume that occurs as the engine warms up and the coolant expands. I am using a typical 15PSI Stant cap. using this model cap since shortly after buying the car in '08 when a local "expert exotic" shop changed the coolant/cap. I've since replaced the coolant/cap several times but I'm now thinking the system hasn't been pressurizing for 11 years! Any thoughts re proper Rad caps?