I checked the new service valve cap and it is aluminum with no rubber washer inside. It looks slightly dirty/corroded, so I give it a good chance to be the culprit. As you say, it could leak from elsewhere. Given the valve is leaking heavily (blows soap bubbles when I checked), the system is relying on the cap to seal it.
The system is old, but I'd like to get another year or two from it. See my other post of why I doubt I will recoup much of the cost of a new system. John