Stant has two versions of radiator caps. Just went through same problem with our 68 Barracuda. They have a version that they call "vented" - the small brass vacuuum vent will not be tight to the rubber seal with this style cap. We never got the cooling system to build pressure. We switched to a "non-vented" cap and it will hold pressure like nobodies business. An OEM style cap for our car is bilt like the non-vented cap, except that on the OEM there is no outter gasket for use with a fluid recovery system. That little seal disk is up tight to the vacuum vent.
Do not confuss vented and non-vented in this case with the "safety vented" which have a lever to release pressure without removing the cap.
