Good answer - what you have is an "adapter fitting" - what you need is a "transition" fitting.
Heat and cold cycles impact copper and CPVC differently, and transition fittings are designed to not seal on the threads, but on a gasket. The only way to fix this is switching to a transition fitting.
The transition fittings typically are CPVC on one side and metal on another.
http://www.ppfahome.org/images/CPVC_Transition_Presentation_FINAL.pps
