Select the process and click on 'Press to continue' tab. You will get the data of Cp and Cv. (just click on the scroll down menu of various properties to find out Cp and Cv).
The characteristic gas constant is given by (Cp-Cv).
Actually for ideal gases this is constant. I am not knowledgeble about the real behaviour of R134a with respect to pressure. I think you should better post it in any of the chemical engineering fora to get better understanding.
Thanks for the great resource, it is bookmarked. Actually I'm still fumbling with the theory and math. What I eventually want to know is how much work can I expect from R134a for a given heat input. I am utilizing the refrigerant in a heat engine loop instead of steam. If I could compare with steam for performance I'd be on my way in sizing the boiler and generating load specs. Thanks again.